Controlling liquid drops with texture ratchets.

Controlled vibration selectively propels multiple microliter-sized drops along microstructured tracks, leading to simple microfluidic systems that rectify oscillations of the three-phase contact line into asymmetric pinning forces that propel each drop in the direction of higher pinning.

Measurement of the cell-substrate separation and the projected area of an individual adherent cell using electric cell-substrate impedance sensing.

This work presents an electrical technique called electric cell-substrate impedance sensing to measure the cell-substrate separation and the projected area of an individual adherent cell. Cell adhesion and cell spreading are fundamental processes of adherent cells. By recording changes in the cell-substrate separation, the projected area, or both properties with time, the dynamics of cell spreading and cell adhesion can be studied. The advantage of this electrical technique is that it enables a measurement of many individual cells simultaneously. This is a great benefit to the study of heterogeneity in cell populations. The research consisted of building a custom impedance sensing setup, designing an in vitro assay to record an impedance spectrum of an individual living cell, and developing a data analysis method to obtain two properties of the cell from curve-fitting of the impedance spectrum. The values of the cell-substrate separation and the projected area of an individual cell were within the expected ranges and in agreement with those obtained from optical microscopy.

Directing droplets using microstructured surfaces.

Systematic variation of microscale structures has been employed to create a rough superhydrophobic surface with a contact angle gradient. Droplets are propelled down these gradients, overcoming contact angle hysteresis using energy supplied by mechanical vibration. The rough hydrophobic surfaces have been designed to maintain air traps beneath the droplet by stabilizing its Fakir state. Dimensions and spacing of the microfabricated pillars in silicon control the solid-liquid contact area and are varied to create a gradient in the apparent contact angle. This work introduces the solid-liquid contact area fraction as a new control variable in any scheme of manipulating droplets, presenting theory, fabricated structures, and experimental results that validate the approach.