Janus Sphere Research Articles

Dynamics and Collective Behavior of Chemically Propelled Janus Sphere Dimers in Complex Solvents.

The propulsion mechanisms and collective dynamics of chemically powered Janus sphere dimers at the micro- and nanoscales, confined in a quasi-two-dimensional geometry, are investigated using a coarse-grained microscopic dynamical model. These active Janus dimers consist of two identical Janus spheres, featuring a catalytic cap on one hemisphere. The chemical reaction taking place on the catalytic surface generates asymmetric concentration gradients of product molecules around the Janus sphere, leading to the self-propulsion of the dimers. Depending on the dimer configuration, they exhibit various motion behaviors such as forward propulsion, rotation, and restricted stochastic motion. Due to chemotactic effects and self-diffusiophoretic forces, ensembles of dimers spontaneously form diverse structures, such as transient clusters, stable rotational ensembles, and antiparallel aligned doublets. This study demonstrates that the configurations of Janus sphere dimers significantly influence their self-propulsion and collective behaviors, providing crucial insights for the design and control of active micro- and nanoscale systems.

Characterizing the Janus colloidal particles in AC electric field and a step towards label-free cargo manipulation

In recent years, researchers have been exploring Janus particles, a unique type of colloidal system, which has gained enormous attention across diverse fields, from soft matter physics to biology. These particles, possessing an asymmetric surface, can be manipulated in the fluid using external energy sources. This versatility makes them suitable for mimicking biological systems and applications like cargo transportation, drug delivery, biosensing, and environmental cleanup. The study focuses on the response of metal-dielectric (Ti-PMMA) Janus spheres in the AC field, created through the Physical Vapor Deposition (PVD) technique. When subjected to a uniform AC electric field, these particles exhibit different dynamic behaviors at various frequency ranges. The particles show diverse responses from induced-charge electrophoresis (ICEP: 500 Hz-80 kHz) to reversed motion (r: ICEP:100 kHz-1 MHz) and linear chain formation (1 MHz). Additionally, they randomly cluster near the lower characteristic frequency (800 Hz) of the ICEP, whereas they exhibit the 3D motion at frequencies (100 Hz) below this lower characteristic value. The mechanisms behind these dynamic phenomena could be explained by considering the concept of electric double layer (EDL), self-dielectrophoresis (sDEP), dipolar interactions, hydrodynamic interactions, and electrothermal effects. These active Janus Colloids are further leveraged to manipulate 1 µm diameter PMMA passive microparticles and E. coli bacteria at 1 kHz, showcasing their potential applications in cargo delivery under various microfluidic research areas. The manipulation of payloads could be explained based on dielectrophoretic trapping due to locally generated field gradients around the surface of the Janus sphere. Therefore, the present work offers an unprecedented opportunity to study such out-of-equilibrium complex active matter systems and create novel functional materials. Moreover, it could also be extended as a fuel-free microrobotics system for various biomedical applications such as sensing and on-demand drug or cargo delivery inside the microfluidics chamber.

Microdynamics of active particles in defect-rich colloidal crystals

Hypothesis. Because they are self-propulsive, active colloidal particles can interact with their environment in ways that differ from passive, Brownian particles. Here, we explore how interactions in different microstructural regions may contribute to colloidal crystal annealing.Experiments. We investigate active particles propagating in a quasi-2D colloidal crystal monolayer produced by alternating current electric fields (active-to-passive particle ratio ∼ 1:720). The active particle is a platinum Janus sphere propelled by asymmetric decomposition of hydrogen peroxide. Crystals are characterized for changes in void properties. The mean-squared-displacement of Janus particles are measured to determine how active microdynamics depend on the local microstructure, which is comprised of void regions, void-adjacent regions (defined as within three particle diameters of a void), and interstitial regions.Findings. At active particle energy EA = 2.55 kBT, the average void size increases as much as three times and the average void anisotropy increases about 40% relative to the passive case. The average microdynamical enhancement, <δ(t)>, of Janus particles in the crystal relative to an equivalent passive Janus particle is reduced compared to that of a free, active particle (<δ(t) > is 1.88 ± 0.04 and 2.66 ± 0.08, respectively). The concentration of active particles is enriched in void and void-adjacent regions. Active particles exhibit the greatest change in dynamics relative to the passive control in void-adjacent regions (<δ(t)> = 2.58 ± 0.06). The results support the conjecture that active particle microdynamical enhancement in crystal lattices is affected by local defect structure.

Application of the method of continued boundary conditions to the solution of the problems of wave diffraction on various types of scatterers with complex structure

The article considers the application of the method of continued boundary conditions to the two-dimensional problem of diffraction of electromagnetic waves by a dielectric body with a cross section of complex geometry and to the problem of diffraction by a Janus sphere in the form of a permeable sphere partially covered by an absolutely soft or an absolutely rigid spherical screen. The results of calculating the scattering pattern for a large set of bodies of different geometry, including fractal-like scatterers, are obtained. It is illustrated that in the case of a smooth body boundary, the algorithm based on the Fredholm equations of the 1st kind makes it possible to obtain results with greater accuracy than for equations of the 2nd kind. The correctness of the method was confirmed by verifying the implementation of the optical theorem for various bodies and by comparing with the results of calculations obtained by other methods.

