On-demand contact line pinning during droplet evaporation

Abstract 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.