Bubble Initiation

The development of bubbles within a liquid or polymer solution is generally called nucleation, although the term actually refers only to those bubbles that 

Bubble nucleation is affected by a number of conditions. Physically, the effects of temperature, pressure, and in some cases humidity are fairly obvious. Other important parameters are surface smoothness of the substrate, surface characteristics of filler particles, presence and concentration of certain surfactants or nucleators, size and amount of second-phase liquid droplets, and the rate of gas generation. 

In many cases, bubbles of gas and other contaminants are already present in the liquid or polymer solution, and these serve as sites into which the gas may diffuse. The number and size of these gas bubbles may be another important factor in bubble development. 

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Han, C. Y., and P. Griffith, 1965a, The Mechanism of Heat Transfer in Nucleate Pool Boiling, Part I, Bubble Initiation, Growth, and Departure, Int. J. Heat Mass Transfer S(6) 887-904. (2)... 

In general an increase in intensity (I) will provide for an increase in the sonochemical effects. Cavitation bubbles, initially difficult to create at the higher frequencies (due to the shorter time periods involved in the rarefaction cycles) will now be possible, and since both the collapse time (Eq. 2.27), the temperature (Eq. 2.35) and the pressure (Eq. 2.36) on collapse are dependent on P i(=Ph + PA)> bubble collapse will be more violent. However it must be realised that intensity cannot be increased indefinitely, since (the maximum bubble size) is also dependent upon the pressure amplitude (Eq. 2.38). With increase in the pressure amplitude (P ) the bubble may grow so large on rarefaction (R g, ) that the time available for collapse is insufficient. 

It has been argued (Appendix 3, Eq. A.21) that the collapse time for a bubble, initially of radius R, is considerably shorter than the time period of the compression cyde. Thus the external pressure Pj (= P + Pjj), in the presence of an acoustic field, maybe assumed to remain effectively constant (Pj ) during the collapse period. Neglecting surface tension, assuming adiabatic compression (i. e. very short compression time), and replacing R, by R, the size of the bubble at the start of collapse, the motion of the bubble wall becomes... 

Originally Bowden s school suggested that heat from the adiabatic compression of such gas bubbles initiated explosion in the surrounding liquid. Johansson coworkers (Ref 4), however, pointed out that heat flow from a compressed gas bubble to the surrounding liquid is much too slow to account for the observed phenomena, particularly at low impact energies. They have shown that to achieve explosion fine droplets... 

Bubbles of gas in liquid expls, suppression of bubble initiation 2 B320... 

Many other baked products, such as cakes, originate as both W/O emulsions and foams. Cake batter comprises a mostly W/O emulsion, with some O/W domains, that is also a foam containing small-sized air bubbles. Initially, the air bubbles are stabilized mostly by fat crystals. As the baking process gets underway the fat melts,... 

A single human lymphoblast (MOLT-3) was retained by a weir (5 p.m) in a PDMS-glass chip. This cell channel was separated from another channel containing SDS lysing solution by a gas bubble (introduced by a thermopneumatic actuator). Removal of the gas bubble initiated the liquid mixing, and cell lysis occurred [842],... 

As yet there is no fluid dynamic model that describes in quantitative detail the bubble formation process but it is barely necessary for a reaction engineering model. It is adequate to assume that entering reactant gas passes in plug flow through the bottom layer of particles, say, one initial bubble diameter deep and thereafter forms bubbles. Initial bubble diameter is readily estimated from the known flow through the orifice and the fact that frequency is about 8/s. Above this distributor layer the two-phase bubble model can be applied. 

Fig. 4. Schematic of cavitation bubbles interacting with a slurry of precipitated gel particles. The configuration shows typical two orifice processing as afforded in the CaviPro 300 processor. Cavitating bubbles initially form, expand in the recovery zone, and collapse with the formation of a microjet and shock wave.

The level of coalescence between particles, the size of the particles, and the packing arrangement dictate the size of air cavities and, thus, the size of the bubble initially formed in the melt. Once formed, the bubbles remain stationary in the melt. A relatively small bubble diameter, combined with the high viscosity of the melt, prevents the movement of the bubbles into the melt. The bubble removal is known to be a diffusion-controlled process. The identification of key parameters in the dissolution of bubbles formed in the melt has been done using a theoretical model that describes this process. The disappearance of the air bubble formed into the melt was modeled based on... 

For the motion of an electron bubble within the superfluid cluster the static, harmonic approximation (Section IV.E) breaks down. Consider an electron bubble initially located at the radial distance d from the cluster boundary. The dynamic spatial distribution p/(r) of the electron bubble in the image potential VW [Eq. (88)] in the absence of dissipation can be described by the probability of the bubble location at distance d cluster surface, where the bubble moves back and forth from d up to d in the image potential. 

As a starting point, the liquid can be taken to be water that has equilibrated with air to obtain its noble gas content. Furthermore, it is assumed that the liquid is saturated with respect to the dominant gas species forming the bubble/gas phase. The column is divided into cells and it is assumed that there is no transport of dissolved gases or fluid between the cells. When a gas bubble, initially with no noble gas content, is introduced into the first cell the distribution of both Ne and Ar can be calculated from Equation (16) assuming complete equilibration between the gas and fluid in that cell only. The volume of the bubble is assumed to be constant and, now with a noble gas content, is moved to the next cell. Equilibrium is again assumed, and the resulting distribution of Ne and Ar between the gas and liquid phases calculated. In this manner the Ne and Ar concentrations and Ne/Ar ratio can be calculated for the gas phase and each water cell as the bubble is sequentially passed through the unit cells of the liquid column. 

Experiments with bubbles of 50 fxm diam showed that such bubbles initiate an explosion provided they contain a gas with a high y. It was also observed that smaller bubbles took less time to initiate reaction because smaller bubbles respond more quickly to shock and the time to reach minimum volume is reduced. A comparison of the efficiency of different sized bubbles subjected to the same shock is given by the sequence in Figure 5. The diameters of the bubbles B, bi, b2, ba, and b4 are 1.75 mm, 50 /rni, 80 jum, 80 /rni, and 250 /xm, respectively. The smallest bubble caused initiation first (frame 3), and at this stage there was very little change in the volume of the largest bubble. In frame 4, initiation took place at other small bubbles. 

Hybrid binding, (maintenance of upstream end of the bubble) (initiation factor interaction, RNA exit tunnel formation), (RNA exit) (RNA exit)... 

Figure 29 illustrates the behavior of each bubble (initially the size and internal pressure are the same for all the bubbles) and the cloud. In the case of the 5-bubble cloud, a calculation indicates that cavities at the center grow similarly to peripheral bubbles, but end up with less distortion. 

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