The present invention relates to a dispense surface for a nitrogen contai ning fluid, e.g. a beverage such as a stout beer or the li ke. I n particular the invention uti lises a com bination of geometric features and nanostructu red surfaces to encourage bubble nucleation to form a head of foa m in nitrogenated beverages.

Background to the Invention

N itrogenated beverages i nclude beers that are, in practice, pressurized with a mixture of nitrogen and carbon dioxide, often about 75-80% nitrogen and 20-25% carbon dioxide at 310 -380 kPa (a pprox. 30-40 psig). The products take advantage of the u nique properties of nitrogen to create a range of desi rable characteristics includi ng a less bitter taste and a creamy long-lasting head, which can be attributed to the small size of nitrogen bubbles. A com mon exa mple of such a product is stout beer and, particu larly, the Guin ness ^® draught brand.

It is well known to those skilled i n the art that the gas mixture exists in a metastable form i n the beer at atmospheric pressure, a nd the dissolved gas does not spontaneously foam. Therefore, a trigger is necessary to initiate nucleation and growth of the bubbles. U nder the a ppropriate trigger conditions, nucleation of the dissolved gas occurs during dispensing of the beer into a glass, yielding bu bbles with the diameter in the range of 50 to 200 μ η . The lower buoya ncy of the small bubbles results in a relatively long ti me for them to rise to the top of the glass, a desirable consumer characteristic called "ti me to black", e.g. the ti me requi red for al l the bubbles to float to the top and form a cream coloured head with a dark beverage underneath. The entire effect of the rapid nucleation of gas bu bbles and subsequent slow rise to form the head is referred to as "su rge and settle" .

The basic phenomena related to the su rge involves the nucleation of gas bubbles within the liquid . The growth rate of the bubbles is proportional to the concentration of dissolved gases in the liquid a nd the rate of diffusion into the growi ng bubble. The final bu bble size is dictated by the gas mixtu re in the bu bble, the residence time of individual bu bbles in the fluid or the time to detach, and the subsequent development of attached cavities, bubble clouds, formation of daughter droplets, etc. Surge is directly related to reductions in pressure to some critical value, (p _c ), which in turn is associated with dynamic effects, either in a flowing liquid or in an acoustic field.

According to current commercial practices, surge may be induced using one of the following methods:

Flow through Orifice Plate: A system with two lines attached to the keg is used to disperse the beer. One line leads to a premixed gas cylinder containing nitrogen and carbon dioxide which is used to propel beer from the keg. The other line leads to the dispensing tap. On its way to the spout of this tap, the beer is forced at high velocity, created by absolute pressure of approximately 3.77 bar (377kPa), through an orifice plate with five small holes having diameter of 0.6 or 0.9 mm. According to Bernoulli's theorem, the contraction of the fluid path accelerates the beer and the pressure drops as it passes through the holes. If the pressure drop is great enough, the local pressure in the vicinity of the vena contracta (the location in the flow field with a minimum cross-sectional area) is less than the vapor pressure of the liquid. Under such conditions the liquid will vaporize and bubbles will nucleate. Although this approach is quite effective it requires considerable velocities to observe the necessary pressure drop capable to promote the surge. For example, the tap system drives the fluid through five holes at approximately 16 m/s. Therefore, this method is only practical in a commercial establishment where a significant volume of draught beer can be sold since it requires considerable capital investment.

Jet Impingement by "Widget": A widget is a plastic capsule with a tiny hole connecting its interior to the surroundings and that floats on the surface of the beer. Such a device is used to replicate "draught" pouring in a canned or bottled product. Upon pressurization during the filling process, the pressure equalizes in the widget, also forcing some beer into the widget as it does so. When the can (or bottle) is opened, the pressure in the headspace and beer rapidly drops to about 1 bar (atmospheric pressure). The contents of the widget then decompress by squirting gas and some beer into the beer. The jet impingement technique overcomes the barrier to nucleation by utilizing kinetic energy present in the high velocity jet as the gas exits the orifice in the widget; the gas jet is fragmented into discrete bubbles by the turbulent flow. In addition, the momentum of the flow is transferred to the liquid, inducing circulation and mixing throughout the liquid. This method has the advantage that it can be packaged into a beverage container but it incurs an additional cost and slows down the production line during the filling process.

