Liquid lamella

Figure 2.45 Design of a multilamination mixer with hydrodynamic focusing (upper left) and flow pattern in such a mixer for a total volume flow of 10 ml h of water (lower left), taken from [141. The right side of the figure shows the orientation of liquid lamellae over a cross-section of the constriction for different Reynolds numbers [142].

Figure 2.48 The top shows the schematic design of a magneto-hydrodynamic mixer with equally spaced electrodes arranged In a micro channel and an external magnetic field oriented along the z-axis. On the bottom theoretical results for the evolution of two parallel liquid lamellae as a function of dimensionless time are shown [146].

The decay of liquid lamellae of circular cross-section at rest was studied in a 2-D model using the VOF method without subcellular tracking of the interface (in the following denoted basic VOF method ) in combination with a correction algorithm... 

OS 43] [R 14] [P 32] Using a three-liquid layer (water/oil/water) flow instead of a two-liquid layer flow at constant channel dimensions decreases the liquid lamellae width and doubles the absolute value of the organic/aqueous interface. As a consequence, mass transport is facilitated compared with the two-flow configuration. Hence it was found that a much higher yield was obtained for the three-liquid layer flow when performing experiments of both flow configurations imder the same experimental conditions (210 s, 0.2 pi min room temperature, 300 W, > 300 nm... 

Figure 1.92 Regular arrangement of liquid lamellae in the focusing chamber of the SuperFocus micro mixer (steel version large arc of interdigital feeds). For better flow visualization, an asymmetric flow ratio (5 1) was chosen, setting the dyed water solution at a lower flow rate [39].

Figure 1.101 Orientation of liquid lamellae in the mixing channel of the slitshaped interdigital micro mixer viewing direction is the flow direction [37]...

Because numerical errors due to discretization of a convective term introduce an additional, unphysical diffusion mechanism, termed numerical diffusion (ND), the diffusion coefficient D was set to zero [152], The resulting concentration fields nonetheless are indicative of the distribution of a solute within the micro channel volume. In this way, convective patterns can be derived for the redistribution of the liquid transverse to the flow direction. Accordingly, the stretching, tilting and thinning of liquid lamellae can be followed. 

This is also evident when considering the convective entanglement of two initially vertical liquid lamellae. For the double-sided structured channel, a considerable increase in the lamellae entanglement and an enlarged interfacial surface area are observed. 

Wet foams in which the liquid lamellae have Typically, in these cases the gas bubbles have... 

The drainage of liquid from liquid lamellae separating bubbles in a foam. See also Fluid Film. 

Wet foams in which the liquid lamellae have thicknesses on the same scale or larger than the bubble sizes. Typically in these cases the gas bubbles have spherical rather than polyhedral shape. Other synonyms include gas dispersion and kugelschaum . If the bubbles are very small and have a significant lifetime, the term microfoam is sometimes used. In petroleum production the term is used to specify crude oil that contains a small volume fraction of dispersed gas. See also Foam. 

In his theory of coalescence Marrucci [364] concerned himself with the kinematics of the thinning process (drainage), which the liquid lamella between neighboring gas bubbles is subjected to, until it bursts. The reason for the stretching of both boundary layers of the film is due to the pressure difference between the liquid in the film and that in the bulk. They are in equilibrium with the difference in surface tension between the film and the liquid bulk ... 

Thermal foam breaking (by direct injection of steam into the foam space or by indirect heating of the walls) aims at bursting the inflated gas bubbles, while simultaneously lowering the viscosity of the liquid lamellae, ft has proved to be ineffective, frequently resulting in (thermal) damage to the product and incrustation of the heating surface. 

In the case of a polyhedral foam with planar hquid lamella, the pressure difference between the bubbles is not large, and consequently Ostwald ripening is not the mechanism for foam instability in this case. With a polyhedral foam, the main driving force for foam collapse is the surface forces that act across the liquid lamella. 

Many poorly foaming liquids with thick film lamella are easily mptured, for example pure water and ethanol films (with thickness between 110 and 453 run). Under these conditions, rupture occurs by growth of disturbances which may lead to thinner sections [17]. Rupture can also be caused by the spontaneous nucleation of vapour bubbles (forming gas cavities) in the structured liquid lamella [18]. An alternative explanation for the rupture of relatively thick aqueous films containing a low level of surfactants is the hydrophobic attractive interaction between the surfaces that may be caused by bubble cavities [19, 20]. 

