Simultaneous multidrop creation with superhydrophobic wells for field environmental sensing of nanoparticles

Facile creation of multiple drops at appropriate volumes on surfaces without the use of sophisticated instrumentation facilitates downstream evaporative preconcentration of liquid samples for analytical purposes. In this work, a superhydrophobic (SH) substrate comprising wells with a perforated mesh base was developed for simultaneous drop creation in a quick and convenient manner. In contrast to the method of pouring liquid directly over the SH wells, consistent liquid filling was readily achieved by a simple immersion approach. This method works well even for challenging situations where well diameters are smaller than 3.4 mm. Despite the poor liquid-retention properties of SH surfaces, inverting the wells did not result in liquid detachment under gravitational force, indicating strong pinning effects afforded by the well architecture. The perforated base of the well allowed the liquid to be completely removed from the well by compressed air. High-speed camera image processing was used to study the evolution of drop contact angle and displacement with time. It was found that the liquid body was able to undergo strong oscillations. Optical spectroscopy was used to confirm the ability of evaporative preconcentration of silver nanoparticles.