Ultrasound: An ultrasonic device (known as a "surger") can initiate nucleation and growth of nitrogen bubbles in beer that is at atmospheric pressure but still contains all the dissolved gases. For example, the beer may be poured into a glass but since the gas is in a metastable state it remains in the fluid and there is little or no head, (known as "unsurged" beer). However, the glass is then placed onto a plate on the surger device which, when triggered, vibrates ultrasonically. The resultant pressure fluctuations caused by sound waves through the beer creates a surge of bubbles. Although the aforementioned techniques are known to be effective, they are costly and not easily incorporated into beverage containers. The elimination of widgets in a canned beverage application would require incorporation of another trigger bubble nucleation mechanism capable of overcoming the energy barrier to nucleation. Therefore alternative mechanisms need to be investigated.

WO2014027028 describes the use of nanostructures to encourage nucleation and growth of nitrogen bubbles in nitrogenated beverages. The nanostructures may be integrated into the inside surface of the beverage container, allowing, for example, a draught experience to be replicated in canned beer without a widget. However, the challenge of this system is optimizing it for rapid nucleation, occurring in the short time to dispense. Generally, in this prior art there are limitations to the number of nucleation sites per area that can be achieved and/or the rate the gas can diffuse into the growing bubble. Once aware of the possibility to utilize surface structures in or on a container surface to affect bubble nucleation, in hindsight other prior art comes into consideration. For example, WO2012054203 suggests using small structures (down to the microscale) for controlling bubble size in a carbonated beverage which is alleged to affect "mouthfeel" when consumed. Structural examples are discussed that reduce drag on the liquid as it is poured from a container. In other words, in the field of carbonated beverages it is desirable to increase fluid flow during pouring while controlling bubble size and, presumably, minimising excessive foaming.

JP2007297129 describes a cap for a beverage can, e.g. for low-malt beer which ordinarily does not have good foaming characteristics for a head to form in a poured glass. Foam bubbles are encouraged to form by use of a paper or non-woven fabric surface to be located in the flow path of beer poured from the can mouth. The paper or non-woven fabric surface generally presents fibrous structures (i.e. at least several micrometers in size) that contact the beer and encourage foam bubbles to form from the carbonated beverage being poured. mary of the Invention

It will become apparent that the present invention seeks to build upon technologies proposed by the inventors of WO2014027028, i.e. the use of nanostructures for encouraging bubble nucleation in a nitrogenated beverage, but with more specific and optimal forms suitable for use with a canned or bottled beverage. These forms promote bubble nucleation, growth and detachment to achieve bubble sizes consistent with nitrogenated beverages; i.e. generally less than 180 μπι diameter average.

In a broad aspect of the invention there is provided a dispense surface for a nitrogenated, i.e. nitrogen-containing, fluid including: a stagnation zone to create low pressure regions; and a nanostructured zone to lower the energy to nucleation and promote bubble nucleation out of the nitrogenated fluid. In practice, the present invention overcomes the deficiencies of prior art systems to create a system that provides surge and settle in nitrogen-containing beverages at low head pressures (e.g. less than 5 psig = 1.36 bar) and that can be readily integrated into packaging. In general, the invention entails:

• a geometry that creates a stagnation zone as beer flows, creating low pressure regions in the vena contracta and beyond;

• a structured surface at and/or beyond the stagnation zone to lower the energy to nucleation, allowing bubble nucleation and cavitation to occur at the low pressure sites;

• preferably an area beyond the column of liquid at the stagnation zone, creating a cascade of low pressure zones, and/or providing regions of high shear or pressure drag to promote bubble detachment.

Brief Description of the Drawings

Figure 1 illustrates a stagnation zone created by pouring beer across a bent spout;

Figure 2 illustrates a stagnation zone created by fluid flowing over a macroscopic feature.

