Abstract
The integration of photosensitively functionalized materials into integrated microfluidic systems allows controlling small amounts of fluids by light. One of the most promising application is the use of optoelectronic substrates that lead to the optical induction of electric fields that in turn allow manipulating microfluidic droplets. In particular, photovoltaic crystals have attractive capabilities in the spatial control of liquid droplets by means of optically-induced space charge distributions. The distribution of these electric carriers acts as virtual electrode creating strong electric fields, which have been effectively demonstrated as a versatile tool for trapping and moving small droplets. Up to several kV mm-1 electric fields can be achieved by the photovoltaic effect, for instance exploiting iron-doped lithium niobate crystals (Fe:LN). Despite promising results, this crystal has never been directly integrated in lab-on-a-chip system for active manipulation of aqueous droplets which are essential for microfluidics applications. Indeed, the development of light-based manipulation of electric fields enable a large number of operational functions within lab-on-a-chip protocols, for instance to merge on-demand droplet pairs. Herein, we propose the integration of Fe:LN inside a microfluidic droplet device in order to realize light-actuated merging of microfluidic confined aqueous droplets. The working principle is based on electro-coalescence of droplet pairs due to the presence of light-induced electric fields. Droplets are generated within a T-junction integrated circuit, and are further merged on-demand by optically-induced virtual electrodes. The droplets' behavior is analyzed in this novel light functionalized chip compared to standard materials devices. We show that the dynamics of the droplets follow the same scaling laws and demonstrate unique light-induced merging by virtual electrodes on Fe:LN.