Effects of Inhomogeneous Substrate and Molecular Orientation on the AC Electrowetting Behavior of Liquid Crystal Droplet

Abstract

In this paper, the dynamic ac electrowetting (EW) behavior of liquid crystal (LC) droplets was investigated on inhomogeneous substrates by inducing LC molecules to orient themselves on a pre-rubbed polyimide substrate. For the first time, we studied the influence of inhomogeneous molecular orientation on the oscillation response of dielectric droplets during ac EW. The experimental results show that the ac EW oscillation amplitude of LC droplet is also inhomogeneous depending on the polyimide-induced molecular orientations on the substrate. The oscillation amplitude that is parallel to the LC molecular orientation is about ∼40% larger than the oscillation amplitude that is vertical to the LC molecular orientation. The oscillation amplitude on the homogeneous un-rubbed polyimide substrate falls in between. Meanwhile, the droplet oscillation amplitude decreases as the ac EW driving frequency increases. An analysis of the EW driving force and various resistance forces for dielectric droplet revealed that intermolecular resistance dominates in the inhomogeneous response of LC droplet, i.e., the lower intermolecular friction between parallel-arranged molecules and the higher intermolecular friction between the cross-arranged molecules cause the inhomogeneous hydrodynamic damping effect on the LC droplet. The findings provide a potential mechanism for optimizing the driving approach for the ac electrowetting of LC droplets by tailoring the substrate and the LC molecular orientation.

Keywords:

• Ac electrowetting
• liquid crystal
• inhomogeneous substrate
• induced molecular orientation
• intermolecular resistance

Acknowledgements

The authors would like to thank Dr Qian Zhao for helpful discussions on liquid crystals. The authors also would like to thank Prof. Y. P. Zhao for helping in the EW experiment. The project was supported by the National Natural Science Foundation of China (Grant Nos 50975154 and 50730007), and National Key Basic Research Program of China (Grant No 2007CB607604).