Surface-tension dependent effect

Couchman and Karasz (11) recently have made some calculations indicating that spherical microphases should exhibit increased glass-transition temperatures because of an increased pressure inside such microphases attributed to the surface tension between microphases. Since there is some doubt about the existence of a surface of tension in the Gibbs sense (12) between chemically linked microphases, we shall simply note that these calculations are the only ones in existence that indicate a possible reason for an increase in the Tg of a glassy microphase and, in addition, that these calculations also postulate differences in Tg with differences in morphology. For example, this surface-tension-dependent effect would not be expected in samples with lamellar morphology, no matter how small the width of each lamella. 

We expect more insight from simulations in the future, particularly in situations where these multicomponent systems show effects of coupling between the different degrees of freedom, surface tensions depending on temperature and concentration, hydrodynamic flow induced by concentration gradients in addition to thermal buoyancy. 

Because surface curvature depends on radius and different atoms have different sizes, and because the atomic surface tension depends on atomic number, the atomic surface tensions also include surface curvature effects, which has recently been studied as a separate effect.7 Local surface curvature may also correlate with nearest-neighbor proximity and thus may be implicitly included to some extent when semiempirical atomic surface tensions depend on interatomic distances in the solute. 

In order to calculate the effective adsorption time teff which is necessary to compare the determined surface tension dependence with data from other methods an exact determination of the so-called deadtime is required. From some easy assumption the Poiseuille approximation results which yields [178]... 

Let us now return to the question of whether we can calculate the surface energies of polymers from first principles. The rough estimates in section 2.1 tell us correctly the order of magnitude of surface tensions and correctly draw attention to the intimate connection between surface energies and the cohesive forces in liquids, but they have a number of drawbacks. Firstly, temperature makes no appearance in these theories, despite the experimental fact that surface tensions depend quite strongly on temperature. Secondly, we have assumed that the density of the liquid near the surface is the same as the bulk density. These shortcomings are seen at their most extreme if we consider a liquid near the liquid-vapour critical point. Here the distinction between liquid and vapour vanishes completely the surface tension of the liquid approaches zero and the system becomes in effect all interface. An improved theory of surface tension must be able to accoxmt for these phenomena, at least qualitatively. 

Surface tension depends on both the temperature and the concentration in the case of solutions. A temperature (or concentration) gradient leads to a surface tension gradient regions of high 7 (7+) pull on regions of low 7 (7-), and the liquid is set in motion. This effect has a great many consequences, some of which we shall now illustrate. 

Surface tension is a factor influencing solvent selection. Solvents affect the surface tension of coatings, which can have important effects on the flow behavior of coatings during application, as discussed in the section on Film Defects. Since surface tensions depend on temperature and concentration of resins in solution, solvent volatility can have a large effect on the development of surface tension differentials. 

The coupled heat and liquid moisture transport of nano-porous material has wide industrial applications in textile engineering and functional design of apparel products. Heat transfer mechanisms in nano-porous textiles include conduction by the solid material of fibers, conduction by intervening air, radiation, and convection. Meanwhile, liquid and moisture transfer mechanisms include vapor diffusion in the void space and moisture sorption by the fiber, evaporation, and capillary effects. Water vapor moves through textiles as a result of water vapor concentration differences. Fibers absorb water vapor due to their internal chemical compositions and structures. The flow of liquid moisture through the textiles is caused by flber-liquid molecular attraction at the surface of fiber materials, which is determined mainly by surface tension and effective capillary pore distribution and pathways. Evaporation and/or condensation take place, depending on the temperature and moisture distributions. The heat transfer process is coupled with the moisture transfer processes with phase changes such as moisture sorption and evaporation. 

The effect of the hydrophilic group on surface tension depends on the structure of the hydrophile and, for ionic surfactants, also on the counterion. For a constant chain length of C7F15, the surface tension of 0.1% solutions of fluorinated surfactants varies between 17 and 47 mN/m, depending on the nature of the hydrophile [56] (Table 4.5). Nonionic fluorinated surfactants usually have lower surface tensions than their ionic counterparts. Nonionic surfactants derived from... 

Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [317]. 

Small drops or bubbles will tend to be spherical because surface forces depend on the area, which decreases as the square of the linear dimension, whereas distortions due to gravitational effects depend on the volume, which decreases as the cube of the linear dimension. Likewise, too, a drop of liquid in a second liquid of equal density will be spherical. However, when gravitational and surface tensional effects are comparable, then one can determine in principle the surface tension from measurements of the shape of the drop or bubble. The variations situations to which Eq. 11-16 applies are shown in Fig. 11-16. 

The oscillating jet method is not suitable for the study of liquid-air interfaces whose ages are in the range of tenths of a second, and an alternative method is based on the dependence of the shape of a falling column of liquid on its surface tension. Since the hydrostatic head, and hence the linear velocity, increases with h, the distance away from the nozzle, the cross-sectional area of the column must correspondingly decrease as a material balance requirement. The effect of surface tension is to oppose this shrinkage in cross section. The method is discussed in Refs. 110 and 111. A related method makes use of a falling sheet of liquid [112]. 

In a foam where the films ate iaterconnected the related time-dependent Marangoni effect is mote relevant. A similar restoring force to expansion results because of transient decreases ia surface concentration (iacteases ia surface tension) caused by the finite rate of surfactant adsorption at the surface. 

It has been shown (16) that a stable foam possesses both a high surface dilatational viscosity and elasticity. In principle, defoamers should reduce these properties. Ideally a spread duplex film, one thick enough to have two definite surfaces enclosing a bulk phase, should eliminate dilatational effects because the surface tension of an iasoluble, one-component layer does not depend on its thickness. This effect has been verified (17). SiUcone antifoams reduce both the surface dilatational elasticity and viscosity of cmde oils as iUustrated ia Table 2 (17). The PDMS materials are Dow Coming Ltd. polydimethylsiloxane fluids, SK 3556 is a Th. Goldschmidt Ltd. siUcone oil, and FC 740 is a 3M Co. Ltd. fluorocarbon profoaming surfactant. 

In order to take into account the effect of surface tension and micro-channel hydraulic diameter, we have applied the Eotvos number Eo = g(pL — pG)d /(y. Eig-ure 6.40 shows the dependence of the Nu/Eo on the boiling number Bo, where Nu = hd /k] is the Nusselt number, h is the heat transfer coefficient, and k] is the thermal conductivity of fluid. All fluid properties are taken at the saturation temperature. This dependence can be approximated, with a standard deviation of 18%, by the relation ... 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

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