Showing papers on "Wetting transition published in 1981"

Surface tension and contact angle of a liquid–solid interface

Abstract: The contact angle between a liquid film and a solid substrate is treated in the van der Waals model used previously by the author to discuss gas adsorption on solid surfaces The model accounts for the occurrence of three types of wetting behavior, ie, complete wetting, partial wetting, and nonwetting, as well as for transitions between these different regimes According to this model, the contact angle depends on two parameters, namely the reduced temperature T/Tc, where Tc is the critical temperature of the liquid, and ew/kTc, where ew is the minimum in the liquid–solid interaction potential For high‐energy surfaces, corresponding to large ew/kTc, the contact angle decreases on increasing temperature; the opposite behavior is predicted to occur on sufficiently low‐energy surfaces With some hypotheses on how ew depends on the nature of the solid and liquid phases present, one can calculate the variation in contact angle with liquid species for a given solid at fixed temperature The theory is compare

Some effects of substrate roughness on wettability

Abstract: The influence of substrate roughness on wettability has been investigated at room and high temperatures using sixteen material combinations, mostly liquid metals and solid ceramics but also water, glycerol and solid nickel. The contact angles assumed by both wetting and non-wetting drops of all but two material combinations increased linearly with the relative steepness of the surface features, the effect being less for experiments conducted at high temperatures. In contrast, the contact angles of good wetting drops of glycerol and exceptionally good wetting drops of Easy-flo decreased when their silica and nickel substrates were roughened. Similarly, contact angles of both wetting and non-wetting drops were decreased by ultrasonic vibration. The experimental data can best be interpreted in terms of the metastable equilibrium configuration models in which an advancing liquid front has to overcome energy barriers associated with surface features. This occurs more readily if these barriers are small relative to the energy of the liquid which our data suggest can be equated with the enthalpy of the liquid. This interpretation enables the effects of substrate roughness at one temperature or with one liquid to be used to predict behaviour at other temperatures and with other liquids.

Tensiometric studies on wetting. I. Some effects of surface roughness (theoretical)

Abstract: The effect of roughness on the wettability of an axisymmetric cylinder is investigated theoretically by making use of equilibrium meniscus shapes on solid surfaces analogously to previous studies for drops on horizontal surfaces. Employing circumferential sinusoidal and saw-toothed grooved structure, and using mechanistic arguments, one can explain wetting hysteresis, the formation of composite surfaces, and the presence of non-equilibrium jumps during contact line motion.On unidirectionally random surfaces the maximum surface slopes mainly determine the value of the advancing, and the minimum slopes of the receding contact angle. These effects of surface slopes diminish with decreasing roughness size. Diminishing roughness size also gives rise to numerous small non-equilibrium jumps imposed upon larger jumps during wetting. The contact angle hysteresis is found to show a nearly linear relationship with the spread in the distribution of solid surface slopes.

Wetting transitions at fluid–solid interfaces

Abstract: Liquid films on solid substrates can exhibit three types of wetting behaviour, as shown by the values of their contact angles, corresponding to complete wetting, partial wetting or non-wetting. Closely related to this is the fact that adsorption isotherms for gases on solid surfaces fall into three classes, depending on the variation of the adsorbate coverage near saturation. Phase transitions between these different wetting classes can occur on varying the temperature or the composition of the materials present. A statistical mechanical theory of such transitions, based on concepts introduced by van der Waals, is surveyed and compared with available experimental evidence.

Tensiometric studies on wetting. II. Some effects of surface roughness (experimental)

Abstract: The wetting of regularly rough solid bodies was investigated by means of capillarography. In this technique solid probes are immersed and withdrawn from liquid–fluid interfaces with steady velocities. Continuous observation of the three phase contact line (TPL) motion and recording of the capillary force are possible simultaneously. The effect of regular axisymmetric grooves on cylindrical rods on wetting hysteresis was found to follow theoretical predictions closely at large roughness dimensions (∼86 µm) in which case the TPL makes jumps from one equilibrium position to another while advancing. At lower rugosities (∼1 µm) the effect of regular roughness for a given concentric groove geometry is complicated by effects attributed to the dominant influence of chemical heterogeneity of the solid surface.

Physical significance of contact angles

Abstract: The physical significance of contact angles has been interpreted on the basis of a model derived from known surface energy relationships. The degrees of non-spreading and spreading have been expressed in terms of the magnitude of contact angles. On the basis of the physical picture, hysteresis of contact angle has been calculated from the experimental values of equilibrium contact angle and surface tension of the liquid. It has been suggested that it is not necessary to assume that hysteresis of contact angles is due to surface roughness of solids. The picture also explains why apparent contact angle on a non-flat solid surface is more than that on a flat solid.

Surface phenomena in enhanced oil recovery

Abstract: Investigations of thin-film effects by means of complete solutions of the augmented Young-Laplace equation are summarized, as are the implications for observable contact angle, certain wettability syndromes, and breakup of nonwetting phase as it is replaced in a porous medium by wetting phase. Apparent contact angle, capillary pressure, and thin-film thickness are found to be interdependent at equilibrium. Contact angle in porous media may depend significantly on pore size, fluid proportions, and filling history. 26 references.

The use of wetting behaviour at an oil-water interface to study changes in a polymer surface

Abstract: This work is a preliminary report on a study of the wetting properties of fibres. We discuss here the results obtained on ceramic fibres mated with a thin continuous film of poly-monochloro-para-xylylene. The wetting tension has been measured at the air-oil and oil-water interfaces in both advancing and receding modes. The results show that the wetting data are extremely reproducible at both interfaces. The results at the air-oil interface are primarily dependent on the geometrical features of the fibre while those at the oil-water interface are sensitive to changes in the polarity of the surface. Both advancing and receding contact angles may be calculated from the wetting force. By immersing the fibre for prolonged periods in the water phase, it is shown that the receding contact angle is as reproducible as the advancing angle but is a continuous function of time. This result is thought to result from the hydration of the fibre.