Photovoltaic Cycling to-and-fro Actuation of a Water-Microdroplet for Automatic Repeatable Solute Acquisition on Oil-Infused Hydrophobic LN:Fe Surface

Acquiring solute from sources without external aids is a challenging manipulation of water microdroplets for developing LN-based biological lab-on-chips. Herein, a cycling to-and-fro actuation of a water microdroplet stimulated by a laser illumination, with the cycling period controllable by the illumination power, is demonstrated on an oil-infused hydrophobic LN:Fe surface. This cycling to-and-fro movement of the microdroplet, featured by stretching, departing, rebounding, and settling stages, stems from the continuous generation and the intermittent screening of the photovoltaic charges during the laser illumination and it is essentially a natural transportation of net charges from laser spot (high potential) to metal surface (low potential). An analytic model is built to describe the balance between the charge generation and consumption at the laser spot and to predict the dependence of the cycling period on experimental parameters. By utilizing this effect, the water microdroplet is optically manipulated for acquiring fluorescent dye (solute) directly from the metal surface (source) in an automatic and repeatable way. The sequential appearances and fading-away of fluorescent spots inside the microdroplet prove the successful acquisition and dissolution of the fluorescent dye in the microdroplet. The reported technique is crucial to LN-based functionality integration aiming at complicated biological operations.