Over the past decade, the most common approach to creating liquid shedding surfaces has been to amplify the effects of nonwetting surface chemistry, using micro/ nanotexturing to create superhydrophobic and superoleophobic surfaces. Recently, an alternative approach using impregnation of micro/nanotextured surfaces with immiscible lubricating liquids to create slippery liquid-infused porous surfaces (SLIPS) has been developed. These types of surfaces open up new opportunities to study the mechanism of evaporation of sessile droplets in zero contact angle hysteresis situations where the contact line is completely mobile. In this study, we fabricated surfaces consisting of square pillars (10−90 μm) of SU-8 photoresist arranged in square lattice patterns with the center-to-center separation between pillars of 100 μm, on which a hydrophobic coating was deposited and the textures impregnated by a lubricating silicone oil. These surfaces showed generally low sliding angles of 1°or less for small droplets of water. Droplet profiles were more complicated than on nonimpregnated surfaces and displayed a spherical cap shape modified by a wetting ridge close to the contact line due to balancing the interfacial forces at the line of contact between the droplet, the lubricant liquid and air (represented by a Neumann triangle). The wetting ridge leads to the concept of a wetting "skirt" of lubricant around the base of the droplet. For the SLIP surfaces, we found that the evaporation of small sessile droplets (∼2 mm in diameter) followed an ideal constant contact angle mode where the apparent contact angle was defined from the intersection of the substrate profile with the droplet spherical cap profile. A theoretical model based on diffusion controlled evaporation was able to predict a linear dependence in time for the square of the apparent contact radius. The experimental data was in excellent quantitative agreement with the theory and enabled estimates of the diffusion constant to be obtained.

The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid and vapour phases. Here we show that liquid dielectrophoresis (L-DEP) induced by non-uniform electric fields can be used to enhance and control the wetting of dielectric liquids. In the limit of thick droplets, we show theoretically that the cosine of the contact angle follows a simple voltage squared relationship analogous to that found for electrowetting-on-dielectric (EWOD). Experimental observations confirm this predicted dielectrowetting behavior and show that the induced wetting is reversible. Our findings provide a non-contact electrical actuation process for meniscus and droplet control.

There has been intense recent interest in photonic devices based on microfluidics that include displays [1,2] and refractive tunable microlenses and optical beamsteerers [3][4][5] that work using the principle of electrowetting [6,7]. Here we report a novel approach to optical devices in which static wrinkles are produced at the surface of a thin film of oil as a result of dielectrophoretic forces [8][9][10]. We have demonstrated this voltage programmable surface wrinkling effect with periodic devices with pitch lengths of between 20 µm and 240 µm and with response times of less than 40 µs. By careful choice of oils, it is possible to optimise either for high amplitude sinusoidal wrinkles at micrometer-scale pitches or for more complex non-sinusoidal profiles with higher Fourier components at the longer pitches. This provides the possibility for rapidly responsive voltage programmable polarisation insensitive transmission and reflection diffraction devices and for arbitrary surface profile optical devices. * E-mail: carl.brown@ntu.ac.uk, Tel: 44 115 8483184 2 The structure of the device is shown in figure 1. The side view, in figure 1(a), shows the glass substrate coated with patterned gold/titanium conducting electrodes, on which there is a thin solid dielectric layer (either photoresist or a dielectric stack), upon which is coated a thin layer of oil. The electrodes were arranged as an array of stripes parallel to the y direction in the xy plane. This geometry allowed every other electrode to be electrically connected as shown in the plan view in figure 1(b).The electrically induced wrinkling at the oil surface will be considered first for a device with an electrode pitch of p = 80 µm. When a small volume (0.1 µL) of 1-decanol was initially dispensed onto the device it formed a spherical cap with a contact angle of 5°. Every other stripe in the electrode array was biased with an A.C. voltage with r.m.s. magnitude V O and the inter-digitated stripes between them were earthed, as shown in figure 1. This creates a periodic electric field profile in the plane of the oil layer which is highly non-uniform. A polarisable dielectric material in a region containing non-uniform electric fields experiences a force, known as a dielectrophoretic force, in the direction of the increase in magnitude of the electric field [8][9][10]. When the r.m.s. electrode voltage was greater than V O = 20 Volts the dielectrophoretic forces spread the oil into a thin film with a uniform thickness, h = 12 µm, across the area covered by the electrodes.Increasing the voltage between neighbouring electrodes gave rise to a periodic undulation at the surface of the oil. The period of the wrinkle was equal to the electrode pitch, 80 µm, and the peaks and troughs of the wrinkle lay parallel to the electrode fingers along the y-direction. This undulation arises because the highest electric field gradients occur in the gaps between the electrodes and so the dielectrophoretic forces in these regions cause the oil to collect there preferentially. The...

