The Dynamics of Wetting

When describing the dynamics of wetting, we must analyse the motion of the contact line. At rest, the contact angle is e- When the contact line is moving at speed U, the contact angle is the dynamical angle 6 (see Fig. 1.3). If 9 e, the capillary force [7sa — (7 cos 0c1+7sl)] is positive and the line moves forward U 0). If 6 d E7 the line moves backwards U d). 

The dynamical equation of the line is the relation 9 — 9 U). It is found by writing down the equilibrium condition between  

Equating the driving force Fd = 7sa (7cos d -f- 7sl) = 7( d l)/2 and the frictional force, we obtain the dynamical equation for the motion of a liquid wedge, which relates U to d  

While the contact line is moving, capillary energy is dissipated in the form of heat by the frictional forces in the liquid. 

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Since drying occurs simultaneously with wetting, the effect of diy-ing can substantially modify the expected impacd of a given process variable and this should not be overlooked. In addition, simultaneously drying often implies that the dynamics of wetting are far more important than the extent. 

An upper limit on the capillary number required for snap-off arises from the dynamics of wetting fluid flow into the constriction. The capillary number must be below the upper limit for a long enough time that sufficient wetting fluid can flow back into the constriction to form a lamella (40). If the volume of wetting fluid is too small, the lamella cannot form. 

Marangoni interfacial stresses which slow the dynamics of wetting. Additional variables which influence adhesion tension include (1) impurity profile and particle habit/morphology typically controlled in the particle formation stage such as crystallization, (2) temperature of granulation, and (3) technique of grinding, which is an additional source of impurity as well. 

If one draws a line of ink on a piece of plastic, the line breaks up into droplets because, for exactly the same reason, a section of cylinder is less stable than a string of spherical caps. In a subsequent chapter devoted to the dynamics of wetting, we will see how this phenomenon controls numerous hydrodynamic instabilities that show up when a liquid flows and... 

Liquids with an adjustable viscosity coefficient r] usually polymer melts with different chain lengths). They are useful for studying time-dependent phenomena. A characteristic velocity V = yfr] controls the dynamics of wetting. That velocity can range from about 1 im/s to 70 m/s (for water). 

The price to pay is that the substrate is not rigid and that it flows. When studying the dynamics of wetting, it becomes necessary to take into account the flows induced in the substrate as liquid A spreads or dewets (chapter 7). °... 

In crop sprays applied to plants or weeds it is essential that the spray solution wets the substrate completely and in many cases rapid spreading may be required. Again the dynamics of wetting becomes a very important factor. 

Despite increasing attention on the dynamics of wetting, understanding the kinetics of the process at a fundamental level has not been achieved. 

The description of liquids interfaces, in particular in the presence of solid surfaces, is an old question at the confluent between physics, chemistry, and engineering. It was examined by pioneers like Young and Laplace, whose formalism remains the basis of the description of capillarity. The dynamics of wetting has been intensively studied in the second part of the twentieth century, when many experiments and theories were developed. These studies emphasized the importance of a precise description of boundary... 

The dynamics of wetting/general features and open questions for low temperature experimentation", (1984). 

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