FLUID-WICKING MEMBRANES BACKGROUND [0001] Certain types of chemical testing utilize a porous material through which a sample fluid flows by capillary action. As an example, lateral flow assays and vertical flow assays are types of tests in which a sample fluid flows through a porous material such as a porous nitrocellulose membrane. Lateral flow assays often include a test line and a control line. The test line can include a suitable test reactant that is reactive with a target molecule in the sample fluid. In some cases, the test line can indicate the presence of the target molecule with a visible color change of the test line. A control line is often located beyond the test line so that the sample fluid reaches the control line after the sample fluid has already flowed past the test line. The control line can indicate that the sample fluid has flowed sufficiently past the test line so that the test can be considered valid. Similarly, vertical flow assays can include a membrane with a test area having a test reactant that can indicate the presence of a target molecule with a visible color change in the test area. A control reactant can also be placed in a control area on the membrane to indicate that the test can be considered valid. BRIEF DESCRIPTION OF THE DRAWINGS [0002] FIG.1 is a cross-sectional view of an example fluid-wicking membrane in accordance with examples of the present disclosure; [0003] FIG.2 is a cross-sectional view of another example fluid-wicking membrane in accordance with the present disclosure; [0004] FIG.3 is a cross-sectional view of yet another example fluid-wicking membrane in accordance with the present disclosure; [0005] FIG.4 is a flowchart of an example method of making a fluid-wicking membrane in accordance with the present disclosure; [0006] FIG.5 is a cross-sectional view of an example lateral flow assay membrane in accordance with the present disclosure; and [0007] FIG.6 is a cross-sectional view of an example vertical flow assay membrane in accordance with the present disclosure. DETAILED DESCRIPTION [0008] The present disclosure describes fluid wicking membranes, methods of making fluid wicking membranes, and flow assay membranes. In one example, a fluid-wicking membrane includes a substrate and a porous coating layer over the substrate. The porous coating layer includes silica particles bound together by a polymeric binder. The silica particles have an average particle size from greater than 1 μm to 50 μm. The porous coating layer also includes a surface-activating agent activating surfaces of the silica particles. The surface-activating agent includes aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. In some examples, the porous coating layer can have a coating thickness from 1 μm to 300 μm. In other examples, the surface-activating agent can be included in an amount from 2 wt% to 20 wt% with respect to the total dry weight of the porous coating layer. In further examples, the polymeric binder can be included in an amount from 0.5 wt% to 20 wt% with respect to the total dry weight of the porous coating layer. In still further examples, the silica particles can be functionalized by an organosilane, wherein a silicon atom of the organosilane bonds to oxygen atoms at a surface of the silica particles. In some examples, the organosilane can include an organic part including a hydrophobic group, a hydrophilic group, a zwitterionic group, a polar aprotic group, a primary amine group, a carboxylic acid group, or a combination thereof. In certain examples, the organosilane can include N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-(triethoxysilylpropyl)-diethylenetriamine, poly(ethyleneimine)trimethoxysilane, aminoethylaminopropyl trimethoxysilane, aminoethylaminoethylaminopropyl trimethoxysilane, N-aminoethyl-3-aminopropylmethyldimethoxysilane, n-(2-aminoethyl)-11-aminoundecyltrimethoxysilane, aminophenyltrimethoxysilane, 3-aminopropyltrimethoxysilane, triethoxysilylundecanal, 3-mercaptopropyltrimethoxysilane, 3-(triethoxysilylpropyl)-p-nitrobenzamide, 3-cyanopropyltrimethoxysilane, mPEG-silane having a number average molecular weight from 300 Mn to 5,000 Mn, mPEG-propionic acid having a number average molecular weight from 300 Mn to 5,000 Mn, a silane-functionalized zwitterionic compound, or a combination thereof. In some examples, the porous coating layer can include a bottom sublayer and a top sublayer over the bottom sublayer, wherein the bottom sublayer can include silica particles having a first average particle size and the polymeric binder in a first binder concentration, wherein the top sublayer can include silica particles having a second average particle size that is less than the first average particle size, and the polymeric binder in a second binder concentration that is greater than the first binder concentration. In other examples, the porous coating layer can include a bottom sublayer and a top sublayer over the bottom sublayer, wherein the bottom sublayer can include silica particles having a first average particle size and the polymeric binder in a first binder concentration, wherein the top sublayer can include silica particles having a second average particle size that is greater than the first average particle size, and the polymeric binder in a second binder concentration that is less than the first binder concentration. [0009] The present disclosure also describes methods of making fluid wicking membranes. In one example, a method of making a fluid wicking membrane includes coating a substrate with a coating composition. The coating composition includes a dispersion of silica particles having an average particle size from greater than 1 μm to 50 μm. The coating composition also includes a surface-activating agent activating surfaces of the silica particles, wherein the surface-activating agent includes aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. The coating composition also includes a polymeric binder. The method further includes drying the coating composition to form a porous coating layer over the substrate. In some examples, coating the substrate can include applying a first coating composition to form a bottom sublayer and applying a second coating composition to form a top sublayer over the bottom sublayer, wherein the first coating composition includes silica particles having a first average particle size and the second coating composition includes silica particles having a second average particle size that is less than the first average particle size. In certain examples, the first coating composition and the second coating composition can be applied simultaneously by curtain coating. [0010] The present disclosure also describes flow assay membranes. In one