Magnetically actuated droplet/marble transportation with tailored surface wettability

Abstract

Magnetically actuated ferrofluid droplet (FD) transport on the open surface for on-demand manipulation is of great importance in bio- and chemical microreactor utilization. However, adhesion-induced friction and droplets quickly evaporate in open areas, making it challenging to use FDs for magnetic manipulation on a large scale. We can effectively address this limitation by modifying the substrate's surface structure or altering the droplet's surface. Here, we fasten a permanent magnet on a stepper motor, moving uniformly to actuate sessile FDs and ferrofluid marbles (FMs) on the hydrophilic/superhydrophobic surface. We performed a comparative analysis of these methods, investigating the response times and contact patterns of FDs and FMs under magnetic actuation, and the influence of solid-liquid surface friction, while simultaneously analyzing the force and contact details. The results show that changing the hydrophobicity of the interface or preparing it as a marble can significantly improve the magnetic responsiveness of FDs. Their magnetic response times are about 1.88 and 1.51 times faster than FDs, while marbles' unique properties make them excellent actuate carriers. Additionally, we have defined: $Mo=\eta c{\phi }_p/2\sqrt[3]{3V/4\pi }$ to evaluate the level of difficulty of the marble actuation. This study is significant for understanding how to use magnetic excitation to precisely control and quickly respond in droplet transportation within microfluidic systems.