arXiv subjects

P. S. Burada

Publications and source records attributed to P. S. Burada.

At least 19 recordsLinked to original sources

arXiv2025

Inertial active particles in a Poiseuille flow: negative mobility and particle separation

The diffusive behavior of small entities is strongly influenced by the flow of the surrounding medium, which is ubiquitous in natural and artificial environments. In this study, we investigate the transport characteristics of the inertial active Brownian particles (ABPs) in a microfluidic channel under a Poiseuille flow. The interplay between the inertia of the particles and the imposed fluid flow leads to interesting diffusive behaviors. For instance, in the overdamped regime ($m \to 0$), particles exhibit a negative average velocity $\langle v \rangle$ due to upstream movement. As $m$ increases, particles tend to move along the flow direction with an increase in $\langle v \rangle$ in the positive direction, exhibiting a maximum at optimal $m$, and diminish for higher $m$ values. The effective diffusion coefficient $D_{eff}$ also shows a peak at this optimal $m$. Interestingly, at higher $m$ values, $D_{eff}$ decreases with increasing the noise strength. The self-propelled velocity of the particles further enhances the upstream movement. Further, the rotation rate of the particles also contributes positively to the upstream motion, and enhances the diffusion of the particles by many orders in the limit of higher $m$. This study reveals that inertia not only modifies swimmer flow interactions but also enables new dynamical regimes, where mass-dependent trajectories can be harnessed for selective control. Such control holds promise for mass based particle separation in precisely engineered environments and lab on a chip devices for technological applications.

cond-mat.soft↗

arXiv2025

Entropic active particle transport in pulsating 3D geometries

We study the transport of active Brownian particles (ABPs) in three-dimensional (3D) oscillatory geometries, which are spatially periodic. We establish a generalized Fick-Jacobs approach, which reduces a 3D system to an effective 1D system based on the assumption that a fast equilibration of particles along the transversal directions of the geometry. The transport characteristics of ABPs are computed semi-analytically and corroborated by numerical simulations. At the optimal frequency of the geometry oscillation, particles exhibit higher average velocity $\langle v \rangle$ and effective diffusion coefficient $D_{\text{eff}}$, resembling the phenomena of stochastic resonance. This effect is further enhanced by the self-propelled velocity of ABPs and the amplitude of geometry oscillations. These findings have significant implications for the development of micro- and nanofluidic devices with enhanced control over particle transport and precise manipulation of small-scale biomedical devices.

cond-mat.soft↗

arXiv2025

Active chiral rotors: hydrodynamics and chemotaxis

An active chiral rotor is a spherical object that can generate chiral flows in a fluid by rotating about an axis. For example, if the flow around the upper hemisphere of the chiral rotor is in a clockwise direction, then the flow in the lower hemisphere is in the anti-clockwise direction, and vice versa. In this paper, we aim to study the combined behaviour of hydrodynamically interacting chiral rotors in the presence of an external chemical gradient. While a single isolated rotor is stationary in a fluid, a pair of rotors can move in linear or circular paths as they hydrodynamically interact with each other. It is observed that the emergent linear or circular trajectories depend on the type of rotors and the orientation of their rotation axes. The dynamics of the rotors are altered in a more complex environment, such as in an external chemical field. Interestingly, we observe two types of motion: chemotaxis and anti-chemotaxis. While in the anti-chemotaxis case, both rotors are driving away from the target, in the chemotaxis case, one of the rotors successfully reaches the chemical target. This study helps to understand the collective behavior of self-propelled microorganisms and artificial swimmers.

