Surface tension, liquids and

Minimum Wetting Rate The minimum liquid rate required for complete wetting of a vertical surface is about 0.03 to 0.3 kg/m s for water at room temperature. The minimum rate depends on the geom-etiy and nature of the vertical surface, liquid surface tension, and mass transfer between surrounding gas and the liquid. See Ponter, et al. Int. J. Heat Mass Tran.fer 10, 349-359 [1967] Trans. Inst. Chem. Eng. [London], 45, 345—352 [1967]), Stainthorp and Allen Trans. Inst. Chem. Eng. [London], 43, 85-91 [1967]) and Watanabe, et al. ]. Chem. Eng. [Japan], 8[1], 75 [1975]). 

The physical properties associated with the parachor, vi2., the liquid surface tension and the density at various temperatures, for benzofuroxan and 5-methylbenzofuroxan are given by Hammick et al. The parachor values were reevaluated by Boyer et al ... 

Generally, the mean droplet size is proportional to liquid surface tension, and inversely proportional to liquid density and vibration frequency. The proportional power index is —1/3 for the surface tension, about -1/3 for the liquid density, and -2/3 for the vibration frequency. The mean droplet size may be influenced by two additional parameters, i.e., liquid viscosity and flow rate. As expected, increasing liquid viscosity, and/or flow rate leads to an increase in the mean droplet size,[13°h482] while the spray becomes more polydisperse at high flow rates.[482] The spray angle is also affected by the liquid flow rate, vibration frequency and amplitude. Moreover, the spray shape is greatly influenced by the direction of liquid flow (upwards, downwards, or horizontally).[482]... 

In practice, the contact angle can be experimentally determined in a rather routine manner, as can the liquid surface tension and even the solid surface energy. The interfacial energy for the liquid-solid system of interest, ysi, can then be calculated using Young s equation. Alternatively, if ysL, Yl, and ys are known as a function of temperature, the contact angle can be predicted at a specified temperature. 

Note that we make a distinction between a solution and a mixture. When we talk of a solution, we imply that the organic solute is not a major component of the bulk liquid. Therefore, that presence of a dissolved organic compound does not have a significant impact on the properties of the bulk liquid. In contrast, in a mixture we recognize that the major components contribute substantially to the overall nature of the medium. This is reflected in macroscopic properties like air-liquid surface tensions and in molecule-scale phenomena like solubilities of trace constitutents. 

Two mechanisms were proposed for the strong upward and downward flows [135], The first was based on interplay of the liquid-liquid and solid-liquid surface tensions and the gravitational and inertia forces. The second is correlated with the neck formation, the droplet break-up and the retossing of the fluid. 

When the surface of the liquid is drawn up on the solid we say that the liquid wets the solid. This results from a substantial attraction between the liquid and the surface of the solid which is indicated by a small value ySL, the solid-liquid surface tension, and a large downward net surface tension force. A device that measures the force on a plate, such as shown in Fig. 6, is called a Wilhelmy balance. 

Beyond the nucleation stage, size change in the system can occur by a number of mechanisms as depicted in Fig. 3.2. The prevailing mechanism depends on such factors as the feed particle size and other solid properties, liquid surface tension and viscosity and the mode of operation (batch or continuous). After nucleation has occurred, the predominating growth mechanisms are ... 

Depending on conditions the apparent critical size cluster may, in fact, be as small as the dimer, A2. Blander and Katz (2) have pointed out some of the ambiguities in the conventional macroscopic theory for AG in terms of bulk liquid surface tensions and densities. For smaller size clusters (n 20), a molecular approach to the nucleatlon problem is clearly preferred. In this regard, Bauer and Frurlp have recently used a somewhat empirical approach to determine the cluster entropies of condensing iron (3). 

This is only an approximation, albeit generally a good one. In fact, the vapor pressure must be slightly greater than the gas-phase pressure to overcome the effects of liquid surface tension and the hydrostatic head of liquid at the heated surface. 

For a hydrophobic porous material with contact angle greater than 90°, the APc is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water-air-polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 pim (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). 

The surface tension of most concentrated aqueous solutions of inorganic salts, such as those employed in OD as strip solutions, is considerably greater than that of pure water. Intrusion of these solutions into microporous, hydrophobic membranes of the types used in OD is, therefore, unlikely under moderate operating pressures. However, some aqueous feeds contain amphiphilic components that may depress the liquid surface tension, and thereby reduce the critical penetration pressure. In such cases it may be necessary to use a membrane with a pore diameter of less than 0.1 p, to prevent liquid intrusion. For most applications however, membranes with a nominal pore diameter of 0.2 p have been found to be suitable. 

Wetting and dispersion depends on the liquid surface tension and 0, the contact angle between the Hquid and solid. W, W and Wg are spontaneous when 0 < 90°, and Wd is spontaneous when 0 = 0. Since surfactants are added in sufficient amounts ( dynamic lowered sufficiently), spontaneous dispersion is the rule rather than the exception. 

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