Figure 3 shows scanning electron microscope (SEM) images of effective structured surfaces according to the invention, created by anodization of aluminum with oxalic acid, showing the multiple length scales of structure present;

Figure 4 illustrates a first preferred embodiment of a dispense surface according to the invention;

Figure 5 illustrates detail of a dispense surface from Figure 4;

Figure 6 illustrates a second preferred embodiment of a dispense surface according to the invention;

Figure 7 illustrates a spoiler that can be incorporated with a dispense surface according to the invention; Figure 8 illustrates a sketch of the surge method and device according to the invention;

Figure 9 illustrates a spoiler attached to flow surface; and

Figure 10 illustrates alternative experimental embodiments for comparative results.

Detailed Description of Preferred Embodiments of the Invention It is known in the art that when a fluid flows through or over/around certain structures stagnation zones can be formed where the flow becomes almost immobile. Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object. The Bernoulli equation shows that the static pressure is highest when the velocity is zero and hence static pressure is at its maximum value at stagnation points. The invention relies on the low pressure regions that are created past the stagnation zone by changing the direction of fluid as it contracts to go around sharp bends. Stagnation zones are created by adding geometric features in the path of the beer flow. One such feature could be a sharp bend created by allowing a column of fluid to fall onto a surface or over a ridge. A stagnation zone can be created by pouring beer from a height above an inclined surface, however, such an embodiment is not so practical if it is to be integrated into a beverage unit package such as an aluminium can.

Figure 1 shows a stagnation zone created by pouring beer B from a beverage container (aluminium can) 11 across a bent spout 12 extending from a rim 13 of the container adjacent and below the open mouth 14. The rim 13 effectively creates a ridge in the flow path of the fluid. According to the invention an inclined surface such as spout 12 incorporates nanostructures, e.g. multiple pits, sufficient contact time with which will break nitrogen gas out of solution. The high drag, created by high shear and/or pressure variation on the surfaces along the spout, facilitates bubble detachment. These effects together result in a poured head in a drinking vessel comprised of small bubbles, generally with an average size less than 180 μ η. The bubbles form in the poured beverage after a required settling time.

A further structure, illustrated by Figure 2, includes provision of a feature like a sharp nub/protrusion 15 which the fluid travels up and over to create a waterfall-like effect, e.g. onto the spout 12 before transfer to a glass G. This may work alone or in conjunction with rim ridge 13 or an additional ridge. Such features can be integrated into a beverage container, either internally or externally. Alternatively, features can be applied to other locations that may interact with the dispensed beer, as will be described below.

An important aspect of the geometric feature is that it must not substantially block the flow. It is preferable that the volumetric flow rate is maintained at greater than 10 mL/s using the pressure sources available which, for a beverage can, means using the hydraulic head pressure, which is no more than 0.5 psig. The geometric features of the invention are differentiated from prior art such as orifice plates since fluid acceleration is being achieved using hydraulic head pressure while maintaining high flow rate, rather than an additional pressure source.

A geometry that creates a stagnation zone is an important consideration of the invention. The stagnation zone produces subsequent low pressure regions in the vena contracta and beyond. These low pressure regions, when combined with a suitable surface, allow pressure-driven nucleation, also known as cavitation. By accelerating the fluid, a stagnation zone also creates regions of high drag, either regions of high shear force and/or pressure drag, which allows bubble detachment. The diameter of a bubble formed from a surface in nitrogenated beer depends mainly on the diameter at detachment. Bubble detachment occurs when the buoyancy and drag forces overcome surface tension.

Low pressure created by the stagnation zone can be estimated from Bernoulli's equation or from fluid dynamics simulations. For example, the velocity and diameter of the liquid stream at the instant of surface impact can be quantified using the full Bernoulli's equation. The flow pattern around the impingement point including the spatial variation in film thickness and velocity can be quantified using a simplified model for the continuity and momentum balance equations represented by a first order ordinary differential equation, which can be solved for the local velocity and thickness of the liquid film. The local velocity is used to determine the energy imparted to the heterogeneous bubbles and the dynamic pressure in the continuous phase which is subtracted from the total pressure to get the local static pressure available for cavitation.

