Depending on the contact line motion, evaporation of colloid-rich droplets can leave a ring-like or a spot-like residue. Herein, we determine this outcome by dynamically controlling the contact line motion using coplanar direct-current electrowetting-on-dielectrics (DC-EWOD). Thanks to the additional control over the contact angle, the contact line of evaporating droplets can be maintained in a pinned state even on homogeneous Teflon-coated surfaces. In the absence of EWOD regulation, polystyrene particles and human embryonic kidney 293 (HEK293) cells suspended in droplets formed dot-like residues, whereas application of the mechanism triggered the occurrence of ring-like patterns of a controllable radius. Unlike chemically or structurally patterned substrates, ring- and dot-like deposits can be switched simply by switching on or off the EWOD regulation at any predetermined time before reaching the control limit. This ability enables cell counting over a wide concentration range or other potential applications requiring controllable contact line movement with little dependence on bulky, complex and expensive instruments.

根据接触线的运动，富含胶体的液滴的蒸发会留下环状或斑点状的残留物。在这里，我们通过使用共面直流电介质上电润湿（DC-EWOD）动态控制接触线的运动来确定此结果。由于对接触角的附加控制，即使在均匀的特氟隆涂层表面上，蒸发液滴的接触线也可以保持固定状态。在没有EWOD调节的情况下，悬浮在液滴中的聚苯乙烯颗粒和人类胚胎肾脏293（HEK293）细胞形成点状残基，而该机制的应用引发了可控制半径的环状图案的出现。与化学或结构图案化的基材不同，可以通过在达到控制极限之前的任何预定时间打开或关闭EWOD调节来简单地开关环状和点状沉积物。这种能力使细胞计数可以在很宽的浓度范围内进行，也可以用于需要可控的接触线移动的其他潜在应用，而对体积庞大，复杂且昂贵的仪器的依赖性很小。