Manipulating aqueous droplets by light-induced virtual electrodes

Abstract:
“The precise spatio-temporal manipulation of droplets is fundamental for many lab-on-a-chip systems with applications in biology, healthcare and chemistry. Different approaches have been investigated, including thermal, chemical and electrical methodologies. Among this latter, electrophoresis (EP) and dielectrophoresis (DEP) play a key role, since they are highly compatible with microfluidic systems and provide sufficiently strong forces to control up to microliter volume aqueous droplets. However, EP and DEP techniques typically require the presence of metallic electrodes to create the desired electric fields, making these approaches less flexible and efficient than those exploiting pure optical techniques. Iron-doped lithium niobate (LiNbO₃:Fe) allows for the generation of strong electric field modulation due to an inhomogenous illumination, thanks to its photovoltaic properties. These photoinduced fields interact as EP and DEP forces with microdroplets, while guaranteeing the flexibility provided by optical field-based modulation. Indeed, the combination with well-known techniques to control and modulate light fields can be exploited to generate virtual electrodes on the material, achieving reliable as well as flexible devices for water droplets control. In our approach, the photoinduced fields generated by the complex illumination of LiNbO₃:Fe are exploited to control motion and trajectory of water droplets inside microfluidic channel. Moreover, the crystal is integrated in standard droplet microfluidic polymeric device, substituting the usual glass substrate and, thus without hindering the portability. This feature combined with the control of positions of aqueous droplets represents a key tool for several applications of customized lab-on-a-chip systems, highlighting the capabilities of LinbO₃:Fe-based virtual electrodes.”