example, a flow assay membrane includes a substrate, a porous coating layer over the substrate, and a test reactant in a test area of the porous coating layer. The porous coating layer includes silica particles bound together by a polymeric binder and a surface-activating agent activating surfaces of the silica particles wherein the surface-activating agent includes aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. In some examples, the flow assay membrane can be a lateral flow assay membrane, wherein the substrate can be non-porous, wherein the test area can be a test line, and wherein the flow assay membrane can also include control reactant in a control line on the porous coating layer. In other examples, the flow assay membrane can be a vertical flow assay membrane, wherein the substrate can be porous, wherein the porous coating layer can include a bottom sublayer including silica particles having a first average particle size and a top sublayer over the bottom sublayer, wherein the top sublayer can include silica particles having a second average particle size that is less than the first average particle size. [0011] In addition to the examples described above, the methods and devices will be described in greater detail below. It is also noted that when discussing the fluid wicking membranes, methods of making fluid wicking membranes, and flow assay membranes, these relative discussions can be considered applicable to the other examples, whether or not they are explicitly discussed in the context of that example. Thus, for example, in discussing silica particles used in a fluid wicking membrane, this description can also apply to flow assay membranes and methods of making fluid wicking membranes, and vice versa. Fluid Wicking Membranes [0012] As mentioned above, the present disclosure describes fluid wicking membranes. These membranes can be used in a variety of applications in which a liquid is transported along the membrane surface, or penetrates through the membrane, or a combination thereof. In particular, the fluid wicking membranes described herein can include a porous coating layer that can transport liquid through the pores in the layer. Some examples of devices that can include these fluid wicking membranes are: lateral flow assays, vertical flow assays, dot blot assays, western blots, and others. In many of these applications, the fluid wicking membranes can replace nitrocellulose membranes that are often used for fluid transport. Nitrocellulose membranes include pores that can wick liquids through capillary action. Therefore, such membranes are often used in flow assays and similar applications. However, nitrocellulose membranes can have low uniformity with respect to thickness of the membrane, pore size, pore distribution, and other properties. Nitrocellulose membranes can be difficult to manufacture, and the manufacturing process can be imprecise. Nitrocellulose membranes can also be difficult to work with due to static electricity build up and flammability of the nitrocellulose material. [0013] The fluid wicking membranes described herein can provide more precise control over membrane thickness, pore size, and pore distribution. The manufacturing process can also be more cost effective compared to nitrocellulose membranes. The materials used in the fluid wicking membranes can also be non-flammable or have low flammability. These materials can include a substrate and a coating composition that is applied to the substrate to form a porous coating layer. The porous coating layer can include silica particles bound together by a polymeric binder. The silica particles can have an average particle size from greater than 1 μm to 50 μm. The void spaces between the silica particles can act as pores for wicking fluids. The silica particles can also be activated by a surface-activating agent. In some examples, the surface activating agent can include aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. The silica particles can also be functionalized in some examples. Functional groups can be attached to the surfaces of the silica particles, such as functional groups for passivating the surfaces or reactive functional groups to provide a variety of chemical reactions. [0014] One example fluid-wicking membrane 100 is shown in FIG.1. The membrane includes a substrate 110 and a porous coating layer 120 over the substrate. The porous coating layer includes silica particles bound together by a polymeric binder. Silica particles with an average particle size from greater than 1 ^m to 50 ^m can provide sufficient void space between the particles for wicking fluids through the porous coating layer. Additionally, the polymeric binder can be included in an appropriate amount so that the silica particles are bound together and so that sufficient void space remains between the silica particles for wicking fluid. Thus, the polymeric binder is not included in such a great amount that all of the spaces between the silica particles are filled in by the polymeric binder. The silica particles in the porous coating layer are also surface-activated by a surface-activating agent. The surface-activating agent can include aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. In some examples, the activated surfaces of the silica particles can be further functionalized with organosilane compounds. [0015] In various examples, fluid-wicking membranes can include a single, uniform porous coating layer on a substrate. In other examples, the porous coating layer can be made up of multiple sublayers. In certain examples, the multiple sublayers can have different compositions. FIG.2 shows one example fluid-wicking membrane 100 that includes two sublayers. A bottom sublayer 122 is applied to the substrate and a top sublayer 124 is applied over the bottom sublayer. Both of the sublayers can include silica particles and a polymeric binder. However, the compositions of the sublayers can differ one from another. In some examples, differences between the sublayers can include the type of silica particles, particle size of the silica particles, functional groups on surfaces of the silica particles, type of polymeric binder, amount of polymeric binder relative to the silica particles, and so on. In certain examples, the particle size of the silica particles can be different in the individual sublayers. Varying the particle size can be used to adjust the pore size in the sublayers, and thereby the rate at which fluids can flow through the pores can be adjusted. Additionally, using more or less polymeric binder can also affect the rate at which fluids flow through the sublayers. In certain examples, the silica particle size and the concentration of polymeric binder