Pores through coalescence

Pores can coalesce just as particles can. (Think of them as inverse particles.) The driving force is the same, namely, reduction of the total energy by reducing the area of the interface. The difference is that in order to shrink, the pore must nucleate a vacancy first. This vacancy must diffuse away through the lattice, along a dislocation or along an interface. 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

Filtration is achieved bv adsorption within the pores in the media. The oil droplets coalesce into larger drops and after the bed reaches oil saturation the coalesced oil passes upward through the riser pipes with the water. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

In addition to these bubble generation events, bubble-bubble coalescence events can increase the average bubble size. As a result, unless injected as a fine foam with bubble sizes below those of the pore channels, or at rates above those possible in actual reservoirs, foam is expected to advance through the pore channels as single layer bubble trains with the bubbles separated... 

Pores with size above 200 im are obtained through a coalescence polymerization routeP ... 

SCHEME 23 Coalescence of silica gel through Ostwald ripening during alkaline aging, resulting in loss of surface area, expanding pore size (initially), and a change from convex to concave pore surface. 

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