About

Richard M. Lueptow is a Professor of Mechanical Engineering and (by courtesy) Chemical and Biological Engineering at Northwestern University. He serves as the Senior Associate Dean and is affiliated with the Department of Mechanical Engineering. His educational background includes a Doctor of Science (Sc.D.) and a Master of Science (S.M.) in Mechanical Engineering from MIT, as well as a Bachelor of Science (B.S.) in Engineering from Michigan Technological University. His current research interests focus on modeling, simulation, and experiments related to the segregation and mixing of bi- and poly-disperse granular materials, both spherical and non-spherical, in various geometries, as well as molecular-level simulation of transport processes in polymeric nanofiltration and reverse osmosis membranes. Lueptow has made significant contributions to the understanding of granular flow dynamics and membrane transport phenomena, with a notable record of recognition including an honorary doctorate from Aix-Marseille University, fellowships in prominent engineering societies, and multiple awards for teaching and research excellence. His professional service includes editorial roles in leading scientific journals, and he is recognized for his leadership in advancing engineering research and education.

Research topics

• Physics
• Thermodynamics
• Chemistry
• Materials science
• Mechanics
• Chemical physics
• Chemical engineering
• Geology
• Environmental engineering
• Classical mechanics
• Mathematics
• Geometry
• Geotechnical engineering
• Geography
• Composite material
• Statistical physics

Selected publications

• Percolation of a cohesive fine particle in a static bed

  arXiv (Cornell University) · 2026-05-19

  preprintOpen accessSenior author

  Percolation of fine particles (fines) in a static bed of larger particles is central to many industrial and natural processes. Non-cohesive fines either pass through the bed or become trapped depending on multiple factors including particle sizes, friction and restitution coefficients, and size-polydispersity. Here we consider the additional factor of cohesion. We use the discrete element method to simulate gravity-driven percolation of cohesive fine particles through a static bed of randomly packed large particles; fines interact with bed particles but not with each other. A large-to-fine particle diameter ratio of 7 geometrically permits non-cohesive fines to pass the narrowest pore throats formed by the large particles so they can freely percolate. However, sufficiently large cohesion and friction lead to non-geometric trapping. Fines are trapped when they fail to rebound after a collision, due to large cohesion, low restitution, and low collision velocity, and any subsequent rolling or sliding is insufficient to cause detachment. This establishes a sequence of local interactions -- collision, adhesion, and post-contact motion -- that governs the ultimate fate of a fine particle. A collisional model that incorporates a trapping probability per collision and a collision frequency predicts the trapping distance in the regime dominated by collision-induced trapping. For non-rebounding collisions, frictional effects are enhanced by cohesion and, when large enough, prevent the fine particle from subsequently detaching. A static equilibrium condition based on force balance predicts whether a fine particle remains stationary after contact. These results show that percolation of cohesive fine particles is not determined by geometric accessibility alone, but also by particle-scale interaction dynamics that can override geometric expectations.

  PublisherDOI

• Percolation of a cohesive fine particle in a static bed

  ArXiv.org · 2026-05-19

  articleOpen accessSenior author

  Percolation of fine particles (fines) in a static bed of larger particles is central to many industrial and natural processes. Non-cohesive fines either pass through the bed or become trapped depending on multiple factors including particle sizes, friction and restitution coefficients, and size-polydispersity. Here we consider the additional factor of cohesion. We use the discrete element method to simulate gravity-driven percolation of cohesive fine particles through a static bed of randomly packed large particles; fines interact with bed particles but not with each other. A large-to-fine particle diameter ratio of 7 geometrically permits non-cohesive fines to pass the narrowest pore throats formed by the large particles so they can freely percolate. However, sufficiently large cohesion and friction lead to non-geometric trapping. Fines are trapped when they fail to rebound after a collision, due to large cohesion, low restitution, and low collision velocity, and any subsequent rolling or sliding is insufficient to cause detachment. This establishes a sequence of local interactions -- collision, adhesion, and post-contact motion -- that governs the ultimate fate of a fine particle. A collisional model that incorporates a trapping probability per collision and a collision frequency predicts the trapping distance in the regime dominated by collision-induced trapping. For non-rebounding collisions, frictional effects are enhanced by cohesion and, when large enough, prevent the fine particle from subsequently detaching. A static equilibrium condition based on force balance predicts whether a fine particle remains stationary after contact. These results show that percolation of cohesive fine particles is not determined by geometric accessibility alone, but also by particle-scale interaction dynamics that can override geometric expectations.