can be carried independently between the sublayers. In one example, the bottom sublayer can have a larger silica particle size and a lower concentration of polymeric binder compared to the top sublayer. This can form a fluid-wicking membrane in which fluid flows more quickly through the bottom sublayer than through the top sublayer. In another example, the bottom sublayer can have a smaller silica particle size and a higher concentration of polymeric binder compared to the top sublayer. This can form a fluid-wicking membrane in which fluid flows more quickly through the top layer than through the bottom sublayer. The concentration of polymeric binder can be in terms of wt% with respect to the total weight of the sublayer. [0016] Any number of sublayers can be included in the porous coating layer. FIG.3 shows another example fluid-wicking membrane 100 that includes a porous coating layer 120 on a substrate 110, where the porous coating layer is made up of three sublayers 122, 124, 126. The individual sublayers can have different compositions as explained above. In some examples, the individual sublayers can have different silica particle sizes, different concentrations of polymeric binder, or both. In certain examples, the silica particle size can increase from the top sublayer 124 to the middle sublayer 126 and from the middle sublayer to the bottom sublayer 122. In further examples, the concentration of polymeric binder can decrease from the top sublayer to the middle sublayer and from the middle sublayer to the bottom sublayer. Conversely, in other examples the silica particle size can decrease from the top sublayer to the middle sublayer and from the middle sublayer to the bottom sublayer. The concentration of polymeric binder can also increase from the top sublayer to the middle sublayer and from the middle sublayer to the bottom sublayer. [0017] The average pore size of pores in the porous coating layer can be sufficient to allow liquids to flow through the pores. In particular, liquid can wick through the pores by capillary action. In some examples, the porous coating layer can have an average pore size from 50 nm to 30 ^m. In further examples, the average pore size can be from 200 nm to 15 ^m or from 1 ^m to 10 ^m or from 5 ^m to 8 ^m. In some cases, the average pore size can be approximately one fourth of the average particle size of the silica particles. The overall volume fraction of the pores, with respect to the total geometric volume of the porous coating layer, can be from 20% to 60%, or from 25% to 48%, or from 30% to 45%, in various examples. In some examples, the average pore size can be measured using a standard measurement technique, such as mercury intrusion porosimetry, gas adsorption porosimetry, capillary flow porometry, and so on. [0018] As mentioned above, in some examples the silica particles in the porous coating layer can have an average particle size from greater than 1 μm to 50 μm. In further examples, the average particle size can be from 2 μm to 40 μm or from 3 μm to 30 μm or from 5 μm to 20 μm. The average particle size can be based on volume of the particle size normalized to a spherical shape for diameter measurement, for example. Particle size can be collected using a Malvern ZETASIZER™ system (Malvern Panalytical, United Kingdom), for example. Particle size information can also be determined and/or verified using a scanning electron microscope (SEM). [0019] The porosity and average pore size in the porous coating layer can also be affected by the concentration of polymeric binder present in the layer. A higher concentration of polymeric binder can result in fewer pores and smaller average pore sizes. Conversely, a lower concentration of polymeric binder can result in more pores and larger average pore sizes. In some examples, the polymeric binder can be included in the porous coating layer in an amount from 0.5 wt% to 20 wt% with respect to the total dry weight of the porous coating layer. In further examples, the concentration of polymeric binder can be from 1 wt% to 18 wt%, or from 2 wt% to 15 wt%, or from 3 wt% to 10 wt%. [0020] The concentration of polymeric binder and the particle size of the silica particles in the porous coating layer can also influence the capillary flow time (CFT) of the membrane. CFT is defined as the time for a water front to traverse a distance of 4 cm across the membrane. Attaching various functional groups to the surfaces of the silica particles can also affect the CFT in some examples. In certain examples, the fluid-wicking membrane can have a CFT from 75 seconds to 500 seconds. In further examples, the fluid-wicking membrane can have a CFT from 100 seconds to 400 seconds or from 200 seconds to 400 seconds. [0021] Turning to more specific details about the ingredients in the porous coating layer, a variety of polymeric binders can be included in the porous coating layer. In some examples, the polymeric binder can include polyvinyl alcohol or poly(vinylpolypyrrolidine). Specific examples can include MOWIOL® 3-85, MOWIOL® 4-88, MOWIOL® 4-98, MOWIOL® 5-88, MOWIOL® 6-98, MOWIOL® 8-88, MOWIOL® 10-98, MOWIOL® 13-88, MOWIOL® 15-99, MOWIOL® 18-88, MOWIOL® 20-98, MOWIOL® 23-88, MOWIOL® 26-88, MOWIOL® 28-99, MOWIOL® 30-92, MOWIOL® 30-98, MOWIOL® 32-88, MOWIOL® 40-88, MOWIOL® 47-88, and MOWIOL® 56-98 from Kuraray (Japan); SELVOL™ E203, SELVOL™ E04/88LA, SELVOL™ E205, SELVOL™ E08/88, SELVOL™ E23, SELVOL™ E103, SELVOL™ E305, SELVOL™ E107, SELVOL™ E310, and SELVOL™ E325 from Sekisui (Japan); LUVITEC® K17, LUVITEC® K30, and LUVITEC® K60 from BASF (Germany). In further examples, the binder can be crosslinked. In certain examples, crosslinking agents such as boric acid, citric acid, dianhydrides, glutaraldehyde, glyoxal, maleic acid, sodium hexametaphosphate, succinic acid, sulfosuccinic acid, or trisodium trimetaphosphate can be added to crosslink the binder. [0022] The silica particles used in the porous coating layer can initially be colloidal silica particles (i.e., silica particles suspended in a liquid). In particular examples, the silica particles can be prepared as a suspension in an aqueous liquid. The silica particles can be fumed silica particles, precipitated silica particles, silica gel particles, or combinations thereof. In some particular examples, the silica particles can be precipitated silica particles. Specific examples of silica particles that can be used include CAB-O-SIL® M-5, CAB-O-SIL® H-5, CAB-O-SIL® LM-150, and CAB-O-SIL® EH-5 from Cabot Corporation (USA); ACEMATT® 82, ACEMATT® HK 125, ACEMATT® HK 400, ACEMATT® HK 440, ACEMATT® HK 450, ACEMATT® 790, ACEMATT® 810, ACEMATT® 3600, ACEMATT® OK 412, ACEMATT® OK 500, and ACEMATT® OK 520 from Evonik (Germany). In some examples, the