A fundamental limitation of liquids on many surfaces is their contact line pinning. This limitation can be overcome by infusing a nonvolatile and immiscible liquid or lubricant into the texture or roughness created in or applied onto the solid substrate so that the liquid of interest no longer directly contacts the underlying surface. Such slippery liquid-infused porous surfaces (SLIPS), also known as lubricant-impregnated surfaces, completely remove contact line pinning and contact angle hysteresis. However, although a sessile droplet may rest on such a surface, its contact angle can be only an apparent contact angle because its contact is now with a second liquid and not a solid. Close to the solid, the droplet has a wetting ridge with a force balance of the liquid–liquid and liquid–vapor interfacial tensions described by Neumann’s triangle rather than Young’s law. Here, we show how, provided the lubricant coating is thin and the wetting ridge is small, a surface free energy approach can be used to obtain an apparent contact angle equation analogous to Young’s law using interfacial tensions for the lubricant–vapor and liquid–lubricant and an effective interfacial tension for the combined liquid–lubricant–vapor interfaces. This effective interfacial tension is the sum of the liquid–lubricant and the lubricant–vapor interfacial tensions or the liquid–vapor interfacial tension for a positive and negative spreading power of the lubricant on the liquid, respectively. Using this approach, we then show how Cassie–Baxter, Wenzel, hemiwicking, and other equations for rough, textured or complex geometry surfaces and for electrowetting and dielectrowetting can be used with the Young’s law contact angle replaced by the apparent contact angle from the equivalent smooth lubricant-impregnated surface. The resulting equations are consistent with the literature data. These results enable equilibrium contact angle theory for sessile droplets on surfaces to be used widely for surfaces that retain a thin and conformal SLIPS coating.

Contact-line pinning is a fundamental limitation to the motion of contact lines of liquids on solid surfaces. When a sessile droplet evaporates, contact-line pinning typically results in either a stick–slip evaporation mode, where the contact line pins and depins from the surface in an uncontrolled manner, or a constant contact-area mode with a pinned contact line. Pinning prevents the observation of the quasi-equilibrium constant contact-angle mode of evaporation, which has never been observed for sessile droplets of water directly resting on a smooth, nontextured, solid surface. Here, we report the evaporation of a sessile droplet from a flat glass substrate treated with a smooth, slippery, omni-phobic covalently attached liquid-like coating. Our characterization of the surfaces shows high contact line mobility with an extremely low contact-angle hysteresis of ∼1° and reveals a step change in the value of the contact angle from 101° to 105° between a relative humidity (RH) of 30 and 40%, in a manner reminiscent of the transition observed in a type V adsorption isotherm. We observe the evaporation of small sessile droplets in a chamber held at a constant temperature, T = (25.0 ± 0.1) °C and at constant RH across the range RH = 10–70%. In all cases, a constant contact-angle mode of evaporation is observed for most of the evaporation time. Furthermore, we analyze the evaporation sequences using the Picknett and Bexon ideal constant contact-angle mode for diffusion-limited evaporation. The resulting estimate for the diffusion coefficient, D E, of water vapor in air of D E = (2.44 ± 0.48) × 10–5 m2 s–1 is accurate to within 2% of the value reported in the literature, thus validating the constant contact-angle mode of the diffusion-limited evaporation model.