Join us | Zhiwei Peng

Micromechanics and microrheology of active matter

Positions: Ph.D., M.S., and undergraduate internships
Start date: Fall 2024 or after
Location: University of Alberta, Edmonton, AB, Canada
Supervisor: Dr. Zhiwei Peng
Method of inquiry: Theory and computation

We seek motivated students to join the Peng Group (est. 2024) in the Department of Chemical and Materials Engineering at the University of Alberta. We develop theoretical machinery and high-performance (CUDA-enabled) computational libraries to study the mechanics of soft and active matter. Topics of interest include transport, microrheology, fluctuations, and collective motion of soft and active matter. Candidates with a strong background and interest in Fluid Mechanics, Thermodynamics, Statistical Mechanics, GPU computing, and/or Applied Mathematics are particularly encouraged to apply.

Active matter is a novel and exciting class of materials that include suspensions of motile bacteria, molecular motors, and synthetic self-propelled colloids. Potential applications of active matter include catalysis, chemical sensing, self-assembling intelligent materials, and targeted drug delivery. Interested candidates are encouraged to look through the attached references to gauge their interest in the field and fit for the position. For further information, please visit our web page at zhiweipeng.com.

Interested candidates should send their CV, a copy of their transcripts, and a brief description of their interests and motivation to Dr. Peng. Please note that only shortlisted candidates will receive a reply and incomplete inquires (e.g. missing CV) may not receive a reply.

For inquiries regarding postdoctoral positions, please email Dr. Peng and include your CV.

References

2024

AI-aided geometric design of anti-infection catheters

Tingtao Zhou, Xuan Wan, Daniel Zhengyu Huang, and 6 more authors

Science Advances, Jan 2024

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Bacteria can swim upstream in a narrow tube and pose a clinical threat of urinary tract infection to patients implanted with catheters. Coatings and structured surfaces have been proposed to repel bacteria, but no such approach thoroughly addresses the contamination problem in catheters. Here, on the basis of the physical mechanism of upstream swimming, we propose a novel geometric design, optimized by an artificial intelligence model. Using Escherichia coli, we demonstrate the anti-infection mechanism in microfluidic experiments and evaluate the effectiveness of the design in three-dimensionally printed prototype catheters under clinical flow rates. Our catheter design shows that one to two orders of magnitude improved suppression of bacterial contamination at the upstream end, potentially prolonging the in-dwelling time for catheter use and reducing the overall risk of catheter-associated urinary tract infection. A geometric design provides a simple solution for catheter-associated urinary tract infections.

2022

Activity-induced propulsion of a vesicle

Zhiwei Peng, Tingtao Zhou, and John F. Brady

Journal of Fluid Mechanics, May 2022

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Modern biomedical applications such as targeted drug delivery require a delivery system capable of enhanced transport beyond that of passive Brownian diffusion. In this work an osmotic mechanism for the propulsion of a vesicle immersed in a viscous fluid is proposed. By maintaining a steady-state solute gradient inside the vesicle, a seepage flow of the solvent (e.g., water) across the semipermeable membrane is generated which in turn propels the vesicle. We develop a theoretical model for this vesicle-solute system in which the seepage flow is described by a Darcy flow. Using the reciprocal theorem for Stokes flow it is shown that the seepage velocity at the exterior surface of the vesicle generates a thrust force which is balanced by the hydrodynamic drag such that there is no net force on the vesicle. We characterize the motility of the vesicle in relation to the concentration distribution of the solute confined inside the vesicle. Any osmotic solute is able to propel the vesicle so long as a concentration gradient is present. In the present work, we propose active Brownian particles (ABPs) as a solute. To maintain a symmetry-breaking concentration gradient, we consider ABPs with spatially varying swim speed and ABPs with constant properties but under the influence of an orienting field. In particular, it is shown that at high activity the vesicle velocity is \bU∼[K_\perp /(\eta_e\ell_m) ]∫\Pi_0^\mathrmswim \bn dΩ, where \Pi_0^\mathrmswim is the swim pressure just outside the thin accumulation boundary layer on the vesicle interior surface, \bn is the unit normal vector of the vesicle boundary, K_\perp is the membrane permeability, \eta_e is the viscosity of the solvent, and \ell_m is the membrane thickness.
Forced microrheology of active colloids