cond-mat.soft↗

arXiv2024

Chiral swimmer with a regular arbitrary active patch

We investigate the low Reynolds number hydrodynamics of a spherical swimmer with a predominantly hydrophobic surface, except for a hydrophilic active patch. This active patch covers a portion of the surface and exhibits chiral activity that varies as a function of $θ$ and $ϕ$. Our study considers two types of active patches: (i) a symmetric active patch (independent of $ϕ$) and (ii) an arbitrary active patch (depends on both $θ$ and $ϕ$). The swimming velocity, rotation rate, and flow field of the swimmer are calculated analytically. The objective of this work is to find the optimal configurations for both patch models to maximize the swimmer's velocity and efficiency. Interestingly, the maximum velocity can be controlled by adjusting the hydrophobicity, patch configuration, and strength of the surface activity. We find that for the symmetric patch model, the swimmer's velocity is $U_{SP} = 1.414 U_s$, where $U_s$ is the velocity of a swimmer whose surface is fully covered with chiral activity as a reference. For the arbitrary patch model, the velocity is $U_{AP} = 1.45 U_s$, which is higher than that of the symmetric patch model. Our results indicate that swimmers with low hydrophobicity exhibit efficient swimming characteristics. Additionally, due to the incomplete coverage of the active patch, the Stokeslet and Rotlet terms appear in the flow field generated by the swimmer, which is a deviation compared to the case of a swimmer whose surface is fully covered with chiral activity. This study provides insights useful for designing synthetic active particles, which can be applied, for example, in targeted drug delivery, chemotaxis, and phototaxis.

physics.flu-dyn↗

arXiv2024

Role of obstacle softness in the diffusive behavior of active Particles

We numerically investigate the diffusive behavior of active Brownian particles in a two-dimensional confined channel filled with soft obstacles, whose softness is controlled by a parameter $K$. Here, active particles are subjected to external bias $F$. Particle diffusion is influenced by entropic barriers that arise due to variations in the shape of the chosen channel geometry. We observed that the interplay between obstacle softness, entropic barriers, and external bias leads to striking transport characteristics of the active particles. For instance, with increasing $F$, the non-linear mobility exhibits non-monotonic behavior, and effective diffusion is greatly enhanced, showing multiple peaks in the presence of soft obstacles. Further, as a function of $K$ and $F$, particles exhibit various diffusive behaviors, e.g., normal diffusion - where the role of obstacles is insignificant, subdiffusion or superdiffusion - where the particles are partially trapped by the obstacles, and particles are ultimately caged by the obstacles. These findings help understand the physical situations wherein active agents diffuse in crowded environments.

cond-mat.soft↗

arXiv2023

Separation of interacting active particles in an asymmetric channel

We study the diffusive behaviour of interacting active particles (self-propelled) with mass $m$ in an asymmetric channel. The particles are subjected to an external oscillatory force along the length of the channel. In this setup, particles may exhibit rectification. In the absence of interaction, the mean velocity $\langle v \rangle$ of the particles shows a maximum at moderate $m$ values. It means that particles of moderate $m$ have higher velocities than the others. However, by incorporating short-range interaction between the particles, $\langle v \rangle$ exhibits an additional peak at lower $m$ values, indicating that particles of lower and moderate m can be separated simultaneously from the rest. Furthermore, by tuning the interaction strength, the self-propelled velocity, and the parameters of the oscillatory force, one can selectively separate the particles of lower $m$, moderate $m$, or both. Empirical relations for estimating the optimal mass as a function of these parameters are discussed. These findings are beneficial for separating the particles of selective $m$ from the rest of the particles.

cond-mat.soft↗

arXiv2023

External chemical gradient leads to efficient swimming of chiral swimmers

External gradients can strongly influence the collective behavior of microswimmers. In this paper, we study the behavior of two hydrodynamically interacting self-propelled chiral swimmers, in the low-Reynolds number regime, under the influence of an external linear chemical gradient. We use the generalized squirmer model called the chiral squirmer, a spherically shaped body with an asymmetric surface slip velocity, to represent the swimmer. We find that the external gradient favors the attraction between the swimmers and, in some situations, leads to a bounded state in which the swimmers move in a highly synchronous manner. Further, due to this cooperative motion, swimmers efficiently reach the chemical target compared to the individual swimmers. This study may help to understand the collective behavior of chiral swimmers and to design synthetic microswimmers for targeted drug delivery.

