Antony S. Dimitrov

Steady-state unidirectional convective assembling of fine particles into two-dimensional arrays

Abstract We formed millimeter-sized two-dimensional particle arrays on inclined microslides using water suspensions of the particles. We found that the array porosity depended on the particle diameter, the particle volume fraction, and the rate of array growth. Deriving an equation for the steady-state array growth process, we found that these parameters, together with the rate of water evaporation, govern the overall array growth process. Our results illustrate a promising technology for producing fine particle coatings on solid surfaces.

CONTACT ANGLE MEASUREMENTS WITH SESSILE DROPS AND BUBBLES

Abstract In this note a simple and precise method for measurement of contact angles in the range 0°–180° on solid surfaces is proposed. In the case when the three-phase contact line is irregular due to surface roughness and inhomogeneity, the method provides an effective averaged value of the contact angle. The method is based on a numerical solution of the Laplace equation for the interfacial profile. The input parameters used are the liquid-gas surface tension and a pair of the following parameters of the drop (bubble) shape: volume, height, contact radius, and equatorial radius. Depending on the magnitude of the contact angle different pairs of parameters are to be measured for the purpose of best accuracy.

Contact angles of thin liquid films: Interferometric determination

A general method for calculation of three-phase contact angles from interferometric experimental data is proposed. The method is applicable for both usual and differential interferometry. It is specified for the case of horizontal films formed in a cylindrical capillary. The method was tested with model and experimental data for the meniscus profile. In particular, it is found that the contact angle of the meniscus coincides with the contact angle of several different lenses floating in the film. This fact implies that the line tension effect in this case is below the threshold of experimental accuracy.

Accuracy of the Differential-interferometric Measurements of Curvature

The radius of curvature at the top of small liquid drops has been determined both visually and by means of differential interferometry in reflected light (microscope Epival Interphako, Carl Zeiss-Jena). Systematic errors in the interferometric measurements have not been found within the investigated interval of curvatures. A comparison between the data for aqueous and mercury drops showed that there is no pronounced effect of the reflectivity of the surface on the accuracy of the interferometric measurements.

Contact angle, film, and line tension of foam films. I. Stationary and dynamic contact angle measurements

It is established experimentally that slowly shrinking small air bubbles attached to a liquid-air interface form a nonequilibrium (dynamic) contact angle. Aqueous solutions of sodium dodecyl sulfate (SDS) are used at two different concentrations of added electrolyte. A black thin liquid film is formed at the top of the bubble. The experimental cell used allows measurements with shrinking and expanding bubbles as well as with bubbles of constant equatorial radius. Hysteresis of the contact angle was observed. The results show that the shrinking of the contact line (advancing meniscus) causes deviation of the contact angle from its equilibrium value. The reliability of some previous work interpreting the change in contact angle with the radius of shrinking bubbles as a line tension effect is discussed.

Dynamics of contact lines in foam films

Abstract Different factors can contribute to the value of the line tension acting at the periphery of a thin foam film. To study one of them, the motion of the contact line, a new experimental technique has been developed. It was established experimentally that slowly shrinking small air bubbles attached to a liquid - air interface form a nonequilibrium (dynamic) contact angles. Aqueous solution of sodium dodecyl sulfate (SDS) are used at two different concentrations of added electrolyte. A black thin liquid film is formed at the top of the bubble. The experimental cell used allows measurements with shrinking, expanding and quiescent (“stopped”) bubbles. Hysteresis of the contact angle has been observed. The results show that the shrinking of the contact line (advancing meniscus) causes deviation of the contact angle from its equilibrium value. The data allow independent calculation of the film and line tensions, γ and ϰ. For a “stopped” bubble both γ and κ relax with time: κ tends to zero and γ tends to its equilibrium value for infinite film known from other experiments. The results suggest that the concept of dynamic line tension must be introduced. The latter was attributed to the motion of the contact line.

Interfacial fluctuation-dissipation processes and contact angles of thin liquid films

Abstract The role of surface viscosity in interfacial fluctuation—dissipation processes is studied. As a result, a new method for calculation of the correlations between interfacial dynamic variables is developed. It is applied to investigate the effect of the suppression of fluctuation capillary waves in thin liquid films on the value of the film contact angle.