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1. EWOD-aided droplet transport on texture ratchets

   https://doi.org/10.1063/1.5142571  

   Sun, Di; Böhringer, Karl F. (March 2020, Applied Physics Letters)

   We report a digital microfluidic device to transport aqueous droplets on an open surface in air using electrowetting-on-dielectric (EWOD) with anisotropic ratchet conveyors (ARCs). ARCs are micro-sized periodic semicircular hydrophilic regions on a hydrophobic background, providing anisotropic wettability. SiNx and Cytop are used as the dielectric layer between the water droplet and working electrodes. By adopting parylene as a stencil mask, hydrophilic patterning on the hydrophobic Cytop thin film layer is achieved without the loss of Cytop hydrophobicity. While the traditional EWOD platform requires the control of multiple electrodes to transport the droplet, our system utilizes only two controlling electrodes. We demonstrate that 15 μl water droplets are transported at a speed of 13 mm/s under 60 Vpeak sinusoid AC signal at 50 Hz. Droplet transport at 20 Hz is also presented, demonstrating that the system can operate within a range of frequencies. 

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2. Enhanced self-cleaning performance on hydrophobic glass surfaces using hydrophilic dagger features coated with silica nanoparticles

   https://doi.org/10.1016/j.solmat.2024.113366  

   Abeywardena, Madupa; Zimmermann, Thomas; Xu, QianFeng; Löbmann, Peer; Mandel, Karl; Stauch, Claudia; Lyons, Alan (April 2025, Solar Energy Materials and Solar Cells)

   Accumulated dust on solar cover glass reduces transmittance, leading to decreased energy efficiency of photovoltaic (PV) modules. Hydrophobic coatings on solar cover glass have been shown to provide anti-soiling properties when exposed to a condensing environment (e.g. dew). The addition of hydrophilic features along the top edge of the hydrophobic coated glass enhances condensation rates and can be used to achieve self-cleaning of the surfaces. However, to date, relatively long times have been required to clean the surfaces. In this study, we developed a new design for hydrophilic features that reduce the time required to clean the surface in laboratory tests as measured by laser scanning microscopy, optical photographs and UV–vis spectroscopy. The dagger-shaped features improve self-cleaning performance by a combination of three factors: a silica nanoparticle (NP) hydrophilic coating which enhances condensation rate due to a low water contact angle (WCA) and nano-scale porosity; the stepwise transition from the low WCA silica NP region to the high WCA silanized hydrophobic region via a bare glass transition zone; and the pointed shape of the hydrophobic dagger features which further minimizes the barrier for transport of droplets from the condensing region to the high-mobility, hydrophobic, region of the surface. The hydrophilic silica nanoparticle-coated dagger features not only improve the self-cleaning efficiency of the hydrophobic surfaces but also increase the overall amount of water harvested. Such coating designs provide an effective approach to reducing maintenance costs as well as increasing the overall energy output of PV panels. 

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3. Directional self-locomotion of active droplets enabled by nematic environment

   https://doi.org/10.1038/s41567-020-01055-5  

   Rajabi, Mojtaba; Baza, Hend; Turiv, Taras; Lavrentovich, Oleg D. (October 2020, Nature Physics)

   null (Ed.)

   Active matter composed of self-propelled interacting units holds a major promise for the extraction of useful work from its seemingly chaotic dynamics. Streamlining active matter is especially important at the microscale, where the viscous forces prevail over inertia and transport requires a non-reciprocal motion. Here we report that microscopic active droplets representing aqueous dispersions of swimming bacteria Bacillus subtilis become unidirectionally motile when placed in an inactive nematic liquid-crystal medium. Random motion of bacteria inside the droplet is rectified into a directional self-locomotion of the droplet by the polar director structure that the droplet creates in the surrounding nematic through anisotropic molecular interactions at its surface. Droplets without active swimmers show no net displacement. The trajectory of the active droplet can be predesigned by patterning the molecular orientation of the nematic. The effect demonstrates that broken spatial symmetry of the medium can be the reason for and the means to control directional microscale transport. 

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4. Controlling flows of an intra-droplet active fluid across droplet interface

   Jolicoeur, Brock; Chen, Yen-Chen; Chueh, Chih-Che; Wu, Kun-Ta (January 2021, Bulletin of the American Physical Society)

   Fluid dynamics of conventional passive fluid are known to be affected by boundary condition. For example, flow rates in a pipe depend on slipperiness of pipe surface. Similarly, active fluid, which consumes fuels locally to flow spontaneously, was reported to self-flow along a meter-long tubing with the flow rate depending on tubing geometry. However, how boundary condition influences fluid dynamics in an active fluid system remains poorly understood. Here, we investigated how a fluid boundary influenced self-organization of confined active fluid by establishing a 3D COMSOL-based nemato-hydrodynamic simulation platform where active fluid was confined in a compressed cylindrical water-in-oil droplet. Since the droplet interface was fluid, the fluid dynamics within and outside the droplet were coupled. Our simulation demonstrated that flow behaviors of intra-droplet active fluid were influenced by the amount of oil that surrounded the droplet: Without altering the droplet geometry, expanding the volume of oil could induce a circulatory flow within the droplet, which resembled our experimental observation. Our work suggested the feasibility of controlling the fluid dynamics of a confined active fluid system across a fluid interface. 

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5. Tunable spontaneous circulation of microtubule-based active fluid confined in a compressed water-in-oil droplet using milli-fluidic devices

   Chen, Yen-Chen; Jolicoeur, Brock; Chueh, Chih-Che; Wu, Kun-Ta (January 2021, Bulletin of the American Physical Society)

   Active matter consumes local fuels to self-propel. When confined in a closed circular boundary, they can self-organize into a circulatory flow. Such coherence originates from the interactions between the active matter and boundaries, and boundary conditions play an important role on self-organization of active fluid. Herein, we probed how fluid boundaries influenced the self-organization of active fluid. The fluid boundaries were created by confining the active fluid in a compressed water-in-oil droplet. Due to surface tension, the droplet shaped into a cylinder-like geometry. Since water and oil were both fluids, their interface was fluid. We systematically probed how droplet shapes and the amount of oil surrounding the droplet influenced the development of circulation. We found that the formation of circulatory flows depended on the thickness of the oil layer surrounding the droplet, implying that the fluid dynamics between the active fluid within the droplet and the oil outside the droplet were coupled. We used a 3D COMSOL-based simulation successfully reproduced such oil-layer dependence. Finally, we developed two milli-fluidic devices to deform the droplet and alter the oil layer thickness manually to trigger and suppress the intra-droplet circulatory flow in real time. 

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