silica particles can have a surface area from 0.045 m ² /g to 120 m ² /g. In further examples, the surface area can be from 100 m ² /g to 500 m ² /g or 160 m ² /g to 300 m ² /g or from 300 m ² /g to 400 m ² /g. [0023] As mentioned above, the silica particles can be surface-activated by a surface-activating agent such as aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof. In some examples, the surface-activating agent can be added when formulating an aqueous dispersion of the silica particles. Additional functionality can be added to the silica particles by attaching organosilanes to surfaces of the silica particles, either while the silica particles are in the dispersion or after the porous coating layer has been formed. [0024] “Surface-activated” refers to the surface of silica after being treated with an inorganic surface-activating agent, such as aluminum chloride hydrate and/or a multivalent metal oxide, in a sufficient amount to modify the net charge of the surface from negative (í) to positive (+). This is not to say that all negatively charged moieties are converted to positive, but that the net charge of the entire surface is positive. [0025] “Aluminum chloride hydrate,” “ACH,” “polyaluminum chloride,” “PAC,” “polyaluminum hydroxychloride,” or the like, refers to a class of soluble aluminum products in which aluminum chloride has been partly reacted with a base. The relative amount of OH compared to the amount of Al can determine the basicity of a particular product. The chemistry of ACH is often expressed in the form Al _n (OH) _m Cl _(3n-m) , wherein n can be from 1 to 50, and m can be from 1 to 150. Basicity can be defined by the term m/(3n) in that equation. ACH can be prepared by reacting hydrated alumina Al(OH) ₃ with hydrochloric acid (HCl). The exact composition depends upon the amount of hydrochloric acid used and the reaction conditions. Typically, the reaction can be carried out to give a product with a basicity of 40% to 60%, which can be defined as (%)=n/6×100. ACH can be supplied as a solution, but can also be supplied as a solid. [0026] There can be other ways of referring to ACH. Typically, ACH includes many different molecular sizes and configurations in a single mixture. An example stable ionic species in ACH can have the formula [Al ₁₂ (OH) ₂₄ AlO ₄ (H ₂ O) ₁₂ ] ⁷⁺ . Other examples include [Al ₆ (OH) ₁₅ ] ³⁺ , [Al ₈ (OH) ₂₀ ] ⁴⁺ . [Al ₁₃ (OH) ₃₄ ] ⁵⁺ , [Al ₂₁ (OH) ₆₀ ] ³⁺ , etc. Other common names used to describe ACH or components that can be present in an ACH composition include Aluminum chloride hydroxide (8Cl); A 296; ACH 325; ACH 331; ACH 7-321; ALOXICOLL®; ALOXICOLL® LR (from Elementis, United Kingdom); Aluminum hydroxychloride; Aluminol ACH; Aluminum chlorhydrate; Aluminum chlorhydroxide; Aluminum chloride hydroxide oxide, basic; Aluminum chloride oxide; Aluminum chlorohydrate; Aluminum chlorohydrol; Aluminum chlorohydroxide; Aluminum hydroxide chloride; Aluminum hydroxychloride; Aluminum oxychloride; Aquarhone; Aquarhone 18; Astringen; Astringen 10; Banoltan White; Basic aluminum chloride; Basic aluminum chloride, hydrate; Berukotan AC-P; Cartafix LA; Cawood 5025; CHLORHYDROL®; CHLORHYDROL® Micro-Dry; CHLORHYDROL® Micro-Dry SUF (Elementis, United Kingdom); E 200; E 200 (coagulant); EKOFLOCK™ 90; EKOFLOCK™ 91 (Feralco, Sweden); GENPAC® 4370 (Chemtrade Solutions, USA); GILUFLOC® 83 (Kurita, Japan); Hessidrex WT; HPB 5025; Hydral; Hydrofugal; HYPER+ION® 1026 (Chemtrade Solutions, USA); Hyperdrol; KEMPAC™ 10; KEMPAC™ 20 (Kemira Oyj, Finland); Kemwater PAX 14; LOCRON®; LOCRON® P; LOCRON® S (Clariant, Switzerland); Nalco 8676; OCAL; Oulupac 180; PAC; PAC (salt); PAC 100W; PAC 250A; PAC 250AD; PAC 300M; PAC 70; Paho 2S; PALC; PAX; PAX 11S; PAX 16; PAX 18; PAX 19; PAX 60p; PAX-XL 1; PAX-XL 19; PAX-XL 60S; PAX-XL 61S; PAX-XL 69; PAX-XL 9; Phacsize; Phosphonorm; (14) Poly(aluminum hydroxy)chloride; Polyaluminum chloride; PRODEFLOC™ AC 190; PRODEFLOC™ AL; PRODEFLOC™ SAB 18; PRODEFLOC™ SAB 18/5; PRODEFLOC™ SAB 19 (Proquial, Spain); Purachem WT; REACH® 101; REACH® 301; REACH® 501 (Elementis, United Kingdom); Sulzfloc JG; Sulzfloc JG 15; Sulzfloc JG 19; Sulzfloc JG 30; TAI-PAC; Taipac; Takibine; Takibine 3000; Tanwhite; TR 50; TR 50 (inorganic compound); UPAX 20; VIKRAM® PAC-AC 100S (Aditya Birla Chemicals, India); WAC; WAC 2; Westchlor 200; Wickenol 303; Wickenol CPS 325 Aluminum chlorohydrate Al ₂ ClHSO ₅ or Al ₂ (OH) ₅ Cl ₂ H ₂ O or [Al(OH) ₂ Cl] _x or Al ₆ (OH) ₁₅ Cl ₃ ; Al ₂ (OH) ₅ Cl] _x Aluminum chlorohydroxide; Aluminum hydroxychloride; Aluminum chloride, basic; Aluminum chloride hydroxide; [Al ₂ (OH) _n Cl _6-n ] _m ; [Al(OH) ₃ ] _n AlCl ₃ ; or Al _n (OH) _m Cl _(3n-m) (where 0<m<3n); for example. In one example, the composition can include aluminum chlorides and aluminum nitrates of the formula Al(OH) _2X to Al ₃ (OH) _8X , where X is Cl or NO ₃ . In another example, the composition can be prepared by contacting silica particles with an aluminum chlorohydrate Al ₂ (OH) ₅ Cl or Al ₂ (OH)Cl ₅ .nH2O. It is believed that contacting a silica particle with an aluminum compound as described above causes the aluminum compound to become associated with or bind to the surface of the silica particles. This can be either by covalent association or through an electrostatic interaction to form a cationic charged silica, which can be measured by a Zeta potential instrument. [0027] “Trivalent or tetravalent metal oxide” or “multivalent metal oxide” refers to compositions that can be used in conjunction with, or instead of, ACH to reverse the charge of a silica surface from negative (í) to positive (+). Specifically, the negative charge on silica can be reversed by adsorbing an excess of positively charged polyvalent metal oxide on the surface. Coatings can include oxides of trivalent and tetravalent metals such as aluminum, chromium, gallium, titanium, and zirconium. For example, acidified silica can be mixed with a basic metal salt (such as Al ₂ O ₃ ) to substantially cover the surface of silica particulates. By surface activation using such a multivalent metal oxide, the silica can carry a positive charge instead of a negative charge at below a pH of 7. [0028] With specific reference to the surface-activating agent, in one example, the surface-activating agent can be aluminum chloride hydrate. In another example, the surface-activating agent can be a trivalent or tetravalent metal oxide, with metals such as aluminum, chromium, gallium, titanium, and zirconium. If, for example, aluminum chloride hydrate is used, it can be present in an amount from 2 wt % to 20 wt % compared to the silica content, and in another example, the aluminum chloride hydrate can be present at from 5 wt % to 10 wt %. Without being bound by any