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The Leidenfrost effect allows droplets to be transported on a virtually frictionless layer of vapor above a superheated substrate. The substrates are normally topographically structured using subtractive techniques to produce saw-tooth, herringbone, and other patterns and bulk heated, leading to significant challenges in energy consumption and controlled operation. Here, we propose a planar lithographic approach to levitate and propel droplets using temperature profiles, which can be spatially patterned and controlled in time. We show that micro-patterned electrodes can be heated and provide control of the pressure profile and the vapor flow. Using these almost featureless planar substrates, we achieve self-directed motion of droplets, with velocities of approximately 30 mms−1, without topographically structuring the substrate or introducing physical walls. Our approach has the potential to be integrated into applications, such as digital microfluidics, where frictionless and contactless droplet transport may be advantageous. [ABSTRACT FROM AUTHOR]

Stimuli responsive elastic instabilities provide opportunities for controlling the structures and properties of polymer surfaces, offering a range of potential applications. Here, a surface actuator based on a temperature and electrically responsive poly( N-isopropyl acrylamide-co-sodium acrylate) hydrogel that undergoes a two-step delamination and buckling instability triggered using micropatterned electrodes is described. The electrically actuated structures entail large out-of-plane displacements that take place on time-scales of less than 1 s, in response to modest triggering voltages (−3-6 V). Alongside these experimental observations, finite element simulations are conducted to better understand the two-step nature of the instability. In the first step, hydrogel films undergo delamination and formation of blisters, facilitated by electrochemical reduction of the thiol groups anchoring the film to the electrodes. Subsequently, at larger reducing potentials, the electrolytic current is sufficient to nucleate a gas bubble between the electrode and the gel, causing the delaminated region to adopt a straight-sided blister shape. Finally, thermally induced deswelling of the gel allows the film to be returned to its flat state and readhered to the electrode, thereby allowing for repeated actuation. [ABSTRACT FROM AUTHOR]

On surfaces well above 100 °C water does not simply boil away. When there is a sufficient heat transfer between the solid and the liquid a continuous vapour layer instantaneous forms under a droplet of water and the drop sits on a cushion of vapour, highly mobile and insulated from the solid surface. This is known as the Leidenfrost effect and the temperature at which this occurs if known as the Leidenfrost transition temperature. In this report, an investigation of discontinuous surfaces, stainless steel meshes, have been tested to determine the effect of the woven material on the Leidenfrost phenomenon. It was found that with increasing the open area of the mesh pushes up the Leidenfrost temperature from 265 °C for an open area of 0.004 mm 2 to 315 °C for open area of 0.100 mm 2 . This allows suppression of the Leidenfrost effect as it can be increase to over 300 °C from 185 °C for a stainless steel surface. [ABSTRACT FROM AUTHOR]