Directing Salt-Drop Movement on Mesoporous Silica Films

Programmable drop sliding on surfaces is of interest for microfluidics, self-cleaning surfaces, water harvesting, or drop nanoreactors. While drop movement along gradients on surfaces or on slippery surfaces has been demonstrated, achieving programmable movement of drops on nanoporous surfaces without structural or chemical gradients requires different mechanisms of symmetry breaking. Using a hydrophilic mesoporous film, we investigate direction-controlled drop sliding of aqueous salt solutions. We observe drop sliding using an aqueous NaCF₃SO₃ solution drop together with symmetry breaking through local airflow. The direction in which the drop moves is controlled by evaporation-induced, asymmetric salt distribution. We investigate how the macroscopic static contact angle below 10°, the use of a continuous airflow, and varying salt concentrations allow the speed, distance, and direction of the drop to be programmed. The motive force increased with increasing NaCF₃SO₃ concentration in the drop and reaches 7 µN for 5 µL drops. The tuning of the drop direction and speed, driven by asymmetric airflow and controlled by ion concentration and airflow intensity-induced local evaporation, provides a new perspective and mechanistic approach to programmable fluid drop transport, especially on mesoporous surfaces.