The objective of this paper is to characterize the droplet deformation during transport and to show how the droplet morphology changes at different gap heights in digital microfluidics systems. The aspect ratio defined as the ratio between the gap height to the electrode side length will be varied within the range of high to extremely low. In this way, we will demonstrate that the droplet morphology significantly changes during transport when the aspect ratio is changed. Analogous to the channel dimensions, the gap height and the electrode side length are the two most important geometrical variables affecting droplet motion in digital microfluidic systems. In this study, the droplet deformation was found to be minimal at higher aspect ratios. Contrarily, the droplet will exhibit severe necking and elongation at very low aspect ratios. The deformation and necking patterns were found to be similar to the necking pattern that happens during splitting. However, the droplet exhibits this necking when it is only driven by one activated electrode during the transport process. On the other hand, the droplet is pulled by driving forces from two opposite directions in the conventional splitting process. Extended simulations are performed to investigate the effect of changing the aspect ratio and the actuation voltage used. The simulations were performed by FLOW-3D software which uses the volume of fluid (VOF) technique. The results confirm the effect of changing the gap height on the droplet behavior during motion. The extreme deformation and necking happens only at very low aspect ratios. In this case, the transport process starts by moving a small portion of the droplet toward the activated electrode. When this small portion advances toward the activated electrode the rest of the droplet is not greatly affected or pulled toward the same direction. It can be noticed that the remaining portion of the droplet is not moving at the initial stages of motion and the droplet start to be squeezed gradually through a neck instead of moving in a bulk form. Further observations demonstrated droplet elongation and delayed response of the back portion of the droplet compared to the motion of the leading edge. Analyzing the motion by analytical models can be inaccurate as they assume that the droplet retains its circular shape. Therefore, CFD simulations can demonstrate the droplet behavior better than the analytical models where the droplet exhibits severe deformation and deviation from the circular shape.

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