Electrowetting of water drops on structured superhydrophobic surfaces are known to cause an irreversible change from a slippy (Cassie-Baxter) to sticky (Wenzel) regime. An alternative approach to using a water drop on a super-hydrophobic surface to obtain a non-wetting system is to use a liquid marble on a smooth solid substrate. A liquid marble is a droplet coated in hydrophobic grains, which therefore carries its own solid surface structure as a conformal coating. Such droplets can be considered as perfect non-wetting systems having contact angles to smooth solid substrates of close to 180 o . In this work we report the electrowetting of liquid marbles made of water coated with hydrophobic Lycopodium grains and show that the electrowetting is completely reversible. Marbles are shown to return to their initial contact angle for both ac and dc electrowetting and without requiring a threshold voltage to be exceeded. Furthermore, we provide a proof-of-principle demonstration that controlled motion of marbles on a finger electrode structure is possible.

A spherical conducting droplet in an alternating electric field is known to undergo shape oscillations. When the droplet is supported by a substrate, the shape is no longer a complete sphere, but shape resonances are still observed. To obtain a completely spherical droplet, some kind of levitation is needed, unless the droplet is in microgravity, and this has previously been provided by gas films or magnetic or other external forces. In this work, we report observations of shape oscillations of a hydrophobic-powder-coated droplet of water. A droplet of water rolled on a hydrophobic powder self-coats such that the water becomes encapsulated as a liquid marble. When the powder is a spherical hydrophobic grain with a contact angle greater than 90°, it adheres to the solid-water interface with more than half of its diameter projecting from the liquid, thus ensuring the encapsulated water does not come into contact with any substrate. These liquid marbles are highly mobile and can be regarded as completely nonwetting droplets possessing contact angles of 180°. In this work, we show that they also provide a new mechanism equivalent to levitating droplets and provide droplets with small contact areas and completely mobile contact lines for studies of shape oscillations. Liquid marbles were created using hydrophobic lycopodium and droplets of water containing potassium chloride and were excited into motion using an electrowetting-on-dielectric configuration with applied frequency swept from 1 to 250 Hz. Both an up-and-down motion and an oscillation involving multiple nodes were observed and recorded using a high-speed camera. The resonant oscillation modes of small liquid marbles were fitted to the theory for vibrations of a free spherical volume of fluid. This work demonstrates the principle that oscillation modes of completely nonwetting droplets can be studied using a simple powder coating approach without the need for an active mechanism for levitation.

Electrowetting on micro-patterned layers of SU8 photoresist with an amorphous Teflon ® coating has been observed. The cosine of the contact angle is shown to be proportional to the square of the applied voltage for increasing bias.However, this does not apply below 40V and we suggest that this may be explained in terms of penetration of fluid into the pattern of the surface.Assuming that the initial application of a bias voltage converts the drop from Cassie-Baxter to Wenzel regime, we have used this as a technique to estimate the roughness factor of the surface.

Transport of a water droplet on a solid surface can be achieved by differentially modifying the contact angles at either side of the droplet using capacitive charging of the solid-liquid interface (i.e., electrowetting-on-dielectric) to create a driving force. Improved droplet mobility can be achieved by modifying the surface topography to enhance the effects of a hydrophobic surface chemistry and so achieve an almost complete roll-up into a superhydrophobic droplet where the contact angle is greater than 150 degrees . When electrowetting is attempted on such a surface, an electrocapillary pressure arises which causes water penetration into the surface features and an irreversible conversion to a state in which the droplet loses its mobility. Irreversibility occurs because the surface tension of the liquid does not allow the liquid to retract from these fixed surface features on removal of the actuating voltage. In this work, we show that this irreversibility can be overcome by attaching the solid surface features to the liquid surface to create a liquid marble. The solid topographic surface features then become a conformable "skin" on the water droplet both enabling it to become highly mobile and providing a reversible liquid marble-on-solid system for electrowetting. In our system, hydrophobic silica particles and hydrophobic grains of lycopodium are used as the skin. In the region corresponding to the solid-marble contact area, the liquid marble can be viewed as a liquid droplet resting on the attached solid grains (or particles) in a manner similar to a superhydrophobic droplet resting upon posts fixed on a solid substrate. When a marble is placed on a flat solid surface and electrowetting performed it spreads but with the water remaining effectively suspended on the grains as it would if the system were a droplet of water on a surface consisting of solid posts. When the electrowetting voltage is removed, the surface tension of the water droplet causes it to ball up from the surface but carrying with it the conformable skin. A theoretical basis for this electrowetting of a liquid marble is developed using a surface free energy approach.

Translational excitation functions have been determined for production of several MnF* statessb 5 Π, c 5 Σ + , d 5 Π, and (most probably) e 5 Σ + sin the reaction of a laser-ablated beam of Mn atoms with gaseous CF 4 . Although all observed channels show high initial thresholds, ∼200-300 kJ mol -1 , reaction appears to be due to excited Mn atoms rather than the ground state, a 6 S. The reagent species appears to be either the first or third metastable level, a 6 D J or a 4 D J . Analysis of the energy dependences, in terms of a multiple line-ofcenters model [Levy, Res. Chem. Kinet. 1993, 1, 163], indicates that at relatively low energies, a common process is responsible for b 5 Π and c 5 Σ + formation, involving a ∼14% forward shift in reaction transition state as collision energy increases. Quite separate processes, without transition state shifts, lead to production of MnF*(d 5 Π) and of MnF*(e 5 Σ + )/"blue" emission at relatively low energies and to enhanced c 5 Σ + production at high energies. It is possible that enhanced production of MnF*(e 5 Σ + ) and perhaps the d 5 Π state from ∼650-700 kJ mol -1 derives from the depletion of MnF*(b 5 Π, c 5 Σ + ). Despite the undoubted negative CF 4 electron affinity, it seems likely that avoided ionic-covalent curve crossings at least play a role in the b 5 Π/ c 5 Σ + production channel.