physics.bio-ph↗

arXiv2022

Near and far-field hydrodynamic interaction of two chiral squirmers

Hydrodynamic interaction strongly influences the collective behavior of the microswimmers. With this work, we study the behavior of two hydrodynamically interacting self-propelled chiral swimmers in the low Reynolds number regime, considering both the near and far-field interactions. We use the chiral squirmer model, a spherically shaped body with non-axisymmetric surface slip velocity, which generalizes the well-known squirmer model. We calculate the lubrication force between the swimmers when they are very close to each other. By varying the slip coefficients and the initial configuration of the swimmers, we investigate their hydrodynamic behavior. In the presence of lubrication force, the swimmers either repel each other or exhibit bounded motion where the distance between the swimmers alters periodically. The lubrication force favors the bounded motion in some parameter regime. This study is helpful to understand the collective behavior of dense suspension of ciliated microorganisms and artificial swimmers.

cond-mat.soft↗

arXiv2022

Diffusion of chiral active particles in a Poiseuille flow

We study the diffusive behavior of chiral active (self-propelled) Brownian particles in a two-dimensional microchannel with a Poiseuille flow. Using numerical simulations, we show that the behavior of the transport coefficients of particles, for example, the average velocity $v$ and the effective diffusion coefficient $D_{eff}$, strongly depends on flow strength $u_0$, translational diffusion constant $D_0$, rotational diffusion rate $D_θ$, and chirality of the active particles $Ω$. It is demonstrated that the particles can exhibit upstream drift, resulting in a negative $v$, for the optimal parameter values of $u_0$, $D_θ$, and $Ω$. Interestingly, the direction of $v$ can be controlled by tuning these parameters. We observe that for some optimal values of $u_0$ and $Ω$, the chiral particles aggregate near a channel wall, and the corresponding $D_{eff}$ is enhanced. However, for the nonchiral particles ($Ω= 0$), the $D_{eff}$ is suppressed by the presence of Poiseuille flow. It is expected that these findings have a great potential for developing microfluidic and lab-on-a-chip devices for separating the active particles.

cond-mat.soft↗

arXiv2021

Mass separation in an asymmetric channel

We present a mechanism to sort out particles of different masses in an asymmetric channel, where the entropic barriers arise naturally and control the diffusion of these particles. When particles are subjected to an oscillatory force, with the scaled amplitude $a$ and frequency $ω$, the mean particle velocity exhibits a bell-shaped behavior as a function of the particle mass, indicating that particles with an optimal mass $m_{op}$ drift faster than other particles. By tuning $a$ and $ω$, we get an empirical relation to estimate $m_{op} \sim (a\,ω^2)^{-0.4}$. An additional static bias, applied in the opposite direction of the rectified velocity, would push the particles of lighter mass to move in its direction while the others drift opposite to it. This study is useful to design lab-on-a-chip devices for separating particles of different masses.

cond-mat.soft↗

arXiv2021

Unsteady chiral swimmer in presence of an external chemical gradient

Unsteadiness occurs in the low Reynolds number swimmers' motion while they start from rest or escape from a predator or attack prey. In this paper, we study an unsteady chiral swimmer's behavior, with a prescribed surface slip velocity, in the low Reynold number regime and its response to an external chemical gradient. Using the general solution of unsteady Stokes equation and appropriate boundary conditions, i.e., slip velocity at the body surface and the ambient velocity field at the far-field, we calculate the migration velocity, rotation rate, and the ow field of the swimmer in a closed-form. We compare the velocity with some previously known results in appropriate limits. The continuous unsteady motion of the swimmer affects its tactic movement. A relevant example of this situation is chemotaxis, where the presence of chemical stimulus alters the swimmer's surface slip velocity. As a result, the swimmer either steers towards the chemical target or moves away from it, depending on the system parameters. Here, the rotation rate controls the success of the chemotaxis. In addition to the successful and unsuccessful cases, we observe a unique orbiting state where the swimmer keeps revolving around the chemical target in diffusive orbits.

