Abstract
When a droplet is pinned between a single plate structure (SPS) and a parallel plate structure (PPS), a liquid bridge is formed, which often occurs in digital microfluidics. The understanding of this liquid bridge formation process is still undeveloped; for this reason, the function of the side surface of the up-plate is ignored. In this paper, the formation of a liquid bridge is studied. First, based on pressure analysis, the formation process of a liquid bridge can be divided into two parts. The first part is the wetting motion, which indicates that the side and bottom surfaces attract the droplets according to their wetting force to form a liquid bridge. The second part is the pressure motion, which indicates whether the droplets enter or exit the PPS unidirectionally according to the pressure difference (capillary force) between the SPS and the PPS. The influence of the contact angle hysteresis (CAH) is simulated, and the results indicate that the CAH on the bottom surface plays a more important role than the CAH on the side surface. In addition, the influence of both the material and geometric parameters on the amount of bridge motion is studied. The results show that the thinner the upper plate is, the larger the PPS gap is, and the better the droplet entering the PPS is. These conclusions can be used to obtain better performance when droplets need to be delivered to the PPS on a microfluidic chip.
Highlights
Droplets in both single plate structures (SPSs) and parallel plate structures (PPSs) are common in daily life
The results indicate that the whole process can be divided into two parts
The bridge moves according to its movement trend (Fig. 5), but it is resisted by contact angle hysteresis (CAH)
Summary
Droplets in both single plate structures (SPSs) and parallel plate structures (PPSs) are common in daily life. As far as we know, most of the research on liquid bridges has been focused on a single structure, for example, single plate, parallel plate or nonparallel plate structures In these studies, the effects of the contact angle (CA) and contact angle hysteresis (CAH) are discussed. There have been no studies on the effects of the material parameters (CA and/or CAH) and the geometric parameters on the formation of a liquid bridge between the SPS and PPS. Berthier et al., in order to judge the movement trend of droplets, compared the pressure of droplets in the SPS or PPS but ignored the effects of the side surface and CAH. To satisfy the application of digital microfluidics, we pay more attention to the droplet motion into the PPS with hydrophobic surfaces where the advancing angle is larger than 90○. Based on the simulation and experimental results, a new bridge-forming model is proposed to predict droplet motion
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