First, we examined the behavior of a mixture of particles in the VIF system. We fabricated a substrate with micropillars (100 µm in diameter and 50 µm in height) of polydimethylsiloxane (PDMS) using a standard micromolding process (Figure ). These pillars were arranged in a 10 × 10 matrix with center-to-center spacing of 200 µm. Spacers with a height of 50 µm were placed at the corners and sides to define the height of the fluid volume bounded by a cover-glass as a lid. After the oxygen-plasma treatment to make the substrate hydrophilic, the PDMS substrate was mounted on a piezo-drive stage connected to the power and signal sources using a double-sided tape (Figure ).