Electrokinetic Janus micromotors moving on topographically flat chemical patterns

Abstract

Ionic and molecular selectivity are considered unique for the nanoscale and not realizable in microfluidics. This is due to the scale-matching problem—a difficulty to match the dimensions of ions and electrostatic potential screening lengths with micron-sized confinements. Here, we demonstrate a microscale realization of ionic transport processes closely resembling those specific to ionic channels or in nanofluidic junctions, including selectivity, guidance and flow focusing. As a model system, we explore electrokinetic spherical Janus micromotors moving over charged surfaces with complex charge distribution and without any topographical wall. We discuss peculiarities of the long-range electrostatic interaction on the behavior of the system including interface crossing and reflection of positively charged particles from negatively charged interfaces. These results are crucial for understanding the electrokinetic transport of biochemical species under confinement, have the potential to increase the precision of lab-on-chip-based assays, as well as broadening use cases and control strategies of nano-/micromachinery.

Selecting active matter according to motility in an acoustofluidic setup: self-propelled particles and sperm cells

“…We therefore abstract from the details of sperm morphology and propulsion mechanism and focus on the active diffusion of sperm cells exploring the analogy between the motion of sperm and artificial self-propelled particles. 38,39…”

“…The behavior of the system consisting of motile microswimmers (artificial self-propelled Janus particles or sperm cells) and passive species (synthetic beads or immotile sperm cells and debris) is simulated by numerically integrating the overdamped Langevin equations: 16,21,22,32,33,35–37,39,40 for i , j running from 1 to the total number N of particles, active and passive; v 0 is self-velocity of active particles. Here, ξ i 0 ( t ) = ( ξ i 0, x ( t ), ξ i 0, y ( t )) is a 2D thermal Gaussian noise with correlation functions 〈 ξ 0, α ( t )〉 = 0, 〈 ξ 0, α ( t ) ξ 0, β ( t )〉 = 2 D T δ αβ δ ( t ), where α , β = x , y and D T is the translational diffusion constant of a passive particle at fixed temperature; ξ θ ( t ) is an independent 1D Gaussian noise with correlation functions 〈 ξ θ ( t )〉 = 0 and 〈 ξ θ ( t ) ξ θ (0)〉 = 2 D R δ ( t ) that models the fluctuations of the propulsion angle θ .…”

“…The behavior of the system consisting of motile microswimmers (artificial self-propelled Janus particles or sperm cells) and passive species (synthetic beads or immotile sperm cells and debris) is simulated by numerically integrating the overdamped Langevin equations: 16,21,22,32,33,[35][36][37]39,40…”

“…We therefore abstract from the details of sperm morphology and propulsion mechanism and focus on the active diffusion of sperm cells exploring the analogy between the motion of sperm and artificial self-propelled particles. 38,39 The equations of motion of self-propelled particles and the details of the simulation method are presented in the ''Methods'' section. It is worth noting that in our model, diffusion of passive and active species, as well as active diffusion of selfpropelled particles, is taken into account.…”