particular theory, it is believed that the aluminum of the aluminum chloride hydrate associates with an oxygen at the surface of the silica particulates. For example, silica (SiO ₂ ) typically includes Si—OH groups at the surface of the individual particulates, which can act as a weak acid, liberating hydrogen and becoming ionized at a pH above about 2. As the pH is raised, the surface of the silica becomes more negative. The addition of aluminum chloride hydrate to silica causes the surface of the silica to become more positive. If enough aluminum chloride hydrate is added, then the net charge of the silica particulates becomes positive. It is believed that the aluminum of the aluminum chloride hydrate can interact with the ionized —Si—Oí group at the surface of the silica, thus, converting some moieties at the surface to a more positive state. Thus, in this state, the silica is said to be “surface-activated.” [0029] In some examples, the process of surface-activating the silica particles can include increasing the density of Si-OH groups present at the surface of the silica particles prior to adding the surface-activating agent. In one example, the silica particles can be treated with nitric acid, hydrogen peroxide, and ammonium hydroxide to increase the density of Si-OH groups at the surface. The silica particles can then be treated with the surface-activating agents described above. In some examples organosilanes can also be attached to the surfaces of the silica particles through the Si-OH groups on the surfaces of the silica particles. [0030] A variety of organic functional groups can be attached to the surfaces of the silica particles by reacting organosilanes with the silica particles. Specifically, organosilane reagents can be added to the surface-activated silica to add additional positively charged moieties to the surface, or to provide another desired function at or near the surface, such as hydrophobic groups, hydrophilic groups, passivating groups, reactive groups, test reactants, control reactants, and others. In certain examples, the fluid-wicking membrane can be designed to be used for an assay such as a lateral flow assay, and the organosilanes can include an organic group that is non-reactive with a target compound that is to be detected by the lateral flow assay. Sometimes this type of assay is used to detect biological molecules such as proteins. In such cases, it can be useful to render the fluid-wicking membrane non-reactive with the target biological molecules so that the target molecules do not non-specifically bind to the membrane. These assays can include a test line where a test reactant is present, and the test reactant can be selected to bind with the target molecule. If the target molecule binds non-specifically with the membrane outside of the test line, then the amount of target molecule that can bind at the test line may be reduced. Thus, the signal strength can be increased by making the membrane non-reactive with the test molecule except for at the test line. [0031] In various examples, organosilanes that can be attached to the silica particles can include hydrophobic groups, hydrophilic groups, zwitterionic groups, polar aprotic groups, primary amine groups, carboxylic acid groups, or combinations thereof. In some examples, passivating organosilanes that can be attached to the silica particles can include mPEG-silane with a number average molecular weight Mn from 300 Mn to 5,000 Mn, or mPEG-propionic acid with a number average molecular weight Mn from 300 Mn to 5,000 Mn. In other examples, the organosilanes can include functional groups that can participate in a condensation reaction to link to additional molecules. In certain examples, condensation reagent groups can include primary amines, thiols, and carboxylic acids. [0032] In other examples, the organosilane reagents can be amine-containing silanes. In a more detailed example, the amine-containing silanes can include quaternary ammonium salts. Examples of amine-containing silanes include 3-aminopropyltrimethoxysilane, N-(2-aminoethyl-3-aminopropyltrimethoxysilane, 3-(triethoxysilylpropyl)-diethylenetriamine, poly(ethyleneimine)trimethoxysilane, aminoethylaminopropyl trimethoxysilane, aminoethylaminoethylaminopropyl trimethoxysilane, N-aminoethyl-3-aminopropylmethyldimethoxysilane, N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane, aminophenyltrimethoxysilane, and the quaternary ammonium salts of the amine coupling agents mentioned above. An example of a quaternary ammonium salt organosilane reagent includes trimethoxysilylpropyl-N,N,N-trimethylammonium chloride. [0033] Other specific examples of organosilanes can include , triethoxysilylundecanal, 3-(triethoxysilylpropyl)-p-nitrobenzamide, 3-cyanopropyltrimethoxysilane, bis(2-hydroethyl)-3-aminopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, bis(triethoxysilylpropyl)disulfide, 3-aminopropyltriethoxysilane, 3-aminopropylsilsesquioxane, bis-(trimethoxysilylpropyl)amine, N-phenyl-3-aminopropyltrimethoxysilane, N-aminoethyl-3-aminopropylmethyldimethoxysilane, 3-ureidopropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, N-(trimethyloxysilylpropyl)isothiouronium chloride, N-(triethoxysilpropyl)-O-polyethylene oxide, 3-(triethoxylsilyl)propylsuccinic anhydride, 3-(2-imidazolin-1-yl) propyltriethoxysilane, and reagents sold under the trade name DYNASYLAN® from Evonik (Germany). [0034] The amount of organosilane included in the porous coating layer can vary depending on the type of organosilanes, the degree of functionalization of the silica particles with the organosilanes, and the size of the silica particles. In some examples, the organosilane can be present in an amount from 0.05 wt% to 3 wt%, or from 0.1 wt% to 1.5 wt%, or from 0.5 wt% to 1 wt%, with respect to the total weight of the porous coating layer. [0035] Several processes may be used to treat the silica particles with the surface-activing agent and the organosilanes. In some examples, a surface-activating agent can be added to water prior to the silica, and then the silica can be added portion-wise over a period of time. Alternatively, the silica can be dispersed in water first, and then the inorganic surface activating agent can be added to the silica dispersion. In this example the surface-activating agent can be added all at once, or portion-wise, depending on the desired result. In another example, both the silica and the surface-activating agent can be added to water simultaneously. In these examples, the net surface charge of the silica can be converted from negative (í) to positive (+). This does not mean that every negatively charged moiety is converted from negative to positive, but that the surface charge as a whole