The spreading of liquid drops on surfaces corrugated with micron-scale parallel grooves is studied both experimentally and numerically. Because of the surface patterning, the typical final drop shape is no longer spherical. The elongation direction can be either parallel or perpendicular to the direction of the grooves, depending on the initial drop conditions. We interpret this result as a consequence of both the anisotropy of the contact line movement over the surface and the difference in the motion of the advancing and receding contact lines. Parallel to the grooves, we find little hysteresis due to the surface patterning and that the average contact angle approximately conforms to Wenzel's law as long as the drop radius is much larger than the typical length scale of the grooves.

Liquid drops on textured surfaces show different dynamical behaviors depending on their wetting states. They are extremely mobile when they are supported by composite solid-liquid-air interfaces (Cassie-Baxter state) and immobile when they fully wet the textured surfaces (Wenzel state). By reversibly switching between these two states, it will be possible to achieve control over the fluid dynamics. Unfortunately, these wetting transitions are usually prevented by surface energy barriers. We demonstrate here a new, simple design paradigm consisting of parallel grooves with an appropriate aspect ratio that allows for the controlled, barrierless, reversible switching of the wetting states upon application of electrowetting. We report a direct observation of the barrierless dynamical pathway for the reversible transitions between the Wenzel (collapsed) and Cassie-Baxter (suspended) states and present a theory that accounts for these transitions, including detailed lattice Boltzmann simulations.

Table of contentWe record contact line motion of ink-jet printed drops spreading and evaporating on surfaces patterned with polygonal microposts and, aided by lattice Boltzmann simulations, explain the drop shapes in terms of interface pinning at posts. AbstractWe present results showing how water drops, produced by ink-jet printing, spread on surfaces patterned with lattices of diamond or triangular posts. Considering post widths typically ~7 m and lattice spacings between 15-40 m, we observe drop shapes with 3,4 and 6-fold symmetry, depending on both the symmetry of the lattice and the shape of the posts. This is a result of the different mechanisms of interface pinning and depinning which depend on the direction of the contact line motion with respect to the post shape. Lattice Boltzmann simulations are used to describe these mechanisms in detail for triangular posts. We also follow the motion of the contact line as the drops evaporate showi ng that they tend to return to their original shape. To explain this we show that the easy direction for movement is the same for sprea ding and drying drops. We compare the behaviour of small drops with that of larger drops created by jetting several drops at the same position. We find that the contact line motion is unexpectedly insensitive to drop volume, even when a spherical cap of fluid forms above the posts. The findings are relevant to m icro-fluidic applications and to the control of drop shapes in i nk-jet printing.

Void formation in one-and two-dimensional complex (dusty) radio frequency (rf) plasmas has been studied using spherical and cylindrically shaped particles. Threshold values for void formation have been determined for plasma power, pressure and number of particles. A comparison between an electropositive argon and an electronegative oxygen plasma shows that void formation in electronegative plasmas is hindered because of a better radial confinement of the particle cloud. No significant dependence on the particle shape (spherical versus cylindrical) has been observed. Comparing one-and two-dimensional structures shows that collective particle behaviour is not responsible for void formation, but possibly does contribute to cloud confinement.