Optoelectrowetting for actuation of nanoliter droplets, revisited

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In the preliminary numerical investigations of the applicant's group, it was proven that the previously widespread assumption that droplet pushing with OEW (optoelectrowetting) is weaker and occurs at higher frequencies than droplet pulling is not generally valid. A closer look at the equations commonly used to describe EWOD (electrowetting) and OEW and appropriate modifications show different solutions for the contact angle change versus frequency when the OEW control light is turned on. The fact that droplet shifting can also be relatively strong and occur at low frequencies (e.g., around 50 Hz) was also evidenced by experiments. This diversity also makes it difficult to optimize the chips and their operation. Nevertheless, it would be helpful for potential applications of droplet actuation using OEW if both pulling and pushing were available as actuation options: pulling, e.g., to fuse droplets, pushing possibly to split droplets.Thus, in order to open up OEW for practical and feasible use, more detailed experimental and numerical investigations must be undertaken. This is what this project is intended to revolve around, without any specific application in the background.Both the applied voltage and its modulation frequency have a strong and not easily predictable influence on the contact angle and the net OEW force. Moreover, the electrical conductivity and the size of the droplets play a role, so that an optimization for droplets of certain properties does not automatically mean an optimization also for droplets of other parameters.Optimization of devices and operation also means finding and using the maximum of the normalized OEW force on the droplets. The light line OEW (LL-OEW) concept proposed by this group has principal advantages in this regard (not only in terms of setup geometry), because the light line allows for a larger part of the droplet's contact line to be illuminated and utilized than with perpendicular incidence of light in the usual OEW concept. The results of the project would expand the toolbox of OEW use for droplet manipulation.