Settling mode of a bottom-heavy squirmer in a narrow vessel.

The lattice Boltzmann-immersed boundary (IB-LB) method is used to numerically simulate the sedimentation motion of a single two-dimensional, bottom-heavy squirmer in a narrow vessel. The effects of the swimming Reynolds number Res = 0.1-3, eccentricity distance l = 0.15d-0.75d, and density ratio of squirmer to fluid γ = 1.1-2.0 on the settlement motion characteristics are investigated and analyzed. The results showed that four settling modes exist: vertical motion, unilateral oscillation, oscillation, and tilt. The bottom-heavy neutral squirmer and puller settle in the vessel during vertical motion when Res is 0.1-1.5. By increasing Res and swimming strength |ß|, the bottom-heavy squirmer becomes more self-driven, shifting its settling mode from vertical motion to unilateral oscillation or oscillation. Increasing l or |ß| does not affect the bottom-heavy neutral squirmer and puller's vertical settling mode but shifts the bottom-heavy pusher's settling mode from unilateral oscillation to oscillation or oscillation to unilateral oscillation. Similarly, altering γ or |ß| has no impact on the eccentric neutral squirmer and puller's settling mode; however, pushers will switch from oscillation mode to attraction mode or from oscillation mode to tilt mode. Additionally, it was found that after the squirmer collided with the bottom wall, the bottom-heavy squirmer settled at the bottom of the vessel in a different state of motion.

Analysis of electro-osmotic flow characteristics at joint of capillaries with step change in zeta-potential and dimension.

The Navier-Stokes equation was used to describe the characteristics of electro-osmotic flow. The corresponding numerical simulations were performed for varying zeta-potential and dimension. The results indicated that a step change in zeta-potential will cause a significant variation in the velocity profile and pressure distribution of the flow. A step change both in zeta-potential and dimension will result in a more violent variation near the joint of the capillary. This variation will reduce the separation efficiency and quality of capillary electrophoresis. The conclusions are helpful to design and fabrication of microfluidic devices, the analysis of data collected from such devices and improvement of the separation efficiency of capillary electrophoresis.