The local pressure at the interface between the liquid and the dispense surface depends upon the thickness of the fluid film. The invention requires a structured flow surface, i.e. incorporating nanostructures, wherein the flow surface is defined as the surface that interacts with the fluid after the stagnation zone. The purpose of the structured flow surface is to lower the energy to bubble nucleation. The surface changes the cavitation inception number. Cavitation is the pressure-analogy to boiling; cavitation inception occurs when the local pressure in the liquid is some point below the saturated vapour pressure. The cavitation number, o, defined by σ = [pL- PV(TL)]/[0.5PLUL ² ], describes the potential for cavitation; where PL and U L are the pressure and velocity at a reference point in the liquid, PV(TL) is the saturated vapor pressure at the reference temperature, and p is the liquid density. As σ is reduced, cavitation will be observed at some value called the cavitation inception number (σ,). Continued reduction of σ below σ-, creates more bubbles. The cavitation inception number for a particular system is defined by the flow characteristics and the surface. The present invention uses surfaces that have a higher σ-, than a standard lacquered aluminum can, i.e. bisphenol-A epoxy-coated or other epoxy-coated aluminum

It is noteworthy that, using traditional materials as the flow surface - such as lacquered aluminum, glass, epoxy, etc. - some gas can be removed from the beer but not as effectively as a widget or orifice plate in the prior art. When beer at 6 °C containing 1.8 volumes of C0 ₂ and 0.072 g/L of N ₂ is poured from a height onto a plain surface, not all the gas is removed from the beer and the resulting bubble size in the head is larger than desired, i.e. generally larger than 220 μΓη. By contrast, if the flow surface is modified so that it has a high density of nanoscale structures (particularly pits at the nanoscale), substantially all the gas can be removed from the beer and a bubble size of approximately 120 μπι diameter average can be produced.

Suitable materials for the flow surface include grooved, pitted, rough or porous surfaces such as created by etching, electrospinning, imprinting, attachment of nanoparticles, and other methods known in the art. Nanocomposite coatings with silica, alumina, or diatomaceous earth are also suitable. Anodized aluminum created from sulfuric or oxalic acid etching are suitable.

In a preferred embodiment, the dispense surface is comprised of anodized aluminum that is highly etched using oxalic acid (AAO), such as shown in the SEM image of Figure 3. The thickness of this layer is preferably 25 μπι or more. Preferably, in an optimised form of the invention, the morphology can be characterized by a multiscale microstructure: e.g. 1 μΓη-scale grooves and/or larger (e.g. 10 to 100) μΓη-scale pockets with walls composed of pits or "pores" (because the surface does have absorptive capacity) approximately 35 nm to 75 nm across, generally represented in Figure 4. The depth of such pits should be more than 15nm, but preferably even deeper, e.g. 50 nm and above. The bulk porosity by nitrogen physical adsorption is 25% and the surface pit density by digital image processing is approximately 80%; the structure has high tortuosity, in the hundreds. There is expected to be no maximum depth limit for effective application of the invention although there will be practical limits imposed by the manufacturing process.

While not being bound by theory, it is believed that a microscale morphology increases the density of nanoporous nucleating sites, which substantially lowers the energy required for bubble nucleation and growth due to the proximity of active surface sites and number of surfaces sites. Surfaces containing only a microstructure (such as in prior art WO2012054203) or containing a low density of nanostructures are insufficient to provide a strong surge using low head pressure in a nitrogenated beverage. A microscale morphology may also serve to accelerate the fluid and increase drag. The optimum surfaces have a very low contact angle with water, i.e. the area easily wetted and observed to wick the fluid. Wicking may also be important in accelerating fluid.

