Hydrodynamics of swimming microorganisms in complex fluids

Swimming motion of microorganisms, such as spermatozoa, plankton, algae and bacteria, etc., ubiquitously occurs in nature. It affects many biological processes, including reproduction, infection and the marine life ecosystem. The hydrodynamic effects are important in microorganism swimming, their nutrient uptake, fertilization, collective motions and formation of colonies. In nature, microorganisms have evolved to use various fascinating ways for locomotion and transport. Different designs are also developed for the locomotion of artificial nano- and microswimmers. In this study, we use several different computational models to investigate the behavior of microswimmers. Microorganisms typically swim in the low Reynolds number regime, where inertia is negligible. They interact with each other, surfaces and external flow field. Microorganisms often swim in complex fluids, exhibiting non-Newtonian behavior, including viscoelasticity and shear-thinning viscosity. These biological materials contain network of glycoprotein fibers and gel-like polymers. Therefore on the scale of microorganisms, their fluid environments are heterogeneous rather than homogenous. In this study, we develop a computational platform to investigate swimming motion of a single and multiple microorganism(s) in the bulk fluid and near surfaces in complex fluids. We also investigate the role of fluid rheological properties and flow field on the migration of inert particles in a channel flow of viscoelastic fluids

Near-wall motion of a squirmer in viscoelastic fluids

Microbial biofilms ubiquitously occur on natural and man-made surfaces and are closely related to various health and environmental issues. During the biofilm formation, the hydrodynamic interaction between microorganisms, surfaces and biofilm itself, which shows viscoelastic properties, are important. Despite this, the hydrodynamic interaction of swimming microorganisms and solid surfaces in viscoelastic fluids is poorly understood. We perform a three-dimensional direct numerical simulation of a microorganism swimming in viscoelastic fluids near a wall. The background viscoelastic fluid is modeled using a Giesekus constitutive equation and the microorganism is modeled using a squirmer model, which is composed of a spherical cell body with a tangential surface motion. We found that the viscoelasticity of the fluid affects the near-wall motion of a squirmer depending on the swimming mode. For a neutral squirmer, the wall-contact time increases with viscoelasticity and reaches a maximum at Wi ~ 1 due to a negative polymeric torque acting on the squirmer and impeding its rotation away from the wall. The neutral squirmer eventually escapes from the wall. On the other hand, the pusher is found to be trapped near the wall in viscoelastic fluids due to the highly stretched polymers behind its body. The near-wall motion of a puller swimmer is less affected in viscoelastic fluids

Dynamics of particle migration in channel flow of viscoelastic fluids

The migration of a sphere in the pressure-driven channel flow of a viscoelastic fluid is studied numerically. The effects of inertia, elasticity, shear-thinning viscosity, secondary flows and the blockage ratio are considered by conducting fully resolved direct numerical simulations over a wide range of parameters. In a Newtonian fluid in the presence of inertial effects, the particle moves away from the channel centreline. The elastic effects, however, drive the particle towards the channel centreline. The equilibrium position depends on the interplay between the elastic and inertial effects. Particle focusing at the centreline occurs in flows with strong elasticity and weak inertia. Both shear-thinning effects and secondary flows tend to move the particle away from the channel centreline. The effect is more pronounced as inertia and elasticity effects increase. A scaling analysis is used to explain these different effects. Besides the particle migration, particle-induced fluid transport and particle migration during flow start-up are also considered. Inertial effects, shear-thinning behaviour, and secondary flows are all found to enhance the effective fluid transport normal to the flow direction. Due to the oscillation in fluid velocity and strong normal stress differences that develop during flow start-up, the particle has a larger transient migration velocity, which may be potentially used to accelerate the particle focusing.National Science Foundation (U.S.) (grant no. CBET-1445955-CAREER

