Making Ultra-Hydrophobic Textured Silicone-Rubber Surfaces

Breath figures are utilized to form microtextured surfaces.

December 1, 2007

Army Research Laboratory, Adelphi, Maryland

Silicone-rubber surfaces microscopically textured in such a manner as to render them ultra-hydrophobic have been fabricated by a method in which breath figures are utilized. Originally, “breath figures” signified fog-like patches formed by condensation of microscopic droplets of water from air exhaled onto cooler surfaces. Now, “breath figures” refers more generally to patches formed by condensation, whether from natural breath or artificial sources. The essence of the method is to use a breath figure to form a pattern of microscopic, approximately hemispherical pits (each pit corresponding to a condensed water droplet) on the surface of a layer of polystyrene, then use the pitted polystyrene surface as a template to cast the silicone rubber having a surface pattern of nanometer- or micron-sized pillars corresponding to the pits.

Figure 1. A Breath Figure Is Preserved as a pattern of pits in the surface of a polymer cast from solution.

Figure 2. Approximately Hemispherical Pillars in a nearly regular pattern having characteristic dimensions of the order of microns were formed by casting silicone rubber in on a polystyrene template that had been fabricated in the process depicted in Figure 1.

The formation of ordered arrays of approximately hemispherical pits on surfaces of polymer films is of interest because of potential applications in the preparation of photonic-bandgap materials, sensors, and patterned light-emitting diodes. While many methods for formation of such arrays of pits are known, the breath-figure method has been a subject of scrutiny because it exploits a simple and robust pattern-formation mechanism. The breath-figure method is implemented in a process in which a polymer film is cast from solution in humid air. The evaporation of the solvent is accompanied by a decrease in temperature at the air/solution interface, resulting in condensation of water.

The condensation proceeds in stages (see Figure 1). First, water droplets nucleate on the surface of the solution. After nucleation, droplets coalesce and grow in such a manner as to minimize interfacial energy differences, resulting in formation of a pattern. Under properly controlled conditions, the pattern of droplets evolves into an approximation of a regular hexagonal lattice. The difference in temperature between the surface of the solution and the ambient humid air is minimized once the surface is covered with water droplets. At this stage, the water droplets sink into the solution (provided that the solution has been properly formulated so that its mass density at this stage is less than that of water). Upon completion of evaporation of the solvent, the pattern of the droplets is preserved as an approximately regular hexagonal array of pits, resembling a honeycomb. The sizes and the degree of uniformity or non-uniformity of the pits can be tailored, to some extent, through choice of such process variables as the relative humidity, the polymer, the solvent, and the proportions of polymer and solvent in the solution.

In one of several experiments, the breath-figure method was used to form a template comprising an approximately regular hexagonal array of pits on the surface of a layer of polystyrene cast from solution in CH2Cl2. A silicone rubber was cast on the template, then peeled off the template, revealing a pattern of pillars (see Figure 2). Contact-angle measurements have demonstrated significant enhancement of the hydrophobicity of silicone-rubber surfaces thus microtextured with pillars: for example, in one case, a smooth silicone-rubber surface exhibited advancing- and receding-side contact angles of 100º and 90º, respectively, while the microtextured version exhibited advancing- and receding-side contact angles of 135º and 127º, respectively.

This work was done by Adam M. Rawlett, Joshua A. Orlicki, and Nicole Zander of the Army Research Laboratory and Afia Karikari, and Tim Long of Virginia Institute of Technology.

ARL-0020