Zhiwei Peng, and John F. Brady

Journal of Rheology, Sep 2022

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Particle-tracking microrheology of dilute active (self-propelled) colloidal suspensions is studied by considering the external force required to maintain the steady motion of an immersed constant-velocity colloidal probe. If the probe speed is zero, the suspension microstructure is isotropic but exhibits a boundary accumulation of active bath particles at contact due to their self-propulsion. As the probe moves through the suspension, the microstructure is distorted from the nonequilibrium isotropic state, which allows us to define a microviscosity for the suspension using the Stokes drag law. For a slow probe, we show that active suspensions exhibit a swim-thinning behavior in which their microviscosity is gradually lowered from that of passive suspensions as the swim speed increases. When the probe speed is fast, the suspension activity is obscured by the rapid advection of the probe and the measured microviscosity is indistinguishable from that of passive suspensions. Generally for finite activity, the suspension exhibits a velocity-thinning behavior—though with a zero-velocity plateau lower than passive suspensions—as a function of the probe speed. These behaviors originate from the interplay between the suspension activity and the hard-sphere excluded-volume interaction between the probe and a bath particle.
Trapped-particle microrheology of active suspensions

Zhiwei Peng, and John F. Brady

The Journal of Chemical Physics, Sep 2022

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In microrheology, the local rheological properties, such as the viscoelasticity of a complex fluid, are inferred from the free or forced motion of embedded colloidal probe particles. Theoretical machinery developed for forced-probe microrheology of colloidal suspensions focused on either constant-force (CF) or constant-velocity (CV) probes, while in experiments, neither the force nor the kinematics of the probe is fixed. More importantly, the constraint of CF or CV introduces a difficulty in the meaningful quantification of the fluctuations of the probe due to a thermodynamic uncertainty relation. It is known that, for a Brownian particle trapped in a harmonic potential well, the product of the standard deviations of the trap force and the particle position is dkBT in d dimensions, with kBT being the thermal energy. As a result, if the force (position) is not allowed to fluctuate, the position (force) fluctuation becomes infinite. To allow the measurement of fluctuations in theoretical studies, in this work, we consider a microrheology model in which the embedded probe is dragged along by a moving harmonic potential so that both its position and the trap force are allowed to fluctuate. Starting from the full Smoluchowski equation governing the dynamics of N hard active Brownian particles, we derive a pair Smoluchowski equation describing the dynamics of the probe as it interacts with one bath particle by neglecting hydrodynamic interactions among particles in the dilute limit. From this, we determine the mean and the variance (i.e., fluctuation) of the probe position in terms of the pair probability distribution. We then characterize the behavior of the system in the limits of both weak and strong trap. By taking appropriate limits, we show that our generalized model can be reduced to the well-studied CF or CV microrheology models.

2020

Upstream swimming and Taylor dispersion of active Brownian particles

Zhiwei Peng, and John F. Brady

Physical Review Fluids, Jul 2020

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Locomotion of self-propelled particles such as motile bacteria or phoretic swimmers often takes place in the presence of applied flows and confining boundaries. Interactions of these active swimmers with the flow environment are important for the understanding of many biological processes, including infection by motile bacteria and the formation of biofilms. Recent experimental and theoretical works have shown that active particles in a Poiseuille flow exhibit interesting dynamics including accumulation at the wall and upstream swimming. Compared to the well-studied Taylor dispersion of passive Brownian particles, a theoretical understanding of the transport of active Brownian particles (ABPs) in a pressure-driven flow is relatively less developed. In this paper, employing a small wave-number expansion of the Smoluchowski equation describing the particle distribution, we explicitly derive an effective advection-diffusion equation for the cross-sectional average of the particle number density in Fourier space. We characterize the average drift (specifically upstream swimming) and effective longitudinal dispersion coefficient of active particles in relation to the flow speed, the intrinsic swimming speed of the active particles, their Brownian diffusion, and the degree of confinement. In contrast to passive Brownian particles, both the average drift and the longitudinal dispersivity of ABPs exhibit a nonmonotonic variation as a function of the flow speed. In particular, the dispersion of ABPs includes the classical shear-enhanced (Taylor) dispersion and an active contribution called the swim diffusivity. In the absence of translational diffusion, the classical Taylor dispersion is absent and we observe a giant longitudinal dispersion in the strong flow limit. Our continuum theory is corroborated by a direct Brownian dynamics simulation of the Langevin equations governing the motion of each ABP.