Abstract
This work is mainly to investigate the surface tension of the droplet and the interfacial tension between the microchip/binding site and the droplet to study the self-alignment of the microchip and the formation of the liquid microlens. The Surface Evolver Program is adopted as an analysis tool in this work, which is developed for analyzing the droplet formation due to surface tension energy and other energies. On the study of the self-alignment of the microchip with the binding site using the droplet, this work explores firstly the droplet deformation and the contact characteristics between the microchip/binding site and the droplet when the microchip is subjected to translation, compression, yawing or rolling in air. Under the effects of the surface tension of the droplet and the interfacial tension between the droplet and its contact surface, the contact line, overflow and no wet regions of the droplet on the microchip/binding site are thoroughly demonstrated. By observing the details of changes in the contact line and the wetted area and calculating the restoring force and restoring torque, the self-alignment of the microchip can be estimated accurately. Moreover, because the surface tension or interfacial tension is different for the droplet in air and in solution, the contact characteristics between the microchip/binding site and the droplet is also different. For the droplet and the microchip in solution, due to the effects of buoyancy, hydrostatic pressure and interfacial tension, the deformation of the droplet, restoring force or restoring torque are also affected, and change the self-alignment of the microchip. These are all analyzed thoroughly in this work. The present analysis model for the self-alignment of the microchip in air or solution can not only improve some drawbacks in the literature, but also predict the critical values of restoring movements to improve the accuracy of the self-alignment of the microchip. The liquid microlens research generally utilizes the electrowetting effect to control the interfacial tension between the liquid microlens and the substrate to change the radius of curvature of the liquid microlens. The changes in the focal lengths can cause different magnifications. The contact angles under different applied voltages calculated by the present model are in excellent agreement with the experimental results in the literature. This work finally elucidates the accurate control of the formation of the liquid microlens under the applied voltage and investigates the functional relationships between the applied voltage and the radius of curvature, focal length, object distance, image distance, the diameter of the entrance pupil, magnification and f-number, to be of much help for the optical design of liquid microlens. The analysis models established for the self-alignment of the microchip and the study for the formation of the liquid microlens in this work are applied not only to supplement an analysis models discussed in the literature, but also to be helpful for accurately controlling the related design parameters. The computed results are useful to improve the fluidic self-assembly technique or the optical design of the liquid microlens.