A magnetically controlled microstructured surface for three-dimensional droplet manipulation

The smart manipulation of droplets has received widespread attention due to its potential applications in many fields. However, it is still challenging to realize robust multidimensional, versatile liquid manipulation using magnetically responsive surfaces. Here, a magnetically controlled surface with a dense array of cone-shaped microstructures is developed by the spray self-assembly method using soft nontoxic materials. The effects of the spray volume and material concentrations on the surface morphology and wettability are systematically investigated. The wettability and adhesion properties of the developed surface can be reversibly switched in the presence of an on/off magnetic field. In situ observation indicated that the driving force acted on the droplet is derived from localized deformation of the microstructures. Moreover, theoretical models of droplet manipulation are proposed to demonstrate the underlying mechanism. Under the actuation of the moving magnetic field, the surface can transport droplets of 1-14 mu l in the vertical direction, and the modified superhydrophobic surface can transport droplets of 3-30 mu l in the horizontal direction and achieve against-gravity droplet climbing with a volume of 10 mu l at a climbing angle of 25 degrees. The environmentally friendly and facilely manufacturable surface presents promising applications in liquid microreactors and the transportation of mixed fluids in biological and chemical research.