Pressure induced transition between superhydrophobic states: configuration diagrams and effect of surface feature size.

The stability of wetting states, namely the Cassie state (partial wetting) and the Wenzel state (complete wetting) of surfaces with protrusions, is determined by comparing the total free energy of a liquid drop in terms of their apparent contact angles for different protrusion features. It is found that when the area fraction of the topographical features and the intrinsic contact angle for a flat surface are large, the Cassie state is favored, but it can be either the metastable or stable state. It is shown that the transition from the Cassie state to the Wenzel state requires the application of a pressure to the meniscus between the surface protrusions. The critical transition pressure increases not only with increasing area fraction and intrinsic contact angle, but also with decreasing protrusion size. During the transition, a high-pressure gas can be trapped around the protrusions that can cause the Cassie state to be recovered after the release of the applied pressure. The analysis shows that a droplet can 'hang' upside-down when the protrusion size is very small; namely, the protrusions can pin the meniscus. These results are discussed relative to the advancing and receding contact angle.

Synthesis of spherical polymer and titania photonic crystallites.

The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 microm in diameter and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.