<p>The amount and size of bubbles in a foam layer that have originated from a solid surface in a gas supersaturated solution is largely determined by the physical properties of that solid and liquid surface and the supersaturation level of the gas in the liquid. The presence of pre-existent nuclei - gas trapped in pores at the solid surface - as well as the wetting properties of the liquid on that surface contribute to the formation of the bubbles. The supersaturation level of gas in the liquid contributes to the rate at which this phenomenon occurs. The main objective of this study is to quantify their contributions in order to establish the relation between a) the physical properties of the supersaturated liquid and solid surface as well as the shapes of the liquid and the solid phases and b) the foam formation in a supersaturated solutions. Therefore: 1) the growth, detachment and rise of individual bubbles forming at an "active" site - a pre-existent nucleus trapped in a cavity in a solid surface - is studied, 2) based on this knowledge the requirements of an ideal "active" surface - a collection of "active" sites - is formulated, and 3) with a selected "active" surface, the hypothesis of the requirements are tested, by determining the foaming properties of the "active" surface in gas supersaturated solution.<p>In the process of bubble formation at an "active" site in a gas supersaturated solution, the following steps can be distinguished:<p>1) The growth of a bubble at an "active" site.<br/>By using a model "active" site in the experiments- the tip of a glass capillary which is closed at one end - the following variables have been studied: the mouth radius of the "active" site, the supersaturation of the surrounding solution, the wetting behaviour of the "active" site, the flow of the liquid along the bubble, and dynamic surface properties.<p>We have observed the effect of the gas supersaturation on the individual bubble growth rate. With three different concentrations of carbon dioxide gas, the growth of individual bubbles on different sized glass capillaries were measured and compared to several growth models. The growth rates of the bubble could be best described by Bisperink's growth model, which takes into account the fact that the bubble is attached to a solid surface, for the movement of the bubble interface into the liquid as well as for the depletion of gas from the layer of liquid surrounding the bubble during growth (chapter 3).<p>During the experiments, it was observed that the formation of extra, unwanted bubbles, also called "parasite bubbles", influenced the bubble growth rate as a result of the entrainment of liquid. Due to this phenomenon, it was difficult to make a theoretical assessment of bubble growth rates. The opposite effect was also observed, namely at the lowest concentration, where after the formation of a single bubble, the liquid surrounding<br/>the bubbles was so much depleted of gas, that the growth rate of the succeeding bubble was also affected (chapter 3). The dynamic surface properties were found not to play an important part in the process of bubble formation as the velocity of growth was slow enough to keep the system close to equilibrium (chapter 3).<p>2) The detachment of a bubble from an "active" site.<br/>By using the same glass capillary as model "active" site the following variables have been studied: the mouth radius of the "active" site, the angle of inclination of the "active" site, the surface tension and the surface rheological properties of the bubble surface, the hinterland effect, the bubble growth rate and the liquid flow around the bubble.<p>The volume of the detaching bubble has a linear relationship with the internal radius of the "active site", for a well- wetted surface and as long as the shape of the bubble resembles a sphere (capillary radius < 0.5 mm). For a larger capillary size, and thus a large bubble size, the bubble resembles a pear shape (as shown on the cover of this thesis) and detaches with a smaller volume than predicted theoretically, as a result of mechanical instabilities (chapter 3).<p>When the capillary or model "active site" is tilted, the bubble size at detachment is decreased as a result of a lower effective adhesive force to the surface. The bubble size as a function of the tilting angle can be predicted theoretically, but experimental assessment of the theory shows some deviation (chapter 4).<p>To simulate the effect of liquid flow on bubbles attached to a solid surface, a horizontal oscillating movement was imparted to the capillary on which the bubbles grow. Increasing both amplitude and frequency of the oscillation decreases the size of the bubble at detachment. A theoretical examination of this phenomenon is given in chapter 4, and one of the conclusions is that the decrease in bubble size cannot be explained alone with the shear and inertial forces, We have suggested that a "surface skin" is formed, and that the bubble becomes unstable and detaches as soon as the surface skin is mechanically ruptured by shear forces parallel to the surface.<p>The internal volume of the cavity, also called the "hinterland volume" was found to have less effect on the bubble growth rate than was expected, as the dry internal walls of the cavity behaved more hydrophobically than expected. However, due to a sometimes considerable retreat of the air-liquid interface into the glass capillary after bubble detachment, it was concluded that a large hinterland volume could, in extreme cases, lead to an almost complete stop of bubble formation (chapter 3).