WPI Wetting Transitions Group - Rafael Garcia P. I.

On this page we discuss:

Contact Angle Experiments

Contact angles and their temperature-dependence are of critical importance to a number of technologies such as nanofabrication and oil recovery. Despite the fact that wetting phenomena and contact angles have been studied by chemists, engineers and physicists for over a hundred years, the science of predicting contact angles and their temperature dependence remain in a very primitive state, with most of the progress being confined to the last twenty years.

Drawing from the work of Dietrich and Napiorski and Cheng, Cole, Saam and Treiner, we find a very simple approximate expression allows us to predict the contact angle of a liquid on a solid surface based on its surface tension and the intermolecular forces between the liquid and the surface.

A key prediction of this equation (shown in the figure above) is that as we increase the temperature, the contact angle will decrease to zero, so that a hydrophobic surface will become hydrophilic simply by virtue of increasing the temperature. However, at least for water and other polar liquids, preliminary work indicates that electrostatic charging of the liquid-solid interface can introduce a correction to this simple equation, modifying this predicted temperature dependence.

Play movie of contact angle decreasing as temperature increases according to theoretical equation shown in the figure shown above.

Thus, one important focus of the work in our laboratory at present is testing this simple equation for a variety of liquids on solid surfaces near room temperature. Our methods range from using quartz microbalance apparatus for measuring adsorption potential parameters to simple optical determinations of the contact angle of droplets on a surface.

Liquid Crystal Films: thin-thick coexistence phenomena

A second kind of wetting transition was first discovered in 2003 and recently confirmed by us for thin films of the liquid crystal 8CB on silicon. The liquid crystal molecules on the surface in 2D act analogously to liquid-vapor system in a container in 3D. As we increase the temperature, for a fixed number of molecules in the container, there is a range of temperatures where two distinct densities coexist inside, a liquid and a gas. Analogously, for a fixed coverage of liquid crystal molecules on the silicon surface, there is a range of temperatures where two distinct film thicknesses coexist on the surface. We were able to show that once inside the temperature range where coexistence is observed, the percentage area covered by thick and thin phases is determined by a lever-rule that conserves the total coverage on the surface.

In the image to the right, we are looking at the reflected light from a liquid crystal film on silicon. The color of the reflected light gives us information on the film thickness because of interfrence. The light yellow-brown areas correspond to thin film patches, the darker green-brown areas correspond to thick films.

The shape of the coexistence region in the temperature-thickness phase diagram is ultimately determined by the molecule-surface interactions as well as intermolecular forces between the liquid crystal molecules, including those associated with liquid crystalline phase transitions. However, the precise explanation is still very controversial and the focus of much theoretical research. One intriguing possibility is that the "bump" near the bulk nematic-to-smectic temperature is caused by Casimir forces due to fluctuations associated with the second-order nematic-to-smectic transition.

Play movie of liquid crystalline film on silicon (with average coverage of 30 nm) as the temperature is increased through the coexistence region shown above left.

In collaboration with Masa Fukuto and Ben Ocko at Brookhaven National Lab, we are extending our measurements to better understand the nature of the phases in different parts of the phase diagram for this system, which may provide important clues as to the cause of the phenomenon. In addition to pursuing more detailed, higher-precision measurement of the shape of the coexistence region boundaries, we are also studying the effect of disorder on the coexistence region.

Liquid Crystal Films: the effect of disorder in 2D

The rapid progress towards minuturization of physical systems in chemistry and electronics, combined with exaggerated importance of surfaces in systems of reduced dimension and the near ubiquity of disorder in physical systems, underscores the urgent need to understand the effect of disorder in quasi-2D physical systems. For example, dust on a surface is one of the main reasons why computer chips are lost upon manufacture.

Our experiments studying thin liquid crystal films mentioned above have revealed that thin films of liquid crystal 8CB on silicon exhibit a novel, two-state coexistence region, in principle describable in the framework of the two dimensional Ising model, which is arguably one of the simplest two-dimensional systems available to experiment. We have also found that, in sufficient concentration, quenched random disorder, in the form of random dust particles distributed on the surface, has a very large effect on the phase transition, making this an ideal system for measuring the effect of quenched random disorder in a quasi-2D system.

Shown in the figure above is the thin-thick coexistence region in 8CB in the presence of large amounts of dust. Although the dust particles themselves are invisible to the naked eye, they are made visible by the changes induced in the surrounding liquid crystal film. We observe that the thin-thick coexistence phenomenon continues even in the presence of dust, as the famous theorist Harris predicted, although its width is dimininished and it is shifted to lower temperatures.

At WPI we are now working to more precisely measure the behavior of this two-state coexistence region as a function of surface disorder characterized by the areal concentration of artifically-introduced aerosil particles. We will attempt to discern whether indeed the shift in the phenomenon obeys the theoretically-predicted dependence on disorder.