physics.flu-dyn↗

arXiv2021

Interacting Brownian particles exhibiting enhanced rectification in an asymmetric channel

Rectification of interacting Brownian particles is investigated in a two-dimensional asymmetric channel in the presence of an external periodic driving force. The periodic driving force can break the thermodynamic equilibrium and induces rectification of particles (or finite average velocity). The spatial variation in the shape of the channel leads to entropic barriers, which indeed control the rectification of particles. We find that by simply tunning the driving frequency, driving amplitude, and shape of the asymmetric channel, the average velocity can be reversed. Moreover, a short range interaction force between the particles further enhances the rectification of particles greatly. This interaction force is modeled as the lubrication interaction. Interestingly, it is observed that there exists a characteristic critical frequency $Ω_c$ below which the rectification of particles greatly enhances in the positive direction with increasing the interaction strength; whereas, for the frequency above this critical value, it greatly enhances in the negative direction with increasing the interaction strength. Further, there exists an optimal value of the asymmetric parameter of the channel for which the rectification of interacting particles is maximum. These findings are useful in sorting out the particles and understanding the diffusive behavior of small particles or molecules in microfluidic channels, membrane pores, etc.

cond-mat.soft↗

arXiv2021

Effective medium model for a suspension of active swimmers

Several active organisms in nature tend to reside as a community in a viscous fluid medium. We analyze the variation of swimming characteristics of an active swimmer present in a dilute and disperse suspension, modeled as an effective Brinkman medium. This idealized representation of a collection of active swimmers allows one to distinguish the impact of the interior domain available to an individual swimmer as well as the contribution of its neighbors. Darcy's law along with the analytical solution enables the effective resistivity to be predicted as a function of the volume fraction which is in close agreement with the well-known Carman-Kozeny equation. This facilitates the successive analysis of the propulsion speed, power dissipation, and swimming efficiency of the targeted swimmer as a function of the volume fraction which is decisive in nutrient transport and uptake or reproduction in a collective environment. A stress-jump condition is also imposed across a cell to indicate a mean effective force due to the nearby swimmers. At suitable values of this stress-jump coefficient, the relative increase of the migration velocity and swimming efficiency is noticeably higher at an optimum occupancy.

cond-mat.soft↗

arXiv2021

Hydrodynamics of chiral squirmers

Many microorganisms take a chiral path while swimming in an ambient uid. In this paper, we study the combined behavior of two chiral swimmers using the well-known squirmer model taking into account chiral asymmetries. In contrast to the simple squirmer model, which has an axisymmetric distribution of slip velocity, the chiral squirmer has additional asymmetries in the surface slip, which contribute to both translations and rotations of the motion. As a result, swimming trajectories can become helical and chiral asymmetries arise in the ow patterns. We study the swimming trajectories of a pair of chiral squirmers that interact hydrodynamically. This interaction can lead to attraction and repulsion, and in some cases even to bounded states where the swimmers continue to periodically orbit around a common average trajectory. Such bound states are a signature of the chiral nature of the swimmers. Our study could be relevant to the collective movements of ciliated microorganisms.