In some embodiments of the invention the nanostructures may themselves be used to create a periodic pressure perturbation at relatively high frequency (in kHz range) according to the flow dynamics. Such a high frequency pressure perturbation may be originated by the low dimensions of the nanostructures (e.g. approx. 10 to 25nm) and the performance is significantly influenced by the circumstances, especially at the nano-scale where a large surface area of the material is exposed. The invention considers a combination of flow surface area and geometry to be effective. The combination of a structured surface in the region isolated to the stagnation zone is insufficient to achieve surge and settle but, surprisingly, it was found that the region beyond the stagnation zone is critically involved in producing the surge and settle effect. In other words, the flow surface must be functionalized with a nanostructured surface. The region beyond the stagnation zone, where bubble nucleation and cavitation is occurring, must also promote bubble detachment by creating a region of high drag. This can be achieved by pouring the beverage from a height onto an inclined surface, where the fluid contracts into a film flow with high velocity. However, in other embodiments it is useful to incorporate macrofeatures that increase shear or, even, increase pressure drag. According to a preferred embodiment, illustrated by Figure 4, a can end 11 includes an integral sharp bend pouring feature by tilting the can to pour beverage from a mouth opening 14 which acutely impacts a dispense surface 12 on the underside of a spout cover/lid 16 located closely over the mouth. The dispense surface 12, shown in detail by Figure 5, includes macrofeatures in the form of visible divets 17. Nanopits are also present across dispense surface 12, but are invisible to the naked eye.

A further embodiment is illustrated by Figure 6, where a dispense surface on the underside of a covering 16 is positioned over a container mouth (not visible) in order to ensure a directional change for pouring beverage (from the tilted container) and contact with surface features according to the invention. In Figure 6 a series of macrofeatures in the form of ridges 17 perpendicular to the direction of flow are provided, over which contacting beverage must flow on its way toward a serving vessel. In accordance with the invention the diversion of pouring beverage caused by the covering 13 and its surface features 17 causes stagnation zones of low pressure regions which, in combination with the nanostructures also formed across the dispense surface encourage bubble nucleation, growth and detachment into the beverage as it pours.

It will be appreciated that arrangements other than those shown in Figures 4 to 6 can be used, for example a pattern of chevrons or triangles can be formed in, or on, the spout 12. When analysing the embodiments of Figures 4, 5 and 6, compared to a case with no macrofeatures, where the flow rate is 60 mL/s and the beer allowed to drain from a can held at fixed angle, the max shear stress increased from approximately 6-8 Pa to up to 20 Pa. Other suitable macroscale geometries for increasing shear include cylinders of various size, shape, and density; combinations of horizontal and vertical slats and venturi shapes. In some embodiments, such as when the stagnation zone and flow surface must be accommodated within a beverage container, a spoiler is desirable. The role of the spoiler is to focus the fluid to impact the flow surface with sufficient velocity and to help create the stagnation zone. The spoiler may be designed according to the Coanda effect and be placed at the exit opening of the can to accelerate the fluid and cause a bending down towards the nanostructed flow surface. The bending of fluid flow results in violent fluid collision on the nano-structural coating although the spoiler itself does not directly produce any surge effect. It does not need to be functionalized with nanostructured material.

Figure 7 schematically shows the design of a spoiler 18. The rounded geometry of the spoiler causes beer to bend down as it flows out of the can end, causing it to hit the functional flow surface 12.

It is believed that a critical requirement of the invention is to develop sufficient impact pressure during pouring. The angle of collision of the beer with the activated solid surface affects the shape and extension of the impact zone, as well as the magnitude of the shear stress acting on solid surface. The latter is important for bubble detachment. In case of a liquid fluid of radius (R) collided normally on a flat rigid surface at an attack angle a, the outward side flow is prevented when the boundary of the contact area spreads out more rapidly than do the compression waves into the drop. As the fluid advances, the expansion of the contact area becomes progressively slower and a point is reached where flow at the periphery of contact becomes possible. The width of contact area at which side flow begins is a function of angle of attack (a).

sin °c—

R where:

a = angle of collision between colliding surfaces as shown in Figure 8

R = radius of beer flow (predetermined by can opener) Based on definition of the shear stress, the stress is given as

T = pg(X - y)s i^ + ^ ^ec - ²

n

Where: Θ is surface incline angle (in degree): θ = \ α +

According to equations (eq.l & eq.2) the maximum value of the shear stress and the extension of stagnation point are achieved for incline angle equal to 45°. Experimental findings are in good agreement with the predictions found in regards to the effect of incline angle (or angle of attack) on shear stress and extension of stagnation zone.