  PublisherOA PDF

• Improved velocity-Verlet algorithm for the discrete element method

  Computer Physics Communications · 2025-01-30 · 18 citations

  article
  PublisherDOI

• Diffusion in Granular Mixtures

  Diffusion fundamentals. · 2025-11-03

  articleOpen access

  Granular materials, composed of discrete macroscopic particles such as sand, are ubiquitous in both natural and industrial contexts.These materials exhibit unique mechanical and transport properties due to their discrete nature and interparticle interactions through contact forces.Transport of fine particles within granular media plays a central role in processes ranging from chute flows and silos to geophysical flows and powder handling 1 .In such systems, the interplay of segregation 2 , confinement 3 , and diffusion 4 leads to complex dynamics that are not fully captured by existing models.A particular challenge arises at large particle size ratios, where fines can navigate void networks within the granular bed, resulting in transport mechanisms distinct from those in monodisperse or low size ratio systems 5 .We investigate fine particle diffusion across varying fine-particle concentrations using large-scale Discrete Element Method (DEM) simulations and find that the diffusion coefficient decreases with increasing concentration and size ratio.Drawing inspiration from kinetic theory, we develop a scaling framework that links particle concentration, size ratio, and bed geometry to diffusion behavior in dense granular beds.The framework has broad relevance for both industrial applications, such as mixers, separators, and hoppers, and fundamental studies of diffusion in heterogeneous media.

  PublisherOA PDFDOI

• Membrane Charge Effects on Solute Transport in Nanofiltration: Experiments and Molecular Dynamics Simulations

  Membranes · 2025-06-18 · 8 citations

  articleOpen accessSenior authorCorresponding

  Polyamide membranes, such as nanofiltration (NF) membranes, are widely used for water purification. However, the mechanisms of solute transport and solute rejection due to solute charge interactions with the membrane remain unclear at the molecular level. Here, we use molecular dynamics simulations to examine the transport of single-solute feeds through charged nanofiltration membranes with different membrane charge concentrations of COO- and NH+2 resulting from the deprotonation or protonation of polymeric end groups according to the pH level that the membrane experiences. The results show that Na+ and Cl- solute ions are better rejected when the membrane has a higher concentration of negatively charged groups, corresponding to a higher pH, whereas CaCl2 is well rejected at all pH levels studied. These results are consistent with those of experiments performed at the same pH conditions as the simulation setup. Moreover, solute transport behavior depends on the membrane functional group distribution. When COO- functional groups are concentrated at membrane feed surface, ion permeation into the membrane is reduced. Counter-ions tend to associate with charged functional groups while co-ions seem to pass by the charged groups more easily. In addition, steric effects play a role when ions of opposite charge cluster in pores of the membrane. This study reveals solute transport and rejection mechanisms related to membrane charge and provides insights into how membranes might be designed to achieve specific desired solute rejection.

  PublisherOA PDFDOI

• Membrane Charge Effects on Solute Transport in Polyamide Membranes

  ArXiv.org · 2025-03-13

  preprintOpen accessSenior author

  Polyamide membranes, such as nanofiltration (NF) and reverse osmosis (RO) membranes, are widely used for water desalination and purification. However, the mechanisms of solute transport and solute rejection due to charge interactions remain unclear at the molecular level. Here we use molecular dynamics (MD) simulations to examine the transport of single-solute feeds through charged nanofiltration membranes with different membrane charge concentrations of COO$^{\text{-}}$ and NH$_2\!^+$ corresponding to different pH levels. Results show that Na$^+$ and Cl$^{\text{-}}$ solute ions are better rejected when the membrane has a higher concentration of negatively charged groups, corresponding to a higher pH, whereas CaCl$_2$ is well-rejected at all pH levels studied. These results are consistent with experimental findings which are performed at the same pH conditions as simulation setup. Moreover, solute transport behavior depends on the membrane functional group distribution. When COO$^{\text{-}}$ functional groups are concentrated at membrane feed surface, ion permeation into the membrane is reduced. Counter-ions tend to associate with charged functional groups while co-ions seem to pass by the charged groups more easily. In addition, steric effects play a role when ions of opposite charge cluster in pores of the membrane. This study reveals solute transport and rejection mechanisms related to membrane charge and provides insights into how membranes might be designed to achieve specific desired solute rejection.

