arXiv subjects

John Jairo Molina

Publications and source records attributed to John Jairo Molina.

7 recordsLinked to original sources

arXiv2020

Dynamics of a chiral swimmer sedimenting on a flat plate

Three-dimensional simulations with fully resolved hydrodynamics are performed to study the dynamics of a single squirmer under gravity, in order to clarify its motion in the vicinity of a flat plate. Different dynamics emerge for different gravity strengths. In a moderate gravity regime, neutral squirmers and pullers eventually stop moving and reorient in a direction perpendicular to the plate; pushers, instead, exhibit continuous motion in a tilted direction. In the strong gravity regime, all types of squirmers sediment and reorient perpendicularly to the plate. In this study, the chirality is introduced to model realistic micro-swimmers, and its crucial effects on the swimmer dynamics are presented.

physics.flu-dyn↗

arXiv2017

Hydrodynamic synchronization of externally driven colloids

The collective dynamics of externally driven $N_{\rm p}$-colloidal systems ($1\le N_{\rm p}\le 4$) in a confined viscous fluid have been investigated using three-dimensional direct numerical simulations with fully resolved hydrodynamics. The dynamical modes of collective particle motion are studied by changing the particle Reynolds number {as determined by the strength of the external driving force} and the confining wall distance. For a system with $N_{\rm p}= 3$, we found that at a critical Reynolds number, a dynamical mode transition occurs from the doublet-singlet mode to the triplet mode, which has not been reported experimentally. The dynamical mode transition was analyzed in detail from the following two viewpoints: (1) spectrum analysis of the time evolution of a tagged particle velocity and (2) the relative acceleration of the doublet cluster with respect to the singlet particle. For a system with $N_{\rm p}=4$, we found similar dynamical mode transitions from the doublet-singlet-singlet mode to the triplet-singlet mode and further to the quartet mode.

cond-mat.soft↗

arXiv2017

Hydrodynamic alignment of microswimmers in pipes

We investigate the dynamics of model microswimmers under confinement, in cylindrical geometries, by means of three dimensional direct numerical calculations with fully resolved hydrodynamics. Such swimmers are known to show collective alignment in bulk, and we confirmed that similar alignment can be observed even in pipes, although the volume fraction dependency shows qualitative differences. By comparing the structural information, we investigated the cause of such differences. We found an order/disorder phase transition for a specific type of swimmers, as the size of the pipe becomes comparable to the size of the swimmers. Such dynamics are found not to depend on the geometry of confinement.

cond-mat.soft↗

arXiv2017

Do hydrodynamically assisted binary collisions lead to orientational ordering of microswimmers?

We have investigated the onset of collective motion in systems of model microswimmers, by performing a comprehensive analysis of the binary collision dynamics using three dimensional direct numerical simulations (DNS) with fully resolved hydrodynamics. From this data, we have constructed a simplified binary collision model (BCM) which accurately reproduces the collective behavior obtained from the DNS for most cases. Thus, we show that global alignment can mostly arise solely from binary collisions. Although the agreement between both models (DNS and BCM) is not perfect, the parameter range in which notable differences appear is also that for which strong density fluctuations are present in the system (where pseudo-sound mound can be observed[1]).

physics.flu-dyn↗

arXiv2016

Collective motion of cells crawling on a substrate: roles of cell shape and contact inhibition

Contact inhibition plays a crucial role in the motility of cells, the process of wound healing, and the formation of tumors. By mimicking the mechanical motion of calls crawling on a substrate using a pseudopod, we constructed a minimal model for migrating cells which gives rise to contact inhibition of locomotion (CIL) naturally. The model cell consists of two disks, one in the front (a pseudopod) and the other one in the back (cell body), connected by a finitely extensible spring. Despite the simplicity of the model, the cells' collective behavior is highly nontrivial, depending on the shape of cells and whether CIL is enabled or not. Cells with a small front circle (i.e. a narrow pseudopod) form immobile colonies. In contrast, cells with a large front circle (i.e. such as a lamellipodium) exhibit coherent migration without any explicit alignment mechanism being present in the model. This suggests that crawling cells often exhibit broad fronts because it helps them avoid clustering. Upon increasing the density, the cells develop density waves which propagate against the direction of cell migration and finally arrest at higher densities.

cond-mat.soft↗

arXiv2015

Purely hydrodynamic origin for swarming of swimming particles

Three-dimensional simulations with fully resolved hydrodynamics are performed to study the collective motion of model swimmers in confinement. We show that certain swimming mechanisms can lead to traveling wave-like collective motion even without any direct alignment mechanism. It is also shown that by varying the swimming mechanism, this collective motion can be suppressed, contrary to the perception that hydrodynamic effects are completely screened at high volume fraction. From an analysis of bulk systems, it is shown that this traveling wave-like motion, which can be characterized as a pseudo-acoustic mode, is mainly due to the intrinsic swimming property of the particles.

cond-mat.soft↗

arXiv2013

Direct numerical simulations of rigid body dispersions. I. Mobility/Friction tensors of assemblies of spheres

An improved formulation of the Smoothed Profile method is introduced to perform direct numerical simulations of arbitrary rigid body dispersions in a Newtonian host solvent. Previous implementations of the method were restricted to spherical particles, severely limiting the types of systems that could be studied. The validity of the method is carefully examined by computing the friction/mobility tensors for a wide variety of geometries and comparing them to reference values obtained from accurate solutions to the Stokes-Equation.

cond-mat.soft↗