Sloshing is the movement of liquid in a container induced by an external force. The phenomenon has been extensively studied at the macroscopic scale because of the importance of liquid control in tanks. Sloshing at the microliter scale should be explored for microdroplets in Lab-on-a-chip applications. We therefore verified the sloshing characteristics of droplets in microliter-sized wells. A water-in-oil droplet in the well was vibrated horizontally at between 20 and 100 Hz. Circular and elliptic wells between 2 and 5 mm sizes were applied for the well geometry. The surface chemistry of the well was additionally changed by using amphiphobic coatings to examine the effect of the interfacial tension between the well surface and the liquid, because we considered it a dominant factor at the microliter scale. Decreasing the well diameter caused the natural frequency to increase and the sloshing to suppress. This agreed with Housner's equation, which is conventionally used for macroscopic scale. The experiments also revealed that decreasing the interfacial tension caused the natural frequency to increase and resulted in drastic suppression of the sloshing, which was characterized by Bond number, similarly with the cases under microgravity. We experimentally verified that at the microliter scale, the interfacial tension is dominant to control the sloshing phenomenon.
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