becomes more positive than negative. After combining the surface-activating agent and the silica, the organosilane reagent can then be added, though this order of addition is not limiting. For example, in one example, after forming the silica/surface-activating agent composition as described above, an organosilane reagent can be added to the surface-activating agent treated silica portion-wise. Such an addition scheme can prevent flocculation of the silica. In this example, the pH can be controlled to maintain the colloidal stability of the silica dispersion. Alternatively, the ACH and the organosilane reagent can be added simultaneously to a silica dispersion. In these reaction schemes, little or no organic solvent may be used. For example, the aqueous environment can include a predominant amount of water, and may include small amounts of organic solvent, surfactant, crosslinking agent such as boric acid, etc. The polymeric binders described above can also be added to the aqueous dispersion to form a coating composition that can be applied to a substrate to form the porous coating layers described herein. [0036] In other examples, the coating composition can be prepared without an organosilane reagent. This coating composition can be applied to a substrate to form a membrane, and then organosilane reagents can be applied to the membrane later. This can be useful if different organosilanes are desired in different areas of the membrane. For example, a lateral flow assay or vertical flow assay can include test reactants in certain areas of the membrane. In some examples, the test reagents can be organosilane reagents that can bond to the silica particles. In other examples, an organosilane linker can be used, which is capable of bonding to the silica particles in the membrane and also bonding with a test reactant. In these examples, the specific organosilanes desired can be applied in a certain area of the membrane, to form a test line or test spot, for example. In further examples, a passivating organosilane reagent can be applied to the remainder of the membrane. Different organosilane reagents can be applied to different areas of the membrane using a variety of application processes. In certain examples, a digital application process such as inkjet printing can be used. [0037] The coating composition used to form the porous coating layer can also include an acid or a base in some examples, to adjust the pH of the composition. In certain examples, the pH can be from 3 to 9. In some examples, the coating composition can include a weak acid or a weak base in an amount from 0.1 wt% to 5 wt%, based on the weight of dry ingredients in the coating composition. [0038] In certain examples, a modified silica dispersion can be prepared first, and then the modified silica dispersion can be included in the coating composition for forming the porous coating layer. In some examples, the modified silica dispersion can include silica particles in an amount from 70 parts by weight to 100 parts by weight, aluminum chlorohydrate in an amount from 1 part by weight to 20 parts by weight, organosilane in an amount from 0.1 part by weight to 10 parts by weight, and a weak acid in an amount from 1 part by weight to 5 parts by weight. In further examples, the modified silica dispersion can also include a sufficient amount of water to bring the total solid content to 15 wt% to 35 wt%. The modified silica dispersion can be combined with additional ingredients to form the coating composition. In some examples, the coating composition can include the modified silica dispersion in an amount from 70 parts by dry weight to 100 parts by dry weight, crosslinker in an amount from 1 part by dry weight to 5 parts by dry weight, a polymeric binder in an amount from 10 parts by dry weight to 20 parts by dry weight, humectant in an amount from 1 part by dry weight to 5 parts by dry weight, and surfactant and/or other additives in an amount from 0.1 part by dry weight to 5 parts by dry weight. A sufficient amount of water can be added to bring the total solid content of the coating composition to 15 wt% to 35 wt%. [0039] The coating composition can be applied to a substrate and dried to form a fluid-wicking membrane. The substrates used in the fluid-wicking membranes described herein can include a variety of materials. In some examples, the substrate can be non-porous. Non-porous membranes can be useful in applications where fluid is to flow along the surface of the membrane without penetrating through the membrane. These can be used in lateral flow assays, for example. Non-limiting examples of non-porous substrates can include polymeric films such as polyethylene film, rigid substrates such as glass, poly(methy methacrylate), and others. In other examples, porous substrates can be used. Porous substrates can be particularly useful when fluid is to penetrate through the membrane. This type of membrane can be used in vertical flow assays, for example. Non-limiting examples of porous substrates can include paper, nonwoven fabric, woven fabric, nitrocellulose, pads including glass and polymeric fibers, among others. [0040] The coating compositions described above can be applied to the substrate using a variety of coating processes, such as curtain coating, air knife coating, blade coating, gate roll coating, doctor blade coating, Meyer rod coating, roller coating, reverse roller coating, gravure coating, brush coating, spray coating, and so on. In some examples, the coating can have a thickness, when dry, from 1 μm to 300 μm, or from 5 μm to 200 μm, or from 10 μm to 200 μm. The coating process can provide a coating layer with good uniformity of coating thickness. In some examples, the coating thickness can be more uniform than is typical for nitrocellulose membranes. The pore size and pore distribution can also be very uniform because the coating can include a homogeneous distribution of the silica particles and polymeric binder throughout the coating layer. In some examples, the coating thickness, pore size, and/or pore distribution can be uniform within a tolerance of 10% of the average value across the coating layer. Methods of Making Fluid Wicking Membranes [0041] The present disclosure also describes methods of making fluid-wicking membranes. In some examples, fluid-wicking membranes can be made by applying a coating composition as described above to a substrate as described above. The coating composition can then be dried to form a porous coating layer. FIG.4 is a flowchart illustrating one example method 200 of making a fluid-wicking membrane. This method includes: coating a substrate with a coating composition including a dispersion of silica particles having an average particle