Description
The controlled and reproducible splitting of liquid droplets is a key function in many fields and for microfluidic applications. In recent years, various strategies have been used to achieve this task. In this work, we present an optofluidic technique based on an engineered surface composed by coating a z-cut iron-doped lithium niobate (Fe:LiNbO3) crystal with a lubricant-infused layer (LIS), which provides a very slippery surface for prolonged use. The illumination of the crystal
with a light spot induces the accumulation of surface charges of opposite signs on the two opposite crystal faces as a result of the photovoltaic effect. If the light spot is intense enough, microliter water droplets (corresponding to a few millimeters in size) placed near the illuminated
area split into two charged fragments: one fragment remains trapped by the bright spot, while the other one moves away from it. The latter fragment does not move randomly, but follows one of three well-defined trajectories separated by 120°, which reflect the anisotropic crystalline
structure of Fe:LiNbO3 . Numerical simulations explain the behavior of water droplets within the framework of the forces induced by the interplay of photovoltaic effects and thermo-piezo
stresses, induced by laser illumination, which originate simultaneously within the illuminated crystal [1]. The time required for the droplet splitting is characterized by varying experimental parameters such as droplet volume, illumination intensity, or mutual distance between the spot and the droplet [2]. Interestingly, the same splitting behavior is also observed for organic liquid droplets. Unlike water, organic liquids do not contain ions and can be polarized by the evanescent field
generated by the surface charges of Fe:LiNbO3. The study of the splitting time can provide a valuable feature in applications that require splitting and coalescence of droplets, such as chemical microreactors and biological encapsulation and screening.
References
[1] S Cremaschini et al, Trifurcated splitting of water droplets on engineered Lithium Niobate surfaces. ACS Appl. Mater. Interfaces 24 (2024), 4271.
[2] S. Cremaschini et al., Optical splitting of droplets on z-cut Lithium Niobate crystals, in preparation.