Hierarchically structured assembly of polymer microsieves, made by a combination of phase separation micromolding and float-casting.

Phase separation micromolding and float-casting are combined to prepare hierarchically structured microsieves.

Experimental investigation of particle-assisted wetting.

In principle, one might prepare thin polymer membranes in an easy and economic way by generating a wetting layer of a polymerizable oil on a water surface, solidifying it, and transferring it to a solid substrate. However, there is hardly any oil that wets a water surface. One efficient way to assist the wetting of the water surface by an oil is to mix the oil with suitable particles. Theoretically, one expects several scenarios, which depend on the contact angles of the particles with the liquid interfaces. On the basis of these calculations, one can draw phase diagrams of particle-assisted wetting. In the current paper, we verify such a phase diagram of particle-assisted wetting experimentally.

Formation of a freely suspended membrane via a combination of interfacial reaction and wetting.

Applying poly(ethoxysiloxane) (a liquid non-water-soluble polymer that can be hydrolyzed and cross-linked by diluted acids) to an air/pH 1 water interface gave rise to thin homogeneous solid layers. These layers were strong enough to be transferable to electron microscopy grids with holes of dimensions up to 150 microm and covered the holes as freely suspended membranes. No homogeneous layers were formed at an air/pH 5 water interface. Brewster angle microscopy images show that the poly(ethoxysiloxane) is not spontaneously forming a wetting layer on water. It initially forms lenses, which slowly spread out within several hours. We conclude that the spreading occurs simultaneously with the hydrolysis and cross-linking of the poly(ethoxysiloxane) and that the reaction products finally assist the complete wetting of the water surface.

Influence of particle hydrophobicity on particle-assisted wetting.

The surface properties of silica particles significantly influence their efficiency in particle-assisted wetting. A series of small particles of controlled surface hydrophobicity was mixed with a nonvolatile oil. These mixtures were applied onto a water surface; the structures formed were subsequently solidified by photopolymerization and observed using scanning electron microscopy. For the most hydrophilic particles, only lenses of pure oil formed, with the particles being submerged into the aqueous phase. The most hydrophobic particles help to form patches of stable homogeneous mixed layers composed of oil and particles. In these cases the particles adhere to the air-oil as well as to the oil-water interfaces. For particles with intermediate hydrophobicity, lenses and patches of mixed layers were observed. These three different observations verify that the hydrophobicity of the particle surface determines the wetting behavior of the oil at the water surface.