size from greater than 1 μm to 50 μm, a surface-activating agent activating surfaces of the silica particles, wherein the surface-activating agent includes aluminum chloride hydrate, a trivalent metal oxide, a tetravalent metal oxide, or a combination thereof, and a polymeric binder 210; and drying the coating composition to form a porous coating layer over the substrate 220. [0042] When applying the coating composition to the substrate, any of the coating processes described above can be used. In certain examples, a curtain coating process can be used. Curtain coating can be particularly useful in some examples that include multiple layers of different coating compositions applied to a single substrate. In these examples, multiple different coating compositions can be applied to a substrate to form multiple porous coating layers with different properties, such as porosity, pore size, capillary flow time, surface energy, and so on. In some cases, a curtain coater can apply multiple coating compositions simultaneously with good control over the thickness of the individual coatings. [0043] In a particular example, a method of making a fluid-wicking membrane with multiple sub-layers in the porous coating layer can include applying a first coating composition to form a bottom sublayer and a second coating composition to form a top sublayer. The properties of the sublayers can be adjusted in several ways. In certain examples, the particle size of silica particles in the coating sublayers can be adjusted so that the individual sublayers have different silica particle sizes. The concentration of polymeric binder in the sublayers can also be adjusted. In one example, the bottom sublayer can have a larger silica particle size and a smaller polymeric binder concentration compared to the top sublayer. In another example, the bottom sublayer can have a smaller silica particle size and a larger polymeric binder concentration compared to the top sublayer. In further examples, three sublayers can be applied using three different coating compositions. In one example, the silica particle size can decrease from the top sublayer to the bottom sublayer and the polymeric binder concentration can increase from the top sublayer to the bottom sublayer. In another example, the silica particle size can increase from the top sublayer to the bottom sublayer and the polymeric binder concentration can decrease from the top sublayer to the bottom sublayer. Flow Assay Membranes [0044] The present disclosure also describes flow assay membranes that can be made using the fluid-wicking membranes described above. The flow assay membranes can include a test reactant in an area of the membrane. The test reactant can be a reactant that is reactive with a particular target molecule that the assay is designed to detect. In various types of assays, a reaction between the target molecule and the test reactant can be detected in a variety of ways, such as by a visible color change, a change in fluorescence, or others. In some examples, the test reactant can bond selectively with the target molecule. In further examples, the test reactant can also be bonded to the membrane surface, for example by bonding to a linking group on an organosilane attached to the surface of a silica particle in the porous coating layer. In specific examples, the test reactant can include an antibody, an aptamer, a peptide, a DNA segment, an RNA segment, and others. [0045] In some examples, the flow assay membrane can be used in a lateral flow assay. Lateral flow assays often include a test reactant forming a test line on the membrane. A sample fluid can be wicked through the membrane until the sample fluid contacts the test line. If the target molecule is present in the sample fluid, then the target molecule can bind to the test line and produce a visible color change at the test line. In some examples, the visible color change is achieved by mixing the sample fluid with a conjugate reactant that bonds with the target molecule and which has a visible color, so that the visible color can intensify as target molecules and bonded conjugate reactants accumulate at the test line. Lateral flow assays can also include a control line on the membrane, positioned beyond the test line. The control line can produce a visible color change when the sample fluid reaches the control line, to verify that the sample fluid flowed a sufficient distance through the membrane for a valid test. Additionally, in some examples, the substrate in the lateral flow assay membrane can be a non-porous substrate. The non-porous substrate can be appropriate because the non-porous substrate can direct the sample fluid to flow along the surface of the membrane instead of penetrating through the membrane. [0046] FIG.5 shows an example lateral flow assay membrane 300. In this example, the membrane includes a substrate 110, a porous coating layer 120, a test line 330, and a control line 340. In this example, the substrate is a non-porous substrate. The lateral flow assay membrane can be combined with additional components to make a full lateral flow assay test. Additional components can include a housing, a sample pad, a conjugate pad, an absorber pad, and others. [0047] In further examples, the flow assay membrane can be a vertical flow assay membrane. Vertical flow assays often include a wicking pad beneath the membrane. A sample fluid can be applied on the top of the membrane, and the sample fluid can wick through the membrane to the wicking pad. The membrane can be porous throughout the thickness of the membrane to allow the sample fluid to penetrate through the membrane. Therefore, in these examples the substrate of the fluid-wicking membrane can be a porous substrate. As the sample fluid passes through the membrane, target molecules can bond to a test reactant located in a test region on the membrane. In some examples, a label fluid can be applied after the sample fluid. The label fluid can include a label reactant that can bond to the target molecules that have been immobilized in the test region of the membrane, producing a visible color change at the test region. [0048] In certain examples, vertical flow assay membranes can include a porous coating layer made up of multiple sublayers. The sublayers can have different properties as described above. In certain examples, the porous coating layer can include a top sublayer that has a smaller average silica particle size and a bottom sublayer that has a larger average silica particle size. The bottom sublayer can have a larger average pore size and a shorter capillary flow time so that sample fluid can be drawn down into the bottom sublayer. The capillary flow time of the top sublayer can be