<p>Surface rheological properties were found to be of influence on the detachment of bubbles under the influence of liquid flow. It was found that the bubbles detached at an earlier stage than could be predicted from forces alone. We therefore speculate that due to periodic surface expansion and compression, a "skin" is formed which, when ruptured, unbalances the bubble and induces it to detach (chapter 4).<p>3) The rise and growth of the detached bubble on its way to the foam layer. In a model experiment, single bubbles released from a tilted capillary tip were studied during their rise through the liquid. Variables were: the bubble size, the surface rheological properties, the liquid viscosity, the travelled distance, the degree of supersaturation and the distance between two consecutive bubbles. After the bubbles detach, they rise through the supersaturated liquid to the foam layer. In a gas supersaturated solution relatively free of surfactant, the bubbles are found to grow considerably during the travelled distance, but bubbles rising in a gas supersaturated solution like beer hardly grow during rise. It was therefore speculated that in beer, a surface "skin" could be formed at the bubble interface which is either mechanically too rigid to grow, or is partly insoluble to gas molecules which reduces the mass transfer of gas through the interface to the bubble (chapter 5).<p>Based on the knowledge obtained in the first part, the second part of this thesis was dedicated to formulating the requirements of an ideal "active" surface, selecting such an "active" surface, and relating the foamability to the material properties of that surface (chapters 6 and 7). From a fimdamental point of view, an ideal "active" surface for the production of foam out of a supersaturated solution has to fulfil the following requirements:<br/>1) The "active" sites are situated at such a lateral distance from each other that coalescence between neighbouring bubbles is prevented.<br/>2) The number of "active" sites is big enough to produce a foam layer of pre-set volume in the required time.<br/>3) The "active" surface is situated in a vertical position allowing that bubbles are formed on both sides.<br/>4) The outside part of the "active" surface is hydrophilic to ensure a controlled small bubble size.<br/>5) The inside surface of the "active" sites are hydrophobic to ensure a long shelf life of such "active" sites.<p>In an empirical set-up, a variety of surfaces was tested on their ability to produce a foam in a gas supersaturated solution. A suitable and very "active" surface, Tyvek, was selected for further study. The structure of this paper like material is very heterogeneous and porous. It is comprised of polyethylene fibres which are pressed together to form a hydrophobic porous matrix. The outside of this paper is made hydrophilic for printing purposes. This kind of structure could produce a great amount of bubbles simultaneously, and was stable in foamability. The liquid is prevented from entering the pores due to the hydrophobic nature of the material and due to the curvature of the liquid inside the pores, which makes the gas inside the Tyvek material stable over long periods of time, when submerged in the liquid (chapter 6).<br/>The degree of internal wetting of the "active" surface is found to affect foam formation considerably. Almost complete wetting results in almost no bubble formation. When the surface is not previously wetted this may lead to extremely turbulent bubble formation. This proves our hypothesis that pre-existent nuclei are needed to help in the formation of bubbles at the levels of gas supersaturation used in this work. The "active" surface is doubly effective when it is placed in an upright position due to its two sides. A horizontal positioning of the surface leads to the formation of very large bubbles trapped underneath the surface, which help to destabilise the foam as soon as the become a part of that foam (chapter 6).<p>The surface area of the Tyvek used, and therefore the amount of "active" sites, has a direct effect on the initial rate of foam formation. For example, an area of 16 cm <sup>2</SUP>forms a foam layer at a much faster rate than an area of 4 cm <sup>2</SUP>. However, this latter surface may stay active for a longer period of time, as the dissolved gas in the solution is not exhausted as rapidly. Eventually, the bubble formation stops when the gas supersaturation in the liquid is exhausted. The surface, however, can be inserted into a fresh supersaturated liquid and the process starts anew (chapter 7).<p>With this work, we have shown that, in order to enable an "active" surface to foam, it should contain pre-existent nuclei trapped within pores or cavities and these should be stable over a long period of time. This can be accomplished with a material that contains pores or cavities with a hydrophobic internal surface. If this is not the case, the liquid will penetrate into the solid surface and dissolve the gas nuclei within. Without the gas nuclei, no foaming will occur at the gas supersaturation levels observed in this work.
|Qualification||Doctor of Philosophy|
|Award date||19 Jun 1997|
|Place of Publication||S.l.|
|Publication status||Published - 1997|