In fields such as medicine and chemistry, methods for transporting microdroplets are currently necessitated, which include the analysis of reagents, mixing, and separation. As microdroplets become finer, their movement becomes difficult to control as a result of surface tension. This has resulted in the use of an excessive amount of reagents. In this study, we evaluated the dynamic characteristics of microdroplets and the excitation force. Microdroplets were dropped onto a tilted glass substrate, and the displacement of the microdroplets was measured while changing the droplet amount, vibration frequency, and vibration direction. Moreover, the behavior of the droplet just before it started to move was observed, and the relationship between the displacement of the minute droplet and the vibration force was compared and examined.

The electrostatic force required for the driving of liquid droplet injected in a microchannel was studied to obtain the guiding principle to reduce the driving voltage of waveguide optical switch based on the movement of droplet. We analytically calculated the relation between the threshold voltage and velocity of droplet and the surface roughness of microchannel, and clarified some unconfirmed parameters by comparing experimental results and aeromechanical analysis. The driving of droplet in a microchannel was best analyzed using the Hagen-Poiseuille flow theory, taking into account the movement of both ends of the droplet. When the droplet is driven by some external force, a threshold of the external force occurs in the starting of movement, and hysteresis occurs in the contact angle of the droplet to the side wall of the microchannel. The hysteresis of contact angle is caused by the roughness of side wall. In our experiment, the threshold voltage ranged from 200 to 350 V and the switching time from 34 to 36 ms. The velocity of droplet was evaluated to be 0.3-0.4 mm/s from these experimental results. On the other hand, the measured angle distribution of side wall roughness ranged from 30 to 110 degrees, and the threshold voltage was evaluated to be 100-320 V, showing a good agreement with experimental results. The reduction of threshold voltage can be realized by smoothing the side wall roughness of microchannel. The switching time of 10 ms, which is required for the optical stream switch, can be obtained by shortening the horizontal spot size down to 1.5 µm.