Hydrophobic-substrate based water-microdroplet manipulation through the long-range photovoltaic interaction from a distant LiNbO:Fe crystal.

Development of photovoltaic water-microdroplet manipulation using LN:Fe crystals has to meet the requirement of the hybrid and heating-avoided design of biological lab-on-chips. To fulfill this, we demonstrate a successful manipulation of a water microdroplet on a hydrophobic substrate by utilizing the long-range photovoltaic interaction from a distant LN:Fe crystal (see Visualization 1). The maximal manipulation distance (MMD) is found to be dependent on the laser-illumination intensity at the LN:Fe crystal and it can be tuned up to a sub-centimeter level (∼4 mm). Basing on the two-center model of light-induced charge transport in the LN:Fe crystal, we establish an analytic model to describe the force balance during the microdroplet manipulation under a long-range photovoltaic interaction. Either shortening the manipulation distance or increasing the illumination intensity can enhance the photovoltaic interaction and increase the velocity of the microdroplet being manipulated. An abrupt shape change followed by a fast repelling movement of the water microdroplet is observed under a strong photovoltaic interaction (see Visualization 2).