Dissipative Particle Dynamics Simulations Of Droplet Manipulation In Microfluidic Chips

Microfluidic system can integrate sample separation, mixture, detection and analysis of a certain chemical/biological process into a small chip in order to achieve quantitative analysis of the products. Nowadays, the technology of microfluidic chip is one of the most active research fields and represents the trends of miniaturization and integration of analytical instruments in 21st century.The driving and controlling of fluid is one of the crucial technologies in microfluidic chips. With decreasing scale of fluid, the specific surface area dramatically increases and thus the interfacial forces have much greater impact on the flows, leading to wide applications of driving method based on surface tension.Being beneficial to give a deep insight into transport phenomena of microfluid driven by gradients of surface tension and wettability, theoretical modeling and simulations are performed to optimize the design of microfluidic devices. Based on dissipative particle dynamics, numerical calculation is performed to simulate the droplet manipulation in microfluidic chips. The characteristics of droplet movement are investigated as well as the three dimensional flow fields inside the droplet. Furthermore, a three-dimensional code for dissipative particle dynamics simulations has been developed to simulate the liquid transportation at meso-scale. The details and some accomplishments are presented as follows.(1) A method was proposed to impose controllable slip boundary conditions in dissipative particle dynamics. Different with the macroscopic flows, the velocity slippage on solid wall should be considered in micro/mesoscopic hydrodynamics. The strength of slippage could be adjusted by variation of parameters in present method, and the validity and the availability of the approach were verified.(2) A dissipative particle dynamics model was proposed for the simulation of moving droplet on solid surface driven by gradient of wettability, and process of droplet transportation was also investigated. Two mechanisms, i.e. the rolling and the rolling plus sliding, were revealed during the droplet moves. The former dominates the motion when the droplet lies on hydrophobic surface, while the latter plays a key role when the droplet contacts with the hydrophilic surface.(3) The three-dimensional flow field of the moving droplet was visualized, which described the shape of droplet contacting with solid surfaces and the dynamical characteristics of the droplet movement clearly.(4) The effect of thermal fluctuation on the droplet transportation was investigated. It was found that the change of thermal fluctuation could hardly influence the motion of droplet when the gradient steepness of wettability was small, but with increasing the gradient steepness, thermal fluctuation could enhance the moving ability of the droplet. However it should be noticed that the thermal fluctuation could not change the parking location of the droplet.(5) Based on Lippmann-Young equation, a dissipative particle dynamics model for droplet oscillations in AC electrowetting was proposed. The oscillations of droplet under AC voltage with various frequencies were investigated. The structure of time-dependent velocity field inside the droplet clearly indicates the characteristics of droplet oscillation. When a low frequency voltage is applied, the distortion of the drop surface can be evidently observed. For some applied voltages with intermediate frequency, although the fluid motion can be ignored in most regions inside the droplet, especially for the top region, the fluid oscillate dramatically near the contact line as the voltage varies. The results were acceptable agreement with the experimental data.(6) Hysteresis phenomenon of contact line dynamics was analyzed during droplet oscillation. It indicates that phase difference between the applied voltage and the movement of the contact line generally increases with increasing the input AC frequency, except for the resonance frequencies. The AC frequency could dramatically influence the amplitude of contact line oscillation. It was found that the amplitude A_R decreases exponentially with increasing the frequency f by the relation of A_R = 1.56×10²Â·f^-4/5.