Droplet Control Using Light-Induced Thermocapillary Force on Microfluidic Chip

Abstract

This thesis described the droplet control using light-induced thermocapillary force on microfluidic chip. Thermocapillary effects occur due to the interfacial tension difference caused by temperature difference around the droplet. Droplet control technique using this effect has a merit of strong force acting on a droplet. In addition, thermocapillary effects could be induced by light such as laser. When the droplet is controlled by this method, the contamination could be minimized due to non-contact nature. Despite these merits, most of previous studies have focused on the potential of this method, and there is limited studies on specific droplet control conditions and practical applications. Therefore, this thesis focused on development of droplet trajectory control technique and merging control technique using light-induced thermocapillary effects. The specific conditions for performing each control were investigated, and on-demand droplet routing and nanoparticle synthesis were developed as practical applications. First, the droplet trajectory control technique using light-induced thermocapillary effects was developed. We intended to estimate the thermocapillary force acting on a droplet and propose empirical equations between the force and simply measurable key parameters. Furthermore, we aim to estimate the droplet trajectory shifting distance and apply to develop an on-demand droplet routing system. Laser beam with 532 nm wavelength was used to induce the thermocapillary effects optically. The mixture of light-absorbing material was used as continuous phase fluid to convert light energy to thermal energy, while deionized water (DI-water) was used as dispersed phase fluid. Empirical equations to estimate the thermocapillary force were suggested, and we applied these equations to estimate the shifting distance and on-demand droplet routing. We found that the shifting distance was linearly dependent to the thermocapillary force, and we successfully developed on-demand droplet routing system with success rate of over 95 %. Next, the droplet merging control system using light-induced thermocapillary force was developed. In literature, droplet merging technique generally employed microstructure to minimize unintended droplet heating. However, we investigated droplet merging conditions without microstructure to overcome the limitation of droplet control range that might occur when microstructure was used. Also, we applied this technique to synthesize CdS nanoparticles with size control. DI-water and red edible dye aqueous solution (1.0 wt. %) were used as dispersed phase fluid to distinguish the droplets. As a result, we found that minimum power to merge droplet was linearly proportional to the droplet velocity and diameter, and empirical equations expressed as function of droplet diameter and velocity were derived. CdS nanoparticles were synthesized as an application of this technique, and the size of particles was controlled by adjusting the power of heat source.