Challenges and attempts to make intelligent microswimmers

The study of microswimmers’ behavior, including their self-propulsion, interactions with the environment, and collective phenomena, has received significant attention over the past few decades due to its importance for various biological and medical applications. Microswimmers can easily access micro-fluidic channels and manipulate microscopic entities, enabling them to perform sophisticated tasks as untethered mobile microrobots inside the human body or microsize devices. Thanks to the advancements in micro/nano-technologies, a variety of synthetic and biohybrid microrobots have been designed and fabricated. Nevertheless, a key challenge arises: how to guide the microrobots to navigate through complex fluid environments and perform specific tasks. The model-free reinforcement learning (RL) technique appears to be a promising approach to address this problem. In this review article, we will first illustrate the complexities that microswimmers may face in realistic biological fluid environments. Subsequently, we will present recent experimental advancements in fabricating intelligent microswimmers using physical intelligence and biohybrid techniques. We then introduce several popular RL algorithms and summarize the recent progress for RL-powered microswimmers. Finally, the limitations and perspectives of the current studies in this field will be discussed

Electroconvective flow in presence of polyethylene glycol oligomer additives

Metal electrodeposition in batteries is fundamentally unstable and affected by different instabilities depending on operating conditions and chemical composition. Particularly at high charging rates, a hydrodynamic instability called electroconvection sets in that aggravates the situation by creating non-uniform ion flux and preferential deposition at the electrode. Here, we experimentally investigate how oligomer additives interact with the hydrodynamic instability at a cation selective interface. From electrochemical measurements and direct visualization experiments, we find that electroconvection is delayed and suppressed at all voltage in the presence of oligomers. Our results also reveal that it is important to consider the role of polymers at the interface, in addition to their bulk effects, to understand the stabilization effect and its mechanism

Near wall motion of undulatory swimmers in non-Newtonian fluids

<p>We investigate the near-wall motion of an undulatory swimmer in both Newtonian and non-Newtonian fluids using a two-dimensional direct numerical simulation. Our results show that the undulatory swimmer has three types of swimming mode depending on its undulation amplitude. The swimmer can be strongly attracted to the wall and swim in close proximity of the wall, be weakly attracted to the wall with a relatively large distance away from the wall, or escape from the wall. The scattering angle of the swimmer and its hydrodynamic interaction with the wall are important in describing the near-wall swimming motion. The shear-thinning viscosity is found to increase the swimming speed and to slightly enhance the wall attraction by reducing the swimmer’s scattering angle. The fluid elasticity, however, leads to strong attraction of swimmer’s head towards the wall, reducing the swimming speed. The combined shear-thinning effect and fluid elasticity results in an enhanced swimming speed along the wall.</p

Suppression of dendrite growth by cross-flow in microfluidics.

Formation of rough, dendritic deposits is a critical problem in metal electrodeposition processes and could occur in next-generation, rechargeable batteries that use metallic electrodes. Electroconvection, which originates from the coupling of the imposed electric field and a charged fluid near an electrode surface, is believed to be responsible for dendrite growth. However, few studies are performed at the scale of fidelity where root causes and effective strategies for controlling electroconvection and dendrite growth can be investigated in tandem. Using microfluidics, we showed that forced convection across the electrode surface (cross-flow) during electrodeposition reduced metal dendrite growth (97.7 to 99.4%) and delayed the onset of electroconvective instabilities. Our results highlighted the roles of forced convection in reducing dendrite growth and electroconvective instabilities and provided a route toward effective strategies for managing the consequences of instability in electrokinetics-based processes where electromigration dominates ion diffusion near electrodes

Suppression of dendrite growth by cross-flow in microfluidics.

Formation of rough, dendritic deposits is a critical problem in metal electrodeposition processes and could occur in next-generation, rechargeable batteries that use metallic electrodes. Electroconvection, which originates from the coupling of the imposed electric field and a charged fluid near an electrode surface, is believed to be responsible for dendrite growth. However, few studies are performed at the scale of fidelity where root causes and effective strategies for controlling electroconvection and dendrite growth can be investigated in tandem. Using microfluidics, we showed that forced convection across the electrode surface (cross-flow) during electrodeposition reduced metal dendrite growth (97.7 to 99.4%) and delayed the onset of electroconvective instabilities. Our results highlighted the roles of forced convection in reducing dendrite growth and electroconvective instabilities and provided a route toward effective strategies for managing the consequences of instability in electrokinetics-based processes where electromigration dominates ion diffusion near electrodes