  PublisherOA PDFDOI

• Rayleigh-Taylor instability in size-bidisperse isodense granular flow down an incline

  Physical review. E · 2025-12-23

  articleOpen access

  Size-bidisperse granular material flowing down a rough incline, or chute, may develop a Rayleigh-Taylor instability even when the two particle species have the same density. Unlike fluid instability, which occurs under quiescent conditions, the granular material must be flowing for the instability to occur. Initially, size segregation results in a monodisperse layer of large particles above a monodisperse layer of small particles with an interfacial layer of mixed particles that has a higher volume fraction and, hence, is denser than the monodisperse layers above and below it. The interfacial layer destabilizes via a Rayleigh-Taylor instability and forms dense descending plumes of mixed particles and ascending plumes of less dense pure small particles. As a result of these plumes, the upper layer of large particles breaks and accumulates above the descending plumes; the small-particle plumes reach the free surface. The instability evolves into persistent longitudinal rolls corresponding to streamwise bands, or stripes, of small and large particles at the free surface. The appearance of the instability depends on the particle and mixture properties as well as the flow conditions. In all cases, the propensity for the appearance of the instability and subsequent band formation can be traced back to the total particle volume fraction, or packing density, in the mixed-particle layer, which depends on the particle size ratio, fraction of each particle species, and thickness of the flow.

  PublisherOA PDFDOI

• Granular segregation across flow geometries: a closure model for the particle segregation velocity

  Journal of Fluid Mechanics · 2025-07-28 · 2 citations

  preprintOpen accessSenior authorCorresponding

  Predicting particle segregation has remained challenging due to the lack of a general model for the segregation velocity that is applicable across a range of granular flow geometries. Here, a segregation-velocity model for dense granular flows is developed by exploiting force balance and recent advances in particle-scale modelling of the segregation driving and drag forces over the entire particle concentration range, size ratios up to 3 and inertial numbers as large as 0.4. This model is shown to correctly predict particle segregation velocity in a diverse set of idealised and natural granular flow geometries simulated using the discrete element method. When incorporated in the well-established advection–diffusion–segregation formulation, the model has the potential to accurately capture segregation phenomena in many relevant industrial applications and geophysical settings.

  PublisherOA PDFDOI

• Mobile-collector capture of particles in a chaotic flow

  PLoS ONE · 2025-08-07

  articleOpen accessSenior authorCorresponding

  Removing dispersed material, such as pollutants, from dynamic fluid environments like the ocean or the atmosphere is challenging when the flow is chaotic. Here the capture of passive tracer particles by a mobile collector (MC) is studied in a model two-dimensional chaotic flow with vortices. Four simple capture strategies for determining the MC direction are considered, all of which rely on periodic measurement of the local particle distribution. The ultimate success of a strategy depends on its associated motion and detection parameters as well as the underlying fluid flow. When the flow is fully chaotic or the relative velocity of the MC is large, the four strategies exhibit nearly equal effectiveness. However, when the flow is less chaotic and the relative MC velocity is small, the collector can become trapped in or outside of a vortex. Changing the particle detection parameters can prevent trapping, which improves capture. In the absence of trapping and for both high and low relative velocities of the MC, a scaling analysis explains the dependence of the capture rate on the relevant dimensionless variables based on timescales for the mobile collector and the underlying flow. For a wide range of parameters and all four capture strategies, the capture timescale depends linearly on a combination of the characteristic kinematic timescale related to the relative motion of the collector and the gradient timescale related to the underlying flow field, confirming that the capture process is properly characterized.

  PublisherOA PDFDOI

• Membrane Charge Effects on Solute Transport in Polyamide Membranes

  SSRN Electronic Journal · 2025-01-01 · 2 citations

  preprintOpen accessSenior author
  PublisherDOI

Recent grants

• GOALI: Charge Interactions in Transport of Mixed Solutes in Nanofiltration Membranes

  NSF · $334k · 2019–2025

• Reactive Membrane Technology for Water Treatment

  NSF · $404k · 2004–2009

• GOALI: Fine Particle De-Mixing in Granular Flows

  NSF · $462k · 2022–2026

• GOALI: Flow driven segregation at the particle level

  NSF · $342k · 2019–2024

Frequent coauthors

• Julio M. Ottino

  184 shared

• Paul B. Umbanhowar

  Northwestern University

  126 shared

• É. Serre

  Centre National de la Recherche Scientifique

  37 shared

• Yi Fan

  Liaoning Technical University

  19 shared

• Hongyi Xiao

  Friedrich-Alexander-Universität Erlangen-Nürnberg

  19 shared

• Umberto D’Ortona

  18 shared

• Ivan C. Christov

  Purdue University West Lafayette

  18 shared

• Denis Martinand

  Centre National de la Recherche Scientifique

  17 shared

Education

• ScD

  Massachusetts Institute of Technology

  1986

• SM

  Massachusetts Institute of Technology

  1980

• BS

  Michigan Technological University

  1978

Awards & honors

• Docteur Honoris Causa (honorary doctorate) from Aix-Marseill…
• Fellow of American Institute of Chemical Engineers, 2023
• American Institute of Chemical Engineers Dow Particle Proces…
• Northwestern University McCormick School of Engineering and…
• Society of Automotive Engineers Teetor Educational Award