The light propagation patterns in optofluidic diffusive waveguides are distinctively different with those in solid waveguides [1].Most significantly,the light focuses periodically with increasing focal lengths due to the diffusion-induced refractive index (RI) distribution.This paper investigated the contributing factors of this unique focusing phenomenon in optofluidic waveguides and then presented a novel method to measure diffusion coefficient in binary solutions using this focusing phenomenon.Figure 1 illustrates the waveguide regimes leading to the light focusing phenomenon.As shown in Figure 1(a),the optofluidic waveguide consisted of a central stream and two cladding streams.Figure 1(b) and (c) show the simulation results of the concentration distribution and light propagation pattern in the microchannel,respectively.In Figure 2(a),the flow rate of central stream was fixed to 1μL/min and 1.5μL/min and the flow rate of cladding streams changes from 0.5μL/min to 1.5μL/min with an interval of 0.1μL/min.The results indicate that with negligible diffusion the first focal length is linearly proportional to the width of core stream.In Figure 2(b),the flow rate ratios of cladding,core and cladding streams were fixed to 1:1:1,2:1:2,and 3:1:3.The core flow rates were tuned from 0.5μL/min to 1.5μL.The results showed that with significant diffusion,the first focal length decreased exponentially with the increase of central flow rate.This focusing phenomenon was used to measure the diffusion coefficient of pure ethylene glycol in water.The difference of the 2nd and 3rd focal length was measured to be 52μm.Figure 3 plots the simulated focal length difference with an increasing diffusion coefficient.According to comparison,the diffusion coefficient of 100% ethylene glycol in water was determined to be 3.2x10-10m2/s,which agreed well with the published result [2].In conclusion,this paper investigated the diffusion-induced focusing phenomenon in optofluidic waveguides and concluded that the central stream width and diffusion rate co-determined the focal length.This paper then proposed a method to measure the diffusion coefficient in binary solutions by using the focusing phenomenon.The diffusion coefficient of pure ethylene glycol in water was measured to be 3.2x10-10m2/s.