Capillary tweezer for programmable droplet manipulation

Precise droplet manipulation is of great importance in various cutting-edge engineering applications, including microfluidic device operations, condensation heat transfer, water harvesting, micro reaction, among others. Most of the existing droplet manipulation techniques, consisting of microchannels, bionic structures, functional surfaces, and external stimuli, tend to lack high-precision controllability, have high fabricating costs, or suffer from liquid cross-contamination. Here we report an effective and multifunction droplet handling method based on the proposed capillary tweezer. A pillar with a magnet bar induces a capillary meniscus of carrier oil, and due to the curvature-induced immersion force, droplets in the carrier oil are attracted to the pillar. By modulating the immersion force with the deformation of a flexible supporting membrane, droplet capturing and releasing can be dynamically switched, like a tweezer. Moreover, droplet transporting and rotating can easily be realized with the pillar motion driven by an external magnet bar. Using this method, programmable droplet manipulations, including non-contact coordinated droplet transportation, selection, digital coalescence, and chemical micro reaction, have been successfully demonstrated. Due to the high simplicity and controllability, the tunable capillary tweezer proposed embraces a great potential in many microfluidics, microfabrication, and bioengineering applications. • Demonstrating a new approach to micromanipulation of droplets based on a capillary tweezer. • Analyzing the principle and control parameter of the capillary tweezer. • Presenting programmable droplet manipulations using the capillary tweezer.