Electrokinetically tunable optofluidic bi-concave lens

This paper numerically and experimentally investigates and demonstrates the design of an optofluidic in -plane bi-concave lens to perform both light focusing and diverging using the combined effect of pressure driven flow and electroosmosis. The concave lens is formed in a rectangular chamber with liquid core liquid cladding (L) configuration. Under constant flowrates, the performance of the lens can be controlled by an external electric fields. The lens consists of a core stream (conducting fluid), cladding streams (non-conducing fluids), and auxiliary cladding streams (conducting fluids). In the focusing mode, the auxiliary cladding stream is introduced to sandwich the biconcave lens for protecting the light rays from scattering at the rough chamber wall. In the diverging mode, the auxiliary cladding liquid has a new role as the low refractive-index cladding of the lens. In the experiments, the test devices were fabricated in polydimethylsiloxane (PDMS) using the standard soft lithography technique. Enthanol, cinnamaldehyde, and a mixture of 73.5% ethylene glycol and 26.5% ethanol work as the core stream, cladding streams and auxiliary cladding streams. In the numerical simulation, the electric force acts as a body force. The governing equations are solved with finite volume method on a Cartesian fixed staggered grid. The evolution of the interface was captured by the lev el set method. The results show that the focal length in the focusing mode and the divergent angle of light beam in the diverging mode can be tuned by adjusting the external electric field at fixed flowrates. The numerical results have a reasonable agreement w ith the experimental results.