In recent years,droplets-based microfluidic systems are widely used in chemical and biological processes [1,2] because they offer large specific surface area,short diffusion distance and can confine reaction in independent environments.The above advantages can reduce the experiment time for determining the diffusivity of solutes.In this study,Rhodamine B was encapsulated into droplets generated by a T-junction micro-channel[3] in order to determine its diffusivity.Rhodamine B was extracted from the dispersed phase to the continuous phase when the droplet passed through the serpentine micro-channel.The diffusivity of Rhodamine B was estimated via the equilibrium time of extraction,which is equivalent to the time required for Rhodamine B molecules to diffuse from the core to the edge of a droplet.The microfluidic device was fabricated by pattering micro-channels onto poly(dimethylsiloxane)PDMS sheets using soft lithography.Aqueous solution of Rhodamine B was utilized as dispersed phase and n-octanol was utilized as continuous phase to generate micro-droplets in the micro-channel.The size of micro-droplets can be readily controlled by changing the ratio between volumetric flow rates of dispersed phase and continuous phase as well as the relative concentration/viscosity between the two phases.Because Rhodamine B is more soluble in n-octanol,it was extracted from micro-droplets to the continuous phase.The amount of Rhodamine Bin the droplet was monitored by its green fluorescence intensity.Rhodamine B dissolved in 20% sucrose aqueous solution as dispersed phase was also applied to measure its diffusivity.The purpose of this experiment is to study how viscosity of dispersed phase affects the diffusivity of Rhodamine B.As shown in Figure 2,the diffusivity of Rhodamine B can be estimated by scale law tdiffusion~R2/DAB and assume that rate of extraction is much greater than the rate of diffusion.The estimated diffusivities of Rhodamine B in water and 20% sucrose aqueous solution are 6.04x10-9m2/s and 5.43x10-9m2/s,respectively.According to these experimental results; the diffusivity of Rhodamine B in 20% sucrose was smaller than that in water due to the increased viscosity of dispersed phase.However,the theoretical value which estimated by Stokes-Einstein equation* D=κT/6μπr is 2.2x10-10 m/s2.The experimental value is larger than the theoretical value because of the internal flow in droplets.The convective mass transport reduced the diffusion time and caused the larger diffusivity.*κ is Boltzmanns constant,T is room temperature,μ is viscosity of Rhodamine B aqueous solution,r is the radius of Rhodamine B molecule.