The flow of homogeneous fluids through porous media: analogies with other physical problems

Fundamental aspects of steady-state conversion of heat to work at the nanoscale

The separation of biological cells by filtration through microstructured constrictions is limited by unpredictable variations of the filter hydrodynamic resistance as cells accumulate in the microstructure. Applying a reverse flow to unclog the filter will undo the separation and reduce filter selectivity because of the reversibility of low-Reynolds number flow. We introduce a microfluidic structural ratchet mechanism to separate cells using oscillatory flow. Using model cells and microparticles, we confirmed the ability of this mechanism to sort and separate cells and particles based on size and deformability. We further demonstrate that the spatial distribution of cells after sorting is repeatable and that the separation process is irreversible. This mechanism can be applied generally to separate cells that differ based on size and deformability.

Cell separation based on size and deformability using microfluidic funnel ratchets

We present a complete theoretical study of the relationship between the structure (tip shape and dimensions) and function (selectivity and rectification) of asymmetric nanopores on the basis of previous experimental studies. The theoretical model uses a continuum approach based on the Nernst-Planck equations. According to our results, the nanopore transport properties, such as current-voltage (I-V) characteristics, conductance, rectification ratio, and selectivity, are dictated mainly by the shape of the pore tip (we have distinguished bullet-like, conical, trumpet-like, and hybrid shapes) and the concentration of pore surface charges. As a consequence, the nanopore performance in practical applications will depend not only on the base and tip openings but also on the pore shape. In particular, we show that the pore opening dimensions estimated from the pore conductance can be very different, depending on the pore shape assumed. The results obtained can also be of practical relevance for the design of nanopores, nanopipettes, and nanoelectrodes, where the electrical interactions between the charges attached to the nanostructure and the mobile charges confined in the reduced volume of the inside solution dictate the device performance in practical applications. Because single tracks are the elementary building blocks for nanoporous membranes, the understanding and control of their individual properties should also be crucial in protein separation, water desalination, and bio-molecule detection using arrays of identical nanopores.

https://iopscience.iop.org/article/10.1088/0957-4484/19/31/315707/pdf

Pore structure and function of synthetic nanopores with fixed charges: tip shape and rectification properties

This review provides an overview of recent advances that have been achieved in understanding the basic physics of friction and energy dissipation in molecularly thin adsorbed films and the associated impact on friction at microscopic and macroscopic length scales. Topics covered include a historical overview of the fundamental understanding of macroscopic friction, theoretical treatments of phononic and electronic energy dissipation mechanisms in thin films, and current experimental methods capable of probing such phenomena. Measurements performed on adsorbates sliding in unconfined geometries with the quartz crystal microbalance technique receive particular attention. The final sections review the experimental literature of how measurements of sliding friction in thin films reveal energy dissipation mechanisms and how the results can be linked to film-spreading behavior, lubrication, film phase transitions, superconductivity-dependent friction, and microelectromechanical systems applications. Materials s...

Friction and energy dissipation mechanisms in adsorbed molecules and molecularly thin films

The logistic differential equation is used to analyze cancer cell population, in the presence of a correlated Gaussian white noise. We study the steady state properties of tumor cell growth and discuss the effects of the correlated noise. It is found that the degree of correlation of the noise can cause tumor cell extinction.

Correlated noise in a logistic growth model.

With the disruptive carbon cycle being blamed for global warming, the plausible electrocatalytic CO2 reduction reaction (CO2RR) to form valuable C2+ hydrocarbons and feedstock is becoming a hot topic. Cu-based electrocatalysts have been proven to be excellent CO2RR alternatives for high energy value-added products in this regard. However, the selectivity of CO2RR to form C2+ products via Cu-based catalysts suffers from a high overpotential, slow reaction kinetics, and low selectivity. This review attempts to discuss various cutting-edge strategies for understanding catalytic design such as Cu-based catalyst surface engineering, tuning Cu bandgap via alloying, nanocatalysis, and the effect of the electrolyte and pH on catalyst morphology. The most recent advances in in situ spectroscopy and computational techniques are summarized to fully comprehend reaction mechanisms, structural transformation/degradation mechanisms, and crystal facet loss with subsequent effects on catalyst activity. Furthermore, approaches for tuning Cu interactions are discussed from four key perspectives: single-atom catalysts, interfacial engineering, metal-organic frameworks, and polymer-incorporated materials, which provide new insights into the selectivity of C2+ products. Finally, major challenges are outlined, and potential prospects for the rational design of catalysts for robust CO2RR are proposed. The integration of catalytic design with mechanistic understanding is a step forward in the promising advancement of CO2RR technology for industrial applications.

Electrochemical CO
 2 Reduction to C
 2+ Products Using Cu-Based Electrocatalysts: A Review

Numerical simulations of pulsating blood flow through models of stenotic and tapered arteries have been performed to investigate the distributions of the wall shear stress. In this paper, a mathematical model of a rigid tapered artery with axially symmetric stenosis is established. The incompressible Navier-Stokes equations are solved numerically by using the finite difference method. The effects of tapering and stenosis are considered. The results show that stenosis disturbs the flow field at the vicinity of the stenosis, especially at the throat and downstream, and easily leads to the formation of a flow separation region in the post-stenotic region.

Numerical Study of Pulsating Flow Through a Tapered Artery with Stenosis

We study the energetics of a thermal motor driven by
temperature differences, which consists of a Brownian particle
moving in a sawtooth potential with an external load where the
viscous medium is periodically in contact with hot and cold heat
reservoir along space coordinate. The motor can work as a heat
engine or a refrigerator under different conditions. The heat flow
via both potential and kinetic energy is considered. The former is
reversible when the engine works quasistatically and the latter is
always irreversible. The efficiency of the heat engine can never
approach Carnot efficiency.

Liang-Gang Liu

Papers

Correlated noise in a logistic growth model.

Electrochemical CO 2 Reduction to C 2+ Products Using Cu-Based Electrocatalysts: A Review

Numerical Study of Pulsating Flow Through a Tapered Artery with Stenosis

Heat flow and efficiency in a microscopic engine

Brownian micro-engines and refrigerators in a spatially periodic temperature field: Heat flow and performances