Liquid capillary forces

As time passes, the thickness of the liquid layer decreases and the quantity of adherent particles increases. This is explained by the fact that for a relatively thin layer of liquid, capillary forces become operative (see Section 17), dictating an increase in the adhesion and in the quantity of adherent particles. 

Capillary Rise (or Fall) of Liquids (Capillary Forces)... 

The greater the viscosity of a liquid, the more slowly it flows. Viscosity usually decreases with increasing temperature. Surface tension arises from the imbalance of intermolecular forces at the surface of a liquid. Capillary action arises from the imbalance of adhesive and cohesive forces. 

A capillary system is said to be in a steady-state equilibrium position when the capillary forces are equal to the hydrostatic pressure force (Levich 1962). The heating of the capillary walls leads to a disturbance of the equilibrium and to a displacement of the meniscus, causing the liquid-vapor interface location to change as compared to an unheated wall. This process causes pressure differences due to capillarity and the hydrostatic pressures exiting the flow, which in turn causes the meniscus to return to the initial position. In order to realize the above-mentioned process in a continuous manner it is necessary to carry out continual heat transfer from the capillary walls to the liquid. In this case the position of the interface surface is invariable and the fluid flow is stationary. From the thermodynamical point of view the process in a heated capillary is similar to a process in a heat engine, which transforms heat into mechanical energy. 

The solution of Eq. (10.50) determines the steady states of the liquid velocity, as well as the position of the meniscus in a heated micro-channel. Equation (10.50) can have one, two or three steady solutions. This depends on the value of the parameter (in the generic case parameter B), which takes into account the effect of the capillary forces. 

The change of velocify due to liquid evaporation l and influence of the capillary forces L versus Xf for > 1 is illustrated in Fig. 10.6. In the case 2> 1 the curves L(Xf) and L(xf) have only one point of intersection, which determines the stationary values of ml = L.st and Xf = Xf,st. It is not difficult to show that this point is stable. Indeed a displacement of the meniscus from its initial position Xf,st to the position x[ ) leads to the situation, when the velocity due to the liquid evaporation Hl exceeds the velocity due to the capillary force u[. This leads to the return of the meniscus to its initial position. If the meniscus displaces to the left, > u, this also leads to the return of the system to its initial state. 

The wall heat flux is the cause for the liquid evaporation, and perturbation of equilibrium between the gravity and capillary forces. It leads to the offset of both phases (heated liquid and its vapor) and interface displacement towards the inlet. In this case the stationary state of the system corresponds to an equilibrium between gravity, viscous (liquid and vapor) and capillary forces. Under these conditions the stationary height of the liquid level is less than that in an adiabatic case... 

The multiplicity of phenomena characteristic of flow in heated micro-channels determined the content of the book. We consider a number of fundamental problems related to drag and heat transfer in flow of a pure liquid and a two-phase mixture in micro-channels, coolant boiling in restricted space, bubble dynamics, etc. Also considered are capillary flows with distinct interfaces developing under interaction of inertia, pressure, gravity, viscous and capillary forces. 

Slip casting is common in the ceramics industry. The material to be cast is milled to a mean particle size of a few microns. A slip is made by mixing the finely divided material with a liquid suspending medium. The slip is then poured into a suitable mold (e.g., of plaster of pans). The liquid in the slip is drawn into the mold by capillary forces and the solids are deposited in a coherent form. For TiBj, ZrBj and CrBj a suspending medium of 5-7 wt% cyclopentadiene in xylene is recommended. A 3 wt% aqueous solution of carboxymethylcellulose is the best dispersing medium... 

In a liquid, intermolecular forces are strong enough to confine the molecules to a specific volume, but they are not strong enough to keep molecules from moving from place to place within the liquid. The relative freedom of motion of liquid molecules leads to three liquid properties arising from intermolecular forces surface tension, capillary action, and viscosity. ... 

Most electrode materials are hydrophilic and readily wetted by aqueous solutions. Two methods are used to create and maintain an optimum gas/solution ratio in the electrode. The first method employs a certain excess gas pressure in the gas space. This causes the liquid to be displaced from the wider pores in finer pores the liquid continues to be retained by capillary forces. The second method employs partial wetproofing of tfie electrode by the introduction of hydrophobic materials (e.g., fine PTFE particles). Tfien the electrolyte will penetrate only those pores in the hydrophilic electrode material where the concentration of hydrophobic particles is low. 

Compared with liquid column chromatography, in PLC there is a certain limitation with respect to the composition of the mobile phase in the case of reversed-phase chromatography. In planar chromatography the flow of the mobile phase is normally induced by capillary forces. A prerequisite for this mechanism is that the surface of the stationary phase be wetted by the mobile phase. This, however, results in a Umitation in the maximum possible amount of water applicable in the mobile phase, is dependent on the hydrophobic character of the stationary RP phase. To... 

Liquid film membranes consist of immiscible solutions held in membrane supports by capillary forces. The chemical composition of these solutions is designed to enhance transport rates of selected components through them by solubility or coupled chemical reaction. 

For most tablets, it is necessary to overcome the cohesive strength introduced into the mass by compression. It is therefore common practice to incorporate an excipient, called a disintegrant, which induces this process. Several types, acting by different mechanisms, may be distinguished (a) those that enhance the action of capillary forces in producing a rapid uptake of aqueous liquids, (b) those that swell on contact with water, (c) those that release gases to disrupt the tablet... 

Capillary forces increase in relationship to the relative humidity (RH) of the ambient air. At greater than 65% RH, fluid condenses in the space between adjacent particles. This leads to liquid bridges causing attractive forces due to the surface tension of the water. 

As an immiscible organic liquid retained by capillary forces in the pore space between the soil particles. This free organic phase is often referred to with the abbreviation NAPL (nonaqueous phase liquid). 

Both adsorptive and capillary forces play an important part in soil-liquid interaction (see Figure 18.3). This is very important for unsaturated soil. The total force (i.e., the sum of capillary force and adsorptive force) is termed the matrix potential, which has a negative gage pressure relative to the external gas pressure on the soil water (more often the gage pressure is referred to as the atmospheric pressure). 

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