Active/Passive Control Method and Mechanisms for Droplet/Liquid-Infused Surface Friction

Microfluidic system is integrational analytical laboratory on a micro-chip to realize the generation, transportation, merging and separation of micro-droplets on the microscopic scale. Microchannels are the core parts of the microfluidic system, and the manipulation of microdroplets is mainly completed in microchannels. Consequently, the interface of microdroplet/microchannel is the mainly contact interface and the solid/liquid interface behaviors will influence the precise actuation of microdroplets. Solid/liquid interfacial friction force is an important parameter of interfacial behavior when microdroplets are transported in microchannels. Therefore, the solid/liquid interfacial behaviors can be effectively adjusted to achieve precise driving of micro-droplets by controlling and optimizing the solid/liquid interfacial friction force. The interfacial friction force can be modified by passive and active control. Generally, modifying the interfacial friction by permanently changing the morphology, structure, or surface energy of a solid surface through physical and chemical methods is a passive control. Moreover, changing the interfacial friction by external energy, such as light, electricity, magnetism et al. is an active control. However, these active control methods are limited by practical applications due to the long response time of interface to external energy. Therefore, it is necessary to explore new active control methods with fast response and wide adjustment range in order to further optimize the function of the microfluidic control system.

In this paper, liquid-infused surfaces were prepared to study the interfacial friction behaviors. The friction force at droplet/liquid-infused surface was adjusted by adjusting the infused fluid viscosity and applied voltage. Moreover, the mechanisms of the interfacial friction force caused by fluid viscosity and applied voltage were studied. Results showed that interfacial friction force increased from about 15 to 40 μN as the infused liquid viscosity increased from 10 to 100 mm²/s. Moreover, the friction force increased from 15 μN to approximately 45 μN when the applied voltage increased from 0 V to 240 V. However, the friction force decreased from 45 μN to 15 μN when the applied voltage was gradually reduced from 240 V to 0 V. Results indicated that the friction force could be dynamically adjusted within a range of 3 times. It was deduced that the variation of friction with fluid viscosity was associated with the enhancement of the hydrogen bonding at the droplet/liquid-infused surface. Furthermore, droplet/liquid-infused surface could be considered as electrowetting system under applied voltage. The effective interfacial tension at droplet/liquid-infused interface varied with voltage, realizing the friction force dynamic adjustment. Therefore, the frictional force at droplet/liquid-infused interface could be effectively adjusted by liquid viscosity and applied voltage. The results might be helpful for providing technical and theoretical guidance to optimal microchannel design and accurately delivering microdroplet.