cond-mat.soft↗

arXiv2020

Confined diffusion in a random Lorentz gas environment

We study the diffusive behavior of biased Brownian particles in a two dimensional confined geometry filled with the freezing obstacles. The transport properties of these particles are investigated for various values of the obstacles density $η$ and the scaling parameter $f$, which is the ratio of work done to the particles to available thermal energy. We show that, when the thermal fluctuations dominate over the external force, i.e., small $f$ regime, particles get trapped in the given environment when the system percolates at the critical obstacles density $η_c \approx 1.2$. However, as $f$ increases, we observe that particles trapping occurs prior to $η_c$. In particular, we find a relation between $η$ and $f$ which provides an estimate of the minimum $η$ up to a critical scaling parameter $f_c$ beyond which the Fick-Jacobs description is invalid. Prominent transport features like nonmonotonic behavior of the nonlinear mobility, anomalous diffusion, and greatly enhanced effective diffusion coefficient are explained for various strengths of $f$ and $η$. Also, it is interesting to observe that particles exhibit different kinds of diffusive behaviors, i.e., subdiffusion, normal diffusion, and superdiffusion. These findings, which are genuine to the confined and random Lorentz gas environment, can be useful to understand the transport of small particles or molecules in systems such as molecular sieves and porous media which have a complex heterogeneous environment of the freezing obstacles.

physics.bio-ph↗

arXiv2019

Diffusion of interacting particles in a channel with reflection boundary conditions

The diffusive transport of biased Brownian particles in a two-dimensional symmetric channel is investigated numerically considering both the no-flow and the reflection boundary conditions at the channel boundaries. Here, the geometrical confinement leads to entropic barriers which effectively control the transport properties of the particles. We show that compared to no-flow boundary conditions, the transport properties exhibit distinct features in a channel with reflection boundary conditions. For example, the nonlinear mobility exhibits a nonmonotonic behavior as a function of the scaling parameter $f$, which is a ratio of the work done to the particles to available thermal energy. Also, the effective diffusion exhibits a rapidly increasing behavior at higher $f$. The nature of reflection, i.e., elastic or inelastic, also influences the transport properties firmly. We find that inelastic reflections increase both the mobility and the effective diffusion for smaller $f$. In addition, by including the short range interaction force between the Brownian particles, the mobility decreases and the effective diffusion increases for various values of $f$. These findings, which are a signature of the entropic nature of the system, can be useful to understand the transport of small particles or molecules in systems such as microfluidic channels, membrane pores, and molecular sieves.

cond-mat.soft↗

arXiv2018

Chemotaxis of ciliated microorganisms: with and without noise

Biological systems like ciliated microorganisms are capable to respond to the external chemical gradients, a process known as chemotaxis which has been studied here using the chiral squirmer model. This theoretical model considers the microorganism as a spherical body with an active surface slip velocity. In presence of a chemical gradient, the internal signaling network of the microorganism is triggered due to binding of the ligand with the receptors on the surface of the body. Consequently, the coefficients of the slip velocity get modified resulting in a change in the path followed by the body. We observe that the strength of the gradient is not the only parameter which controls the dynamics of the body but also the adaptation time play a very significant role in the success of chemotaxis of the body. Path of the body is smooth if we ignore the discreteness in the ligand-receptor binding which is stochastic in nature. In presence of the later, the path is not only irregular but the dynamics of the body changes. We calculate the mean first passage time, by varying strength of the chemical gradient and adaptation time, to investigate the success rate of chemotaxis.

physics.bio-ph↗

arXiv2016

From active stresses and forces to self propulsion of droplets

We study the self-propulsion of spherical droplets as simplified hydrodynamic models of swimming microorganisms or artificial microswimmers. In contrast to approaches, which start from active velocity fields produced by the system, we consider active surface force or body force densities or active stresses as the origin of autonomous swimming. For negligible Reynolds number and given activity we first calculate the external and the internal ow fields as well as the center of mass velocity and an angular velocity of the droplet at fixed time. To construct trajectories from single time snapshots, the evolution of active forces or stresses must be determined in the laboratory frame. Here, we consider the case of active matter, which is carried by a continuously distributed, rigid but sparse (cyto)-sceleton that is immersed in the droplet's interior. We calculate examples of trajectories of a droplet and its sceleton from force densities or stresses, which may be explicitely time dependent in a frame fixed within the sceleton

cond-mat.soft↗