Surface tension

About: Surface tension is a research topic. Over the lifetime, 25410 publications have been published within this topic receiving 695471 citations.

A Fractal Analysis of Stiction in Microelectromechanical Systems

Abstract: The strong adherence (stiction) of adjacent surfaces is a major design concern in microelectromechanical systems (MEMS). Advances in micromachine technology greatly depend on basic understanding of microscale stiction phenomena. An analysis of the different stiction micromechanisms and the elastic deformation of asperities at MEMS interfaces is developed using a two-dimensional fractal description of the surface topography. The fractal contact model is scale independent since it is based on parameters invariant of the sample area size and resolution of measuring instrument. The influence of surface roughness, relative humidity, applied voltage, and material properties on the contributions of the van der Waals, electrostatic, and capillary forces to the total stiction force is analyzed in light of simulation results. It is shown that the effects of surface roughness and applied voltage on the maximum stiction force are significantly more pronounced than that of material properties. Results for the critical pull-off stiffness versus surface roughness are presented for different material properties and microstructure stand-free surface spacings. The present analysis can be used to determine the minimum stiffness of microdevices required to prevent stiction in terms of surface roughness, apparent contact area, relative humidity, applied voltage, and material properties.

Wettability and Contact Angles

Abstract: In everyday life one encounters many different systems where it is apparent that various processes take place at the line of contact between a liquid phase and a solid phase. The molecules in the liquid phase can move larger distances than those in the solid phase. Therefore, since the molecules in a solid phase are well fixed, this gives rise to the fact that it is in general not possible to study the surface forces of the solid phase in the same way we have described for the liquid phase elsewhere in this book. In spite of this, there exist many examples where we have to attribute a definite value of surface tension to the solid surface, in order to be able to describe the different reactions taking place at the solid surface in contact with liquid phase.

Interfacial phenomena controlling particle morphology of composite latexes

Abstract: A thermodynamic analysis and a mathematical model were derived to describe the free energy changes corresponding to various possible morphologies in composite latex particles. Seeded batch emulsion polymerization was carried out at 70°C using as seed monodisperse polystyrene latex particles having different surface polarity. The surface polarity was estimated by contact angle measurement at the latex “film”/water interface for octane as the probe liquid. Methyl methacrylate and ethyl methacrylate were polymerized in a second stage seeded emulsion polymerization using polystyrene particles as seed in the presence of a nonionic stabilizer, nonyphenol polyethylene oxide (Igepal Co-990). Two types of initiators, potassium persulfate (K2S2O8) and azobisiobutyronitrile (AIBN), were used to change the interfacial tension between the second stage polymer (in monomer) and water interface. The values of the interfacial tension of polymer solutions in the second stage monomer vs. the aqueous phase, measured by drop-weight–volume method under conditions similar to those prevailing during the polymerization, correlated well with the determined particle surface polarity and the observed TEM particle morphology. The results showed that, rather than the polymer bulk hydrophilicity, the surface particle polarity is the controlling parameter in deciding which phase is inside or outside in the composite particle. The variation of the polymer phase interfacial tension with polymer concentration was also estimated. Based on experimentally measured interfacial tensions, composite particle configurations were predicted. The predicted morphologies showed good agreement with the observed particle morphologies of the composite latexes.

The First Rideal Lecture. Microemulsions, a field at the border between lyophobic and lyophilic colloids

Abstract: Microemulsions can be regarded as thermodynamically stable dispersions of droplets of one phase in another phase. There are continuous transitions to swollen micelles, when the droplets become extremely small, and to coarse emulsions when the droplets are large. The mechanism leading to the formation of microemulsions is the tendency to extend the interfacial area until the concentrations of surfactants are sufficiently low that non-negative interfacial tension is achieved. The role of the combination of a surfactant and a cosurfactant is seen in their leading to a decrease in the interfacial tension to potentially negative values. An expression is given for the Gibbs energy of the whole system, for the special case of an ionized surfactant and a non-ionized cosurfactant. The minimization of this Gibbs energy leads to the stability conditions for the microemulsion. The factors which promote O/W and W/O emulsions, respectively, are briefly discussed. The electrical contribution to the interfacial tension depends on the particle size. This leads to a narrow size distribution. It is finally pointed out that, in principle, fine dispersions of solids in a liquid may be thermodynamically stable in the presence of large amounts of easily adsorbed compounds.

Exact solitary water waves with capillary ripples at infinity

Abstract: We prove the existence of solitary water waves of elevation, as exact solutions of the equations of steady inviscid flow, taking into account the effect of surface tension on the free surface. In contrast to the case without surface tension, a resonance occurs with periodic waves of the same speed. The wave form consists of a single crest on the elongated scale with a much smaller oscillation at infinity on the physical scale. We have not proved that the amplitude of the oscillation is actually nonzero; a formal calculation suggests that it is exponentially small.