Thermocapillary effect

Figure 7-17. Streamlines for thermocapillary motion of a gas bubble for a = 0.8 (a2pg//3), where the bubble velocity is reduced to 20% of its value in the absence of thermocapillary effects. The stream-function values are calculated from Eq. (7-244) with coefficients C and D from Eqs. (7-248). Contour values are plotted in increments of 0.7681.

Statement of the problem. Let us consider the motion of a viscous fluid in an infinite layer of constant thickness 2h. The force of gravity is directed normally to the layer. The lower plane is a hard surface on which a constant temperature gradient is maintained. The nonuniformity of the temperature field results in two effects that can bring about the motion of the fluid, namely, the thermogravitational effect related to the heat expansion of the fluid and the appearance of Archimedes forces, and the thermocapillary effect (if the second surface is free) produced by tangential stresses on the interface due to the temperature dependence of the surface tension coefficient. 

To emphasize the role of a chemical reaction, such thermocapillary effects will be called chemo-thermocapillary. The problem on the steady-state translational Stokes flow past a drop is conventionally divided into three parts. 

As far as the thermal part of the problem is concerned, to describe the thermocapillary effect, it does not suffice to consider only the zero approximation with respect to low Peclet numbers, because in this case the temperature would be constant along the drop surface. Therefore, instead of Eqs. (5.10.2), we suggest... 

In the limit case 0 -4 oo (high viscosity of the drop substance), the thermocapillary effect does not influence the motion, B -> the flow around the drop will be the same as for a hard sphere, and (5.11.3) implies the Stokes law (2.2.5). For m = 0 (no heat production or independence of the surface tension on temperature), the thermocapillary effect is absent, and (5.11.3) yields a usual drag force for a drop in the translational flow (2.2.15). 

The breath figures technique is one of the most widely employed methods for the fabrication of organized porous polymer films [30, 31] and, as fiuther depicted in detail, in this approach the template consists of an ordered array of water droplets that can be removed by simple evaporation. Indeed, the simultaneous evaporation of a volatile solvent and condensation of water vapor in combination with thermocapillary effects and Marangoni convection allow the formation and precise organization of water droplets at the polymer solution-air interface [30]. This array of water droplets will evaporate upon complete evaporation of the solvent of the polymeric solution, and the surface will reflect its presence in the form of pores. 

How bifurcation in microchannels is discussed in this entry. In this entry, flow bifurcation refers to geometrical bifurcation. Specifically, the transport of droplets in microchannel where a mother branch bifurcates into two daughter branches with thermocapillary effects is examined. 

Marangoni effect Thermal Marangoni effect Thermocapillary effect... 

Transport of Droplets by Thermal Capillarity, Fig. 1 Model of thermocapillary effect of a liquid droplet in a transient temperature field... 

Nguyen NT, Huang XY (2005) Thermocapillary effect of a liquid droplet in transient temperature field. Jpn JApplPhys 44 1139-1142... 

Oron A. and Rosenau Ph. (1994). On a nonlinear thermocapillary effect in thin liquid layers. J. Fluid Mech., 273, 361-374. ... 

Thermocapillary effect Marangoni effect Thermal Ma-rangoni effect... 

The above two scaling laws both imply that thermocapillary effects may become a dominant driving mechanism for microfluidics. They have led researchers to explore using thermocapillarity to manipulate fluids in microfluidic situations. 

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