Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high-speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie-Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low-friction droplet transport.

like tunable non-wettability and responsive properties, which ultimately can lead to new avenues in applications and functionalities in microfluidic devices and cell handling.Thiol-ene chemistry has emerged recently as a viable route toward polymer microfabrication due to its compatibility with existing microfabrication techniques and minimal requirement for investment. [2] Thiol-ene refers to the UV-initiated click chemistry reaction that occurs between thiol and allyl groups in a mixture containing two chemical components in its simplest form, one with thiol groups and the other with allyl groups mixed together in liquid form at stoichiometric or offstoichiometric ratio. It offers remarkable properties like fast curing where 2 mm thick thiol-ene layer is fully cured by UV dose of approximately 100 J m −2[3] and the ability to tune the mechanical properties of the material from low Young's modulus value of less than 11 MPa [4] to high Young's modulus value of 1750 MPa [2a] by controlling the crosslinking density. Furthermore, rapid and direct surface modification for adjusting the surface energy can be achieved with simple post-functionalization of hydrophobic groups. [2] Thiol-ene is therefore a promising material for polymer-based functional materials including micropatterned responsive surfaces.Magnetically responsive micropatterned surfaces actuated by external non-invasive stimuli allow instantaneous tuning of surface properties such as adhesion, [5] structural coloration, [6] and surface wettability. [7] Notable applications of magnetic pillars that have been reported include tactile force sensing, [8] liquid flow sensing, [9] mixing in microfluidic devices, [10] mixing of droplets, [11] switchable adhesive surfaces, [5a] cell analysis, [12] directional wetting, [13] real-time manipulation of light, [6] micromanipulation of beads, [14] and droplet transport. [15] The fabrication schemes in the reported cases involve either magnetic particle self-assembly under magnetic field or replica molding with polydimethylsiloxane (PDMS) as the common soft elastomeric material. Thiol-ene based materials have not been explored yet for responsive superhydrophobic surfaces.Here, we develop magnetic thiol-ene micropillar arrays that demonstrate fast responsiveness, suitable for droplet transport applications. The advantage of using thiol-ene is the tunability of elastic and surface properties. The elastic properties of the thiol-ene pillars are tuned by controlling the crosslinking Tunable and responsive surfaces offer routes to multiple functionalities ranging from superhydrophobic surfaces to controlled adhesion. Inspired by cilia structure in the respiratory pathway, magnetically responsive periodic arrays of flexible and magnetic thiol-ene micropillars are fabricated. Omnidirectional collective bending of the pillar array in magnetic field is shown. Local non-contact actuation of a single pillar is achieved using an electromagnetic needle to probe the responsiveness and the elastic properties of the pillars by comp...

Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm–550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.

The production of nanostructured plastic items by injection molding with ridges down to 400 nm in width, which is the smallest line width replicated from nanostructured steel shims, is presented. Here we detail a micro-fabrication method where electron beam lithography, nano-imprint lithography and ion beam etching are combined to nanostructure the planar surface of a steel wafer. Injection molded plastic parts with enhanced surface properties, like anti-reflective, superhydrophobic and structural colors can be achieved by micro- and nanostructuring the surface of the steel molds. We investigate the minimum line width that can be realized by our fabrication method and the influence of etching angle on the structure profile during the ion beam etching process. Trenches down to 400 nm in width have been successfully fabricated into a 316 type electro-polished steel wafer. Afterward a plastic replica has been produced by injection molding with good structure transfer fidelity. Thus we have demonstrated that by utilizing well-established fabrication techniques, nanostructured steel shims that are used in injection molding, a technique that allows low cost mass fabrication of plastic items, are produced.

No abstract