(161f) Droplet Locomotion over Oil Immersed Superhydrophobic Surfaces

Microfluidic devices provide a technology for engineering and manipulating droplets at the micro- and nano-scales and open up new avenues for research. In particular, nano- and picoliter volumes of aqueous droplets of reagents and samples are generated in an oil continuous phase on a chip and then maneuvered, merged and divided to undertake chemical diagnostic analyses in dropet microfludics. The droplet microfluidic platform offers the abiity to encapsuate samples into droplet "packets" to prevent cross contamination. Additionally, this format has high surface area to volume ratios, shorter diffusion times and lower reaction volumes thereby enabling precise control over reactions as an alternative to conventional analytical techniques. Apart from these new applications, droplet microfluidic devices present new opportunities to conduct detailed studies of interfacial phenomena such as accurate measurement of slip lengths and ultralow values of interfacial tension which might otherwise be challenging. Devices in droplet microfluidic arrangements are typically fabricated from glass or polydimethyl- siloxane (PDMS), and water droplets immersed in oil drag over these surfaces. This surface movement has distinct disadvantages because such surfaces are not smooth and droplets can stick onto the surface. Sticking interferes with the trafficking of droplets and can cause droplets to break into pieces which can lead to cross contamination of samples.

In this presentation, we study a novel solution in which the water droplets are designed to move along superhydrophobic surfaces which are submerged in the continuous oil phase of the microfluidic channel. A superhydrophobic surface is a substrate which forms a high contact angle( >150⁰) when a water droplet is placed on the surface. Such high contact angles are achieved by tuning the chemistry and the surface roughness simultaneously. The low energy coating renders the surface hydrophobic and the surface roughness enhances the contact angle to achieve superhydrophobicity. High contact angles are obtained because a water droplet placed on such a surface lies on top of the asperities in a "Cassie-Baxter" wetting state leaving a plastron of air between the drop and the surface. Along with a high contact angle, an superhydrophobic surface also possesses low contact angle hysteresis (CAH). Low CAH eliminates pinning of water droplets to a substrate and ensures low adhesion between the substrate and the aqueous drop. When a superhydrophobic surface is submerged in oil, the oil wets into the surface topology. Since oils are known to have a lower surface energy than water, they are retained in the topology to create an oil infused surface. Hence a superhydrophobic surface can, in principle, function optimally and repel water even when immersed in oil. Water droplets easily move over the oil cushion of these surfaces, with reduced adhesion and friction, and importantly reduced contact angle hysteresis. A number of investigations have examined the movement of water droplets over superhydrophobic surfaces in air, in which case the droplets move over an air cushion. In addition, the movement of water droplets over superhydrophobic surfaces situated in air and infused with an oil layer have been studied. But there is very little understanding about the physics of the motion of water droplets over superhydrophobic surfaces in which the droplet and surface are completely immersed in a continuous oil phase. Our aim is to measure the mobility of the drops on the oil infused surface, and obtain a slip coefficient for water moving over an oil infused superhydrophobic surface by simulating the droplet hydrodynamics. Dielectrophoretic forces are used to actuate motion of droplet pairs over the surface.

Superhydrophobic surfaces are first fabricated by dip-coating glass slides into a nano particle polymer composite solution. The polymeric resin binds the nano particles to the substrate and to each other and creates a resilient surface topology. The superhydrophobic substrates are subsequently incorporated into the microuidic devices as the bottom wall and flow focusing is used to form a train of monodisperse droplets of water in a mineral oil. After attaining steady state, the flow is stopped allowing the initially suspended droplets to gravitationally settle to the bottom and equilibrate. An alternating current electric field is then applied by insertion of copper electrodes across the channel. The applied electric field induces surface charges of opposite signs on droplet pairs. This generates a dielectrophoretic force between the drops which eventually leads to their coalescence. The droplet motion and merging was controlled by adjusting the electric field. The trajectories of the merging droplet pairs were recorded with a high speed camera. With the use of image visualization and droplet tracking, the distance between the droplets was measured as a function of time. Comparison with control experiments (microfluidic device with a plain glass bottom) provides substantial evidence about the superior mobility of water drops on superhydrophobic surfaces. The slip coeffcient is obtained from the experimental trajectories by simulating the hydrodynamic motion of the droplets towards each other under inuence of the electrophoretic driving force and comparing the trajectories from this motion to the experimental trajectories. The measured values of the slip length of water moving over the oil infused surfaces are shown to be two orders of magnitude larger than over control surfaces of glass. Finite element simulations were undertaken in the context of computing the hydrodynamic drag and the di-electrophoretic forces. Since the droplets under consideration are micron sized, the shapes are approximated as spherical caps of dierent dimensions in the limit of a low Bond number. The computed values are hydrodynamic drag on a single drop are reported as a function of the contact angle and the slip length while the dielectrophoretic force is evaluated numerically for a droplet pair. An approximation for the viscous drag due to the approach of the identical droplets is formulated as the product of the isolated droplet drag multiplied by the resistance of two approaching spheres in an innite medium (Jerey-Stimson formula). The finite element simulations are validated by comparison with well asymptotic expressions (Davis force) in the limit of a completely non-wetting droplet. The measured values of drag and slip length are found to be in agreement with the corresponding calculated values.