In this research, an in-plane hydrodynamically reconfigurable optofluidic microlens is porposed . The microlens is formed by the laminar flow of a high-refractive-index stream and two low-refractive-index streams as shown in Figure 1. A mathematical model based on the quadrupolar flow theory is established for prediction of the stream dispersion and focal length [2-3]. The calculated streamlines are shown in Figure 2. These streamlines indicate the potential interface position of the cladding and core streams. By properly controlling the flow rate ratio of the streams, the shape of the high-refractive-index stream can be adjusted into biconvex, flat and biconcave, and the curvature of the stream interfaces can be precisely manipulated.
The fabricated microfluidic chip is shown in Figure 3. In the experiment, silicone oil (refractive index = 1.579) is selected as the high-refractive-index stream and 28.9% calcium chloride solution is used as the low-refractive-index stream. The refractive index of the calcium chloride solution is matched to that of PDMS (refractive index = 1.403) to avoid light scattering. Figure 4 shows the variation of the mircolens versus the flow rate ratio between calcium chloride solution and silicone oil. The interfaces between the fluids change with the adjustment of the flow rate of calcium chloride solution. When the flow rate ratio is low, the stream of silicone oil squeezes the streams of calcium chloride solution and forms a biconvex microlens. The curvatures of the interfaces between the fluids are symmetrical and positive and become small with the increase of the flow rate ratio. The interfaces turn to flat when the flow rate ratio is 0.45. If the flow rate ratio is kept increasing, the streams of calcium chloride solution expands and the stream of silicone oil shrinks. The interfaces become concave, resulting in negative curvatures. A biconcave microlens is formed. The output light beam is observed in the beam-tracing chamber, as shown in Figure 5. When the flow rate ratio is small, the shape of the microlens is biconvex and a focused beam can be observed in the beam-tracing chamber. With the increase of the flow rate ratio, the convergent angle shrinks. Afterward, the shape of the microlens gradually turns into biconcave and the output beam becomes divergent. Furthermore, the divergent angle expands with the flow rate ratio. The light beam is stable and no fluctuation is observed.
The proposed optofluidic microlens is a promising candidate for various microfluidic or lab-on-a-chip applications including biomedical sensing, cellular imaging and on-chip photonic signal processing.