Periklis Papadopoulos

A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar's circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water-air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.

For a liquid droplet to slide down a solid planar surface, the surface usually has to be tilted above a critical angle of approximately 10°. By contrast, droplets of nearly any liquid "slip" on lubricant-infused textured surfaces - so termed slippery surfaces - when tilted by only a few degrees. The mechanism of how the lubricant alters the static and dynamic properties of the drop remains elusive because the drop-lubricant interface is hidden. Here, we image the shape of drops on lubricant-infused surfaces by laser scanning confocal microscopy. The contact angle of the drop-lubricant interface with the substrate exceeds 140°, although macroscopic contour images suggest angles as low as 60°. Confocal microscopy of moving drops reveals fundamentally different processes at the front and rear. Drops recede via discrete depinning events from surface protrusions at a defined receding contact angle, whereas the advancing contact angle is 180°. Drops slide easily, as the apparent contact angles with the substrate are high and the drop-lubricant interfacial tension is typically lower than the drop-air interfacial tension. Slippery surfaces resemble superhydrophobic surfaces with two main differences: drops on a slippery surface are surrounded by a wetting ridge of adjustable height and the air underneath the drop in the case of a superhydrophobic surface is replaced by lubricant in the case of a slippery surface.

To predict the properties of superamphiphobic layers we analyzed the wetting of a square and a hexagonal array of vertical pillars composed of spheres (radius R) partially sintered together. Apparent contact angles above 150° are obtained by pinning of a non-polar liquid surface at the underside of the top sphere resulting in a Fakir or Cassie state. Analytical equations are derived for the impalement pressure in the limiting case A0 ≫ R2, where A0 is the area of the regular unit cell containing a single pillar. The case of close pillars is investigated numerically. By balancing forces at the rim of a drop, we calculate the apparent receding contact angle. To describe drag reduction of a flowing liquid we calculate the apparent slip length. When considering pressure-induced flow through cylindrical capillaries of radius rc, significant drag reduction occurs only for thin capillaries. The mechanical stability with respect to normal forces and shear is analyzed. Nanoscopic silica glass pillars would be able to sustain the normal and shear stresses caused by capillary and drag forces. For a high impalement pressure and good mechanical stability A0 should be small and R (respectively the neck diameter) should be large, whereas a large A0 and a small R imply low contact angle hysteresis and high slip length.

The structure and the associated dynamics have been investigated in a series of oligopeptides of gamma-benzyl-l-glutamate using DSC, WAXS, FTIR, NMR and dielectric spectroscopy, and rheology, respectively. The peptides with degrees of polymerization below 18 are mixtures of a lamellar assembly of beta sheets and of columnar hexagonal arrangement of alpha helices, whereas for longer chains, the intramolecular hydrogen bonds stabilize only the alpha-helical conformations. Multiple dielectrically active processes were found. Starting from low temperatures, the two Arrhenius processes (gamma and beta), with apparent activation energies of 20.6 and 50.2 kJ/mol, respectively, associate with the local relaxation of the side-chain methylene units (gamma process) and with more cooperative motions of the side chain dipoles sensitive to the 7/2 helical packing (beta process). The glass transition is manifested in the thermal properties with a step in the heat capacity and with an intense dielectric process bearing characteristics (molecular weight dependence, temperature dependence of relaxation times) known from amorphous polymers. Based on these findings, the alpha process is attributed to the relaxation of amorphous segments located between and at the end of helically ordered segments. Two slower processes were identified with opposite molecular weight dependence. The weak intermediate mode with an M2 molecular weight dependence of the characteristic relaxation times suggests amorphous-like chains, whereas the strong slower process originates from the loss of dipole orientational capacity caused by structural defects and reflects the migration of helical sequences along the chains. This identifies the helices as structures extending over rather short fragments of chains (i.e., of low persistence length). The viscoelastic response indicated that the structural defects arise from locally aggregated chains that inhibit the flow of oligopeptides.