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 A 0 [ R 2 , where A 0 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 r c , 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 A 0 should be small and R (respectively the neck diameter) should be large, whereas a large A 0 and a small R imply low contact angle hysteresis and high slip length.

We theoretically investigate the apparent contact angle and contact angle hysteresis of a droplet placed on a liquid infused surface. We show that the apparent contact angle is not uniquely defined by material parameters, but also has a dependence on the relative size between the droplet and its surrounding wetting ridge formed by the infusing liquid. We derive a closed form expression for the contact angle in the limit of vanishing wetting ridge, and compute the correction for small but finite ridge, which corresponds to an effective line tension term. We also predict contact angle hysteresis on liquid infused surfaces generated by the pinning of the contact lines by the surface corrugations. Our analytical expressions for both the apparent contact angle and contact angle hysteresis can be interpreted as 'weighted sums' between the contact angles of the infusing liquid relative to the droplet and surrounding gas phases, where the weighting coefficients are given by ratios of the fluid surface tensions.

Characterization of the wetting properties is a prerequisite for a fundamental understanding and the targeted development of superhydrophobic and superamphiphobic layers. To fabricate super liquid-repellent layers, two requirements need to be met: The surfaces have to be of low energy and their nano-and microstructure needs to be designed in a way that leads to the entrapment of air. The challenge is to design and produce suitable nano-and microstructures to control wetting. Here we describe important methods to quantify wetting properties of super liquid-repellent layers. These properties include the apparent advancing and receding contact angles, the roll-off angle, tensile and lateral adhesion, the impalement pressure, and the observation of drop impact. The most important one is the apparent receding contact angle because it also limits lateral adhesion. The link of these properties to the nano-and microscopic structure of the layer is discussed. Limits, problems, and future challenges are pointed out.

No abstract