longer to allow more time for target molecules in the sample fluid to bond to the test reactants, which can be present on the top surface of the membrane. [0049] FIG.6 shows an example vertical flow assay membrane 400. The membrane includes a porous substrate 110 with a porous coating layer 120 applied thereon. The porous coating layer includes a bottom sublayer 122 and a top sublayer 124. A test reactant is applied to the top sublayer in a test area 430. The individual silica particles 128 are shown in this figure, represented as circles. In this example, the bottom sublayer includes silica particles having a larger average particle size compared to the silica particles in the top sublayer. The larger silica particles leave larger spaces between the particles, which can allow for faster flow of fluid through the bottom sublayer. A sample fluid can flow more slowly through the top sublayer so that target molecules in the sample fluid can have more time to react with test reactants in the test area. In further examples, additional components can be added to make a complete vertical flow assay. These additional components can include a housing, wicking pad, and other components. In certain examples, the wicking pad can be omitted because the vertical flow assay membrane described herein can operate without a wicking pad. In particular, the porous substrate and bottom sublayer having a larger pore size can be sufficient to cause sample fluid to wick through the top sublayer of the membrane. [0050] In a particular example, a vertical flow assay membrane as shown in FIG.6 can be incorporated in a vertical flow assay. The test reactant can be a capture antibody that is immobilized on the top sublayer of the porous coating layer of the membrane. In some examples, capture antibodies can be immobilized by linking the antibodies to organosilanes that are bonded to the surface of the silica particles in the porous coating layer. After immobilizing the capture antibody, the membrane can be placed in a housing and stored at appropriate conditions to avoid contamination or rehydration of the antibodies. The vertical flow assay can be used to perform a test by introducing a sample fluid onto the membrane. The sample fluid can include an antigen (the target molecule) that will bind to the capture antibodies. The top layer of the porous coating layer, having a small pore size, can act as a filter for the sample fluid. The antigen in the sample fluid can come into contact with the capture antibodies in the top sublayer. The antigens can then bind with the capture antibodies. In some examples, washing and blocking of the membrane can be performed after the sample fluid has been applied to the membrane. A gold conjugated antibody (label reactant) that specifically binds to the target antigen can then be applied to the membrane. The gold conjugated antibody can bind to the antigens that were captured by the capture antibody, producing a visible color change in the test area. [0051] In further examples, multiple different test reactants, such as multiple different antibodies, can be immobilized in different areas of the membrane to allow for multiplexing. Additionally, different organosilanes can be attached to silica particles in different sublayers of the porous coating layer to provide different chemical reactivity in the individual sublayers. [0052] It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. [0053] As used herein, particle size with respect to the silica particles, or any other particles, can be based on volume of the particle size normalized to a spherical shape for diameter measurement, for example. Particle size can be collected using a Malvern ZETASIZER™ system (Malvern Panalytical, United Kingdom), for example. Particle size information can also be determined and/or verified using a scanning electron microscope (SEM). [0054] As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable based on experience and the associated description herein. [0055] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though individual members of the list are individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. [0056] Concentrations, dimensions, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, but also all the individual numerical values or sub-ranges encompassed within that range as if individual numerical values and sub-ranges are explicitly recited. For example, a weight ratio range of about 1 wt% to about 20 wt% should be interpreted to include the explicitly recited limits of about 1 wt% and about 20 wt%, but also to include individual weights such as 2 wt%, 11 wt%, 14 wt%, and sub-ranges such as 10 wt% to 20 wt%, 5 wt% to 15 wt%, etc. EXAMPLES [0057] The following examples illustrate the technology of the present disclosure. However, it is to be understood that the following are illustrative of the application of the principles of the presented technology. Numerous modifications and alternatives may be devised without departing from the present disclosure. The appended claims are intended to cover such modifications and arrangements. Thus, while the disclosure has been provided with particularity, the following describes further detail in connection with what are presently deemed to be the acceptable examples. Example 1 – Modified Silica Dispersion [0058] An example modified silica dispersion was prepared from the following ingredients: 3 parts by dry weight of aluminum chlorohydrate (surface-activating agent), 9 parts by dry weight of an organosilane, 1.25 parts by dry weight of a weak acid, 100 parts by dry weight of ACEMATT® OK 412 silica particles from Evonik (Germany), and a sufficient amount of deionized water to provide a solid content of 25 wt% in the dispersion. The modified silica dispersion had a pH of 4.54, a zeta potential of 5.75 mV, and a viscosity 406.8 cP. Example 2 – Coating Composition [0059] An example coating composition was prepared using the modified silica dispersion from Example 1. The coating composition included the following ingredients: 100 parts by dry weight of the modified silica dispersion of Example 1, 2.5 parts by dry weight of a crosslinker, 14.2 parts by dry weight of a polymeric binder, 1 part by dry weight of polyethylene glycol (humectant), and 0.5 part by dry weight of a surfactant. The coating composition had a solid content of 22 wt%, a pH of 4.54, and viscosity of 774 cP. This coating composition may be coated onto a substrate to make a fluid-wicking membrane as described above. [0060] While the present technology has been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the technology of this disclosure. It is intended, therefore, that the disclosure be limited by the scope of the following claims.