In interdigital multilamination micromixers, the small thickness of the lamellae leads to short diffusion paths, resulting in fast mixing. Further thinning of the liquid lamellae should lead to shorter diffusion paths and faster mixing. The IMM single mixer applied this concept by shrinking the channel width in the slit. A further extension of this concept leads to the... 

For maximum mechanical stability, the interfacial film resulting from the adsorbed surfactants should be condensed, with strong lateral intermolecular forces, and should exhibit high film elasticity. The liquid film between two colliding droplets in an emulsion is similar to the liquid lamella between two adjacent air sacs in a foam (Chapter 7) and shows film elasticity for the same reasons (Gibbs and Marangoni effects). 

A foam structure can always be formed in a liquid if bubbles of gas are injected faster than the liquid between bubbles can drain away. Even though the bubbles coalesce as soon as the liquid between them has drained away, a temporary dispersion is formed. An example would be the foam formed when bubbles are vigorously blown into a viscous oil. Such a foam, comprising spherical, well-separated bubbles, is referred to as a wet foam, or kugelschaum. Wet foams in which the liquid lamellae have thicknesses on the same scale as the bubble sizes are sometimes referred to as gas emulsions . Here, the distinction of whether this is a foam or not relates to stability. But it is complicated by the fact that, as for other types of colloidal dispersions, no foams are thermodynamically stable. Eventually they all collapse. 

Steam-based processes in heavy oil reservoirs that are not stabilized by gravity have poor vertical and areal conformance, because gases are more mobile within the pore space than liquids, and steam tends to override or channel through oil in a formation. The steam-foam process, which consists of adding surfactant with or without noncondensible gas to the injected steam, was developed to improve the sweep efficiency of steam drive and cyclic steam processes. The foam-forming components that are injected with the steam stabilize the liquid lamellae and cause some of the steam to exist as a discontinuous phase. The steam mobility (gas relative permeability) is thereby reduced, and the result is in an increased pressure gradient in the steam-swept region, to divert steam to the unheated interval and displace the heated oil better. This chapter discusses the laboratory and field considerations that affect the efficient application of foam. 

Lamella Leave-Behind A mechanism for foam lamella generation in porous media. When gas invades a liquid-saturated region of a porous medium, it may not displace all of the liquid, but rather leave behind liquid lamellae that will be oriented parallel to the direction of the flow. A foam generated entirely by the lamella leave-behind mechanism will be gas-continuous. See also Lamella Division, Snap-Off. 

Plateau Border The region of transition at which thin fluid films are connected to other thin films or mechanical supports such as solid surfaces. For example, in foams Plateau borders form the regions of liquid situated at the junction of liquid lamellae. Sometimes referred to as a Gibbs ring or Gibbs—Plateau Border. 

Snap-Off A mechanism for foam lamella generation in porous media. When gas enters and passes through a constriction in a pore, a capillary pressure gradient is created and causes liquid to flow toward the region of the constriction, where it accumulates and may cause the gas to pinch-off or snap-off to create a new gas bubble separated from the original gas by a liquid lamella. See also Lamella Division, Lamella Leave-Behind. 

Wet foams in which the liquid lamellae have thicknesses on the same scale as the bubble sizes are sometimes referred to as gas emulsions. Typically in these cases, the gas bubbles have spherical rather than polyhedral shape. 

If the rate of flow of low-molar-mass liquids is increased very sharply, additional velocity components occur because of the surface roughness of the walls. As the flow rate increases, these disturbances of laminar flow finally become so large that they are no longer dampened by the viscosity of the liquid. The individual liquid lamellae no longer flow in a parallel manner, and... 

Figure 20 Schematic of the evolution of a thin liquid lamella between two approaching droplets (147,151) (a) droplets mutual approach with slight deformation of interfaces (b) dimple formation on smfaces (c) near plane-parallel film (d) thermal or mechanical fluctuations at interface (e) black (common) film formation (f) growth of black film or Newton film to equUibrium radius.

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