In seeking to understand experimental results, the conditions that provided excellent results, moderate results, and bad results were analysed. CFD showed that for good results, the impact pressure generally exceeded 250 Pa. The higher impact pressure generally had the effect of providing a larger region of low pressure zones and high shear past the stagnation zone. For the most effective examples, the shear over the entire flow region was 10-13 Pa and higher near the stagnation zone. The present invention can be incorporated into an external attachment for achieving a surge and settle effect in nitrogen-containing beers. For example, a schematic sketch of one prototype is shown in Figure 8. The nanostructured surface is contained as part of a rigid body 19, such as aluminum or plastic, that is mounted at an angle to the exit spout of dispense unit, i.e. pouring beverage B. Alternatively, it may be mounted onto a stand 20 or attached to the can. Beer poured over this surface into a glass G generates a head height of 18 to 20 mm and time to black of 45 to 65 s.

The experimental data obtained from pouring experiments indicates that the stagnation zone, fluid velocity and surface area are pivotal factors that determine overall performance of the invention. The fluid velocity on the surface depends on the impingement velocity and the geometric contraction after the stagnation zone. As fluid velocity increases, surge efficiency is enhanced. This is believed to be due to both more effective nucleation and cavitation (low pressure) and more effective detachment (shear). It is possible to control the fluid velocity by introducing a spoiler concept in the final design. The spoiler is a means of accelerating the fluid prior to impact with the surface. Because of the geometric limitation within a beverage can design, the stagnation point must be within a short distance of the can opening. Therefore a spoiler element can be utilised to direct the fluid onto an activated (i.e. nanostructured) surface.

An example of a can end design that incorporates a spoiler and nanostructured flow surface is shown in Figure 9. The spoiler 18 has a curvature with a radius from 2 to 5 mm and a position from the dispense surface 12 of less than 1 mm. The beer flows out the opening over the spoiler, where it is bent down onto the tab, which contains nanostructured AAO. The point where the fluid bends down is a stagnation zone. The total area of the structured surface 12 is approximately 4 cm ² which has proven sufficient for encouraging bubble formation according to the invention. Indeed the minimum effective area is approximately 2.5 - 5 cm ² . The can end was attached to a can filled with unsurged beer. The beer was caused to flow over the feature. The results of three tests showed that the design was effective (Table 1). All the gas was removed. All Gas Removed? (Activity on pour time, sec head height, mm Time to Black, sec Surger after Pour)

All removed, no additional activity

20 17 40

22 16 40 All removed, no additional activity

20 18 40 All removed, no additional activity

Table 1: Results from pouring over functional tab with spoiler integrated into can

Several additional spouts were designed to attach to the exit of a can end, in order to give comparative results. In each design, the beer exits the can opening and flows over an area of the spout. This spout can be modified with AAO. In one design, the spout is smooth with a smooth flow line over the surface and no stagnation zone. In another design, a bend is added to the spout which creates a stagnation zone. The stagnation zone can be further sharpened and the impingement velocity increased by adding a spoiler at the can exit. Finally, the length of the flow surface was reduced. The results are shown in Figure 10 and Table 2.

Table 2: Results from testing different spouts In the above examples, the high shear was provided by generation of thin film flow after the stagnation zone. In some designs it may not be possible to suitably thin out the fluid, often because impingement velocities are limited by the can geometry and pressure head. In this case, additional macroscopic features may be added onto the nanostructured surface to increase shear forces and facilitate bubble detachment. Likewise, macroscopic features may be incorporated to provide a larger region of low pressure.

