Dual-mode droplet merging in a straight microchannel by using thermocapillary effects

Thermocapillary effects induced by interfacial tension gradient established by the temperature difference has been paid attention due to its strong driving force exerted on a droplet. Nevertheless, previous studies utilizing the thermocapillary effects have limitations in that they use a passive structure to assist droplet merging. We aim to develop a dual-mode droplet merging technique using thermocapillary effects in a simple straight channel without passive structure. In order to merge droplets by thermocapillary effects, a laser beam and a mixture of photothermal materials and oleic acid were used. We found that a minimum power of heat source to merge droplet is linearly dependent on both of droplet diameter and droplet velocity. As a result, we were able to perform the active, on-demand droplet merging with a success rate of 95 % by controlling the laser power. As an application of droplet merging by thermocapillary effects, we synthesized nanoparticles on a passive mode of continuous merging and controlled the size of nanoparticles by varying power of heat source.