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Mohamed Laradji

Publications and source records attributed to Mohamed Laradji.

9 recordsLinked to original sources
arXiv2026

Stress-Boundary-Memory Feedback Drives Vortical-Polar Transitions in Softly Confined Active Matter

We computationally investigate how environmental sensitivity of active matter interacts with soft confinement to shape collective dynamics. In our model, the active constituents are represented as self-propelled particles (SPPs), implemented as nematic, disjoint ring polymers whose direction of motion can reverse without tumbling. Coarse-grained molecular dynamics simulations reveal that collective dynamics arise from a three-way feedback between active stresses, boundary elasticity, and particle-level memory. With increasing driving force, FD, this feedback generates a sequence of collective dynamical regimes. At low FD, SPP motion is dominated by thermal fluctuations. At intermediate FD, coherent vortical motion emerges with intermittent, noise-driven reversals. With further increase in FD, reversals are suppressed, yielding sustained unidirectional vortical motion. At sufficiently high FD, the system transitions to a polar state characterized by strong nematic ordering of the SPPs, symmetry breaking of the enclosure shape, and persistent polar collective motion. In this regime, the SPPs accumulate at the leading edge of the enclosure, driving sustained ballistic propulsion. These results demonstrate how environmental sensitivity and soft confinement jointly regulate emergent collective states and identify boundary elasticity as a control parameter governing the balance between vortical and ballistic dynamics.

cond-mat.soft↗
arXiv2023

Collective Vortical Motion and Vorticity Reversals of Self-Propelled Particles on Circularly Patterned Substrates

The collective behavior of self-propelled particles (SPPs) under the combined effects of a circularly patterned substrate and circular confinement is investigated through coarse-grained molecular dynamics simulations of polarized and disjoint ring polymers. The study is performed over a wide range of values of the SPPs packing fraction $\barϕ$, motility force $F_D$, and area fraction of the patterned region. At low packing fractions, the SPPs are excluded from the system's center and exhibit a vortical motion that is dominated by the substrate at intermediate values of $F_D$. This exclusion zone is due to the coupling between the driving force and torque induced by the substrate, which induces an outward spiral motion of the SPPs. For high values of $F_D$, the SPPs exclusion from the center is dominated by the confining boundary. At high values of $\barϕ$, the substrate pattern leads to reversals in the vorticity, which become quasi-periodic with increasing $\barϕ$. We also found that the substrate pattern is able to separate SPPs based on their motilities.

cond-mat.soft↗
arXiv2019

Some results on double triangle descendants of $K_5$

Double triangle expansion is an operation on $4$-regular graphs with at least one triangle which replaces a triangle with two triangles in a particular way. We study the class of graphs which can be obtained by repeated double triangle expansion beginning with the complete graph $K_5$. These are called double triangle descendants of $K_5$. We enumerate, with explicit rational generating functions, those double triangle descendants of $K_5$ with at most four more vertices than triangles. We also prove that the minimum number of triangles in any $K_5$ descendant is four. Double triangle descendants are an important class of graphs because of conjectured properties of their Feynman periods when they are viewed as scalar Feynman diagrams, and also because of conjectured properties of their $c_2$ invariants, an arithmetic graph invariant with quantum field theoretical applications.

math.CO↗
arXiv2011

Computer Simulation of Cytoskeleton-Induced Blebbing in Lipid Membranes

Blebs are balloon-shaped membrane protrusions that form during many physiological processes. Using computer simulation of a particle-based model for self-assembled lipid bilayers coupled to an elastic meshwork, we investigated the phase behavior and kinetics of blebbing. We found that blebs form for large values of the ratio between the areas of the bilayer and the cytoskeleton. We also found that blebbing can be induced when the cytoskeleton is subject to a localized ablation or a uniform compression. The results obtained are qualitatively in agreement with the experimental evidence and the model opens up the possibility to study the kinetics of bleb formation in detail.

physics.bio-ph↗
arXiv2008

Lipid Flip-Flop Driven Mechanical and Morphological Changes in Model Membranes

We study, using dissipative particle dynamics simulations, the effect of active lipid flip-flop on model fluid bilayer membranes. We consider both cases of symmetric as well as asymmetric flip-flops. Symmetric flip-flop leads to a steady state of the membrane with an effective temperature higher than that of the equilibrium membrane and an effective surface tension lower than that of the equilibrium membrane. Asymmetric flip-flop leads to transient conformational changes of the membrane in the form of bud or blister formation, depending on the flip rate.

cond-mat.soft↗
arXiv2006

Anomalously Slow Domain Growth in Fluid Membranes with Asymmetric Transbilayer Lipid Distribution

The effect of asymmetry in the transbilayer lipid distribution on the dynamics of phase separation in fluid vesicles is investigated numerically for the first time. This asymmetry is shown to set a spontaneous curvature for the domains that alter the morphology and dynamics considerably. For moderate tension, the domains are capped and the spontaneous curvature leads to anomalously slow dynamics, as compared to the case of symmetric bilayers. In contrast, in the limiting cases of high and low tensions, the dynamics proceeds towards full phase separation.

cond-mat.soft↗
arXiv2006

Domain Growth, Budding, and Fission in Phase Separating Self-Assembled Fluid Bilayers

A systematic investigation of the phase separation dynamics in self-assembled multi-component bilayer fluid vesicles and open membranes is presented. We use large-scale dissipative particle dynamics to explicitly account for solvent, thereby allowing for numerical investigation of the effects of hydrodynamics and area-to-volume constraints. In the case of asymmetric lipid composition, we observed regimes corresponding to coalescence of flat patches, budding, vesiculation and coalescence of caps. The area-to-volume constraint and hydrodynamics have a strong influence on these regimes and the crossovers between them. In the case of symmetric mixtures, irrespective of the area-to-volume ratio, we observed a growth regime with an exponent of 1/2. The same exponent is also found in the case of open membranes with symmetric composition.

cond-mat.soft↗
arXiv2004

Dynamics of Domain Growth in Self-Assembled Fluid Vesicles

The dynamics of phase separation in multi-component bilayer fluid vesicles is investigated by means of large-scale dissipative particle dynamics. The model explicitly accounts for solvent particles, thereby allowing for the very first numerical investigation of the effects of hydrodynamics and area-to-volume constraints. We observed regimes corresponding to coalescence of flat patches, budding and vesiculation, and coalescence of caps. We point out that the area-to-volume constraint has a strong influence on crossovers between these regimes.

cond-mat.soft↗