Transport Phenomena of Nano-Swimmers: DPD study

• Main Authors: Da-sheng Lai, 賴達昇
• Other Authors: Heng-kwong Tsao
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/52231339366700984688

碩士 === 國立中央大學 === 化學工程與材料工程學系 === 103 === Nano-swimmers in nature such as microorganisms can utilize different propulsion mechanisms to achieve directed locomotion in a viscous environment.For nature microswimmers, such as E. coli, the locomotion can be described by a run-and-tumble model in which straight runs are depicted by swimming speed (va) and tumbles associated with complete randomizations in the direction take place with mean duration (t). For sufficiently large time and length scales, the diffusive behavior is displayed for run-and-tumble organisms. In this study, the diffusive behavior of nanoswimmers and the flow rate were investigated by active colloids with DPD simulation (Dissipative Particle Dynamics simulation). In an unbounded system, nano-swimmers were observed to display two different diffusion behaviors from a variety of motions. The first type is called as pusher with a random dipole force. Active colloids acted on a surrounding solvent bead to move at each time step. The mean square displacement of active colloids and solvent were linear dependent with the time. After calculation, the diffusion coefficient could be determined by slope and adjusted the relationship between the concentration of active colloids (fp) and related factors, including active force (Fa) and run time(t). For an active colloid’s diffusion coefficient relationship is Dp=D0+f(fp)*v^2*t, and the solvent’s diffusion coefficient relationship is Ds=D0+fp*v^2. The second type was called as external force with a point force. For this type, active colloids didn’t act on the surrounding solvent beads. In this way, the mean square displacement of active colloids was exponential dependent with the time (t^a). When a was between 1 and 2, this condition was called superdiffusion, and the active colloid’s diffusion index (ap) was generally exponential dependent with fp. For the bounded system under gravity, the effect of flow to nano-swimmers. There were two shapes of active colloids. One was spherical and another was rod-like. For a spherical colloid, the higher active force was, the lower flow rate was. It was assumed that active colloids would aggregate close to the boundary so as to enhance the friction. And the flow rate was decreasing. However, for the rod-like colloid with rising active force, the flow rate was not decreasing continuously. The flow rate was observed to increase in the beginning. It was predicted that the rod-like colloids became directional so that the active rods exhibit polar order. And the average orientation was reversed to the direction of gravity. Exactly speaking, when the active rod was in the middle of the system, the orientation of colloids was the same with the direction of gravity. Otherwise, when the active rod was near to the boundary, the orientation of colloids was opposite to the direction of gravity. This phenomenon was due to the difference of flow rate.