According to Figures 4, 5 and 6 tabs/spouts 12 were fashioned and attached at the can exit so that that the beer impinged on the can as it exited. The tabs were constructed from aluminium 31015 sheet of 500 μπι thickness, trapezoidal in shape with a length between 3.0 cm to 3.5 cm and having flanges 27 along the outer edges, e.g. less than 0.5 cm, to act as flow guides and to maintain a gap between the can end and tab. For experiments, two versions were constructed: narrow and wide. The narrow version was approximately 1.0 cm wide at the top and 2.0 cm wide at the bottom; the area of beverage contact was approximately 7.25 cm ² . The wide version was approximately 2.0 cm wide at the top and 3.5 wide at the bottom; the area of contact was approximately 12 cm ² . A macroscopic feature was formed into the aluminum. In the case of Figures 4 and 5, the features are protrusions in a staggered array, 0.08 cm tall, 0.13 cm wide and 0.30 cm apart. In Figure 6 the features are horizontal slats: 4 slats per cm, going nearly the width of the tab. The area of the macrofeatures was the same for both narrow and wide versions. It will be apparent that the protruding side faces the flowing liquid. The tabs were anodized as above using 0.3 mol/L oxalic acid at room temperature, at 37 V using Al as the reference electrode and constant current of 8 ± 2 mA/cm2 for 14 h.

These tabs were attached to either a custom can end with a 2 cm diameter irregular aperture in the center or a standard can end. They were attached at an approximately 12 degree angle with respect to the can end. Unsurged beer was tested as described in the previous examples. As shown in Table 3, the addition of macroscopic features was very effective in generating quality surge when combined with AAO (a nanostructured surface). The angle of attachment, stand-off height, and pouring angle were also found to be important considerations. The pouring angle is defined as 90 degrees for a can that is turned so that its long axis is parallel to the surface and 180 degree for fully inverted. Ideally, pouring technique needs to maximize the impact pressure and provide a large region of high shear. CFD simulations showed that impact pressure was approximately 120 Pa, the maximum shear force was approximately 20 Pa, and the average shear over the region was 10 Pa. In the table, "n/a" means not available because no surge was observed.

Average bubble size was estimated by measuring the size of bubbles at three locations within the head from top to bottom. For the beer initiated with a surger, this value is generally between 125 urn to 140 μπι on average.

The impact pressure can be increased by making the exit aperture smaller and adding a vent in the can so that the flow rate is 30-60 mL/s. Computational Fluid Dynamics (CFD) model simulations show that impact pressure can be increased to more than 300 Pa. If one of the effective tabs is attached over this modified exit, longer surge times are provided; approximately 45 to 60 s time to black. The bubble size in such a produced head has a range of 105 to 155 μπι, with average of approximately 125 μ η. The preferred solution, therefore, should entail a beverage package that is capable of dispensing beverage at a flow rate greater than 30mL/s (up to or exceeding 50-60mL/s) which causes an impact pressure to a dispense surface greater than 300 Pa (and up to 450 Pa or more), thereby inducing sheer stress in the liquid of greater than 20 Pa. Downstream nanopits/pores (20 to 85nm across) further encourage bubble nucleation and break out. It is estimated that the pressure in the liquid is in the order of less than 0.45 atmospheres.

Using a tab that is not macrostructured but coated with AAO is less effective. In the final head, a small region at the top of the head has small bubbles (approximately 120 μπι), but the average is much higher, approximately 180 μ η.

It was also found that these macrostructured surfaces were effective in the configurations described earlier. The height at which effective surge was observed could be reduced to 1.5 cm. Design Flow Pour Time (s) Head Height Time to Black Avg Bubble

Surface (mm) (s) Size

Smooth, Wide Tab; Attached to AAO 14-16 s 6 n/a 180 Wide Mouth

Narrow Tab; Array of Bare 14-16 s n/a n/a >190 Protrusions on Wide Mouth aluminum

Wide Tab; Array of Protrusions AAO 14-16 s 22 30-45 150 on Wide Mouth

Narrow Tab; Horizontal Slats on AAO 14-16 s 20-22 45-55 Not Custom End measured

Narrow Tab; Horizontal Slats on AAO 14-16 s 20-22 30-40 130 Wide Mouth

Table 3: Results from testing with macrostructured tabs.

The invention can be applied to any suitable vessel for dispensing a nitrogenated beverage; i.e. the material or type of container is not a limitation. In practice the vessel is likely to be an aluminium can, glass bottle or PET bottle. A dispense surface according to the invention could be incorporated with any known technique or those developed as technology in this area (creation of nanostructures) progresses.