Magnetically induced reversible transition between cassie and wenzel states of superparamagnetic microdroplets on highly hydrophobic silicon surface

In this work, we report a magnetic technique for reversible wetting-dewetting transitions of microdroplets on highly hydrophobic surfaces. A superparamagnetic microdroplet can be reversibly switched between the Cassie state and the Wenzel state on a highly hydrophobic microstructured silicon substrate by the application of the magnetic field. The transition can be controlled by both the intensity of the magnetic field and the concentration of the superparamagnetic Fe ₃O ₄ nanoparticles in the microdroplet. The magnetic force needed during the transition from the Cassie state to the Wenzel state was found to be apparently smaller than that needed in the reverse process. Such asymmetry is ascribed to the higher energy of the Cassie state compared with the Wenzel state, the change of the gravitational potential energy, and the adhesion hysteresis. This report provides a novel method of dynamically controlling liquid/solid interactions, which can not only help us to understand further the transition between the Cassie state and the Wenzel state but also potentially be used in some important applications, such as lab-on-a-chip devices and chemical microreactors.