Single-Hole Janus Hollow Sphere.

Cross-linked epoxy resin (EP) single-hole Janus hollow spheres are prepared by cross-linking induced phase separation within an emulsion droplet and selective modification. The droplet is composed of an EP oligomer, toluene, and hexadecane. 2-Ethyl-4-methylimidazole is used as the cross-linker added to the aqueous phase. During the cross-linking, hexadecane forms an eccentric core in the cross-linked EP sphere. A single hole forms across the shell after dissolving the solvents, and a single-hole hollow sphere is achieved. The hole and cavity size are controlled by adjusting the solvent content and cross-linker concentration. Furthermore, frozen wax is used as the core material instead of hexadecane to effectively protect the sphere's interior surface. Selective modification of the exterior and interior surfaces is thus permitted. As an example, a responsive single-hole Janus hollow sphere is prepared by the favorable growth of a silica-polyoxyethylene composite layer onto the exterior surface and a selective grafting of poly(2-diethylaminoethyl methacrylate) (PDEAEMA) by atom-transfer radical polymerization (ATRP) onto the interior. The Janus sphere is water-dispersible and controllably captures and releases oil from the aqueous environment as triggered by the pH value.

Anisotropic stresslet and rheology of stick–slip Janus spheres

A Janus sphere with a stick–slip pattern can behave quite differently in its hydrodynamics compared with a no-slip or uniform-slip sphere. Here, using the Lorentz reciprocal theorem in conjunction with surface harmonic expansion, we rigorously derive the extended Faxén formula for the stresslet of a weakly stick–slip Janus sphere, capable of describing the anisotropic nature of the stresslet with an arbitrary axisymmetric stick–slip pattern in an arbitrary background flow. We find that slip anisotropy not only causes a variety of additional contributions to the stresslet, but also naturally renders a stresslet–rotation coupling that may turn a suspension of couple-free stick–slip Janus spheres into a dipolar one under the actions of an external couple. Moreover, to correctly account for the impacts of slip anisotropy on the stresslet, it is necessary to include at least the first four surface harmonic contributions. As a result, the anisotropies of both the stresslet and torque on the sphere in a linear flow field are purely reflected by a symmetric quadrupole and hexadecapole. These hydrodynamic quantities can be further mediated by an antisymmetric dipole and octupole due to the gradients of the imposed strain field. The average bulk stress and effective viscosity for a suspension of stick–slip spheres are also determined, showing characteristics quite distinct from those of a suspension of near spheres. If the spheres possess permanent dipole moments, in particular, additional stresslets and couplets can be generated by an applied external couple on each sphere and added into the bulk stress, accompanied by non-Jeffery orientational orbits of such dipolar stick–slip spheres. In addition to the above, the extended Faxén stresslet and torque relations found in this work will also provide the formulae needed for tackling problems involving hydrodynamically interacting stick–slip spheres on which small slip anisotropy may have profound impacts.

Diffusive mass transfer from a Janus sphere

A highly accurate approximate solution is developed for the problem of diffusive mass transfer from a two-sided sphere. The problem is primarily motivated by the Marangoni-driven motion of a chemically active spherical surfer along a water-air interface, but it is also related to the self-diffusiophoresis of catalytic colloids, as well as to the conduction heat transfer and electrostatics problems involving partially insulated spheres.

Self-propulsion in 2D confinement: phoretic and hydrodynamic interactions.

Chemically active Janus particles generate tangential concentration gradients along their surface for self-propulsion. Although this is well studied in unbounded domains, the analysis in biologically relevant environments such as confinement is scarce. In this work, we study the motion of a Janus sphere in weak confinement. The particle is placed at an arbitrary location, with arbitrary orientation between the two walls. Using the method of reflections, we study the effect of confining planar boundaries on the phoretic and hydrodynamic interactions, and their consequence on the Janus particle dynamics. The dynamical trajectories are analyzed using phase diagrams for different surface coverage of activity and solute-particle interactions. In addition to near wall states such as 'sliding' and 'hovering', we demonstrate that accounting for two planar boundaries reveals two new states: channel-spanning oscillations and damped oscillations around the centerline, which were characterized as 'scattering' or 'reflection' by earlier analyses on single wall interactions. Using phase-diagrams, we highlight the differences in inert-facing and active-facing Janus particles. We also compare the dynamics of Janus particles with squirmers for contrasting the chemical interactions with hydrodynamic effects. Insights from the current work suggest that biological and artificial swimmers sense their surroundings through long-ranged interactions, that can be modified by altering the surface properties.

Self-assembly of amphiphilic Janus spheres using the lattice Boltzmann method

We discuss the relevance of the lattice Boltzmann method (LBM) for soft matter alongside other simulation methods. We set up a popular problem of self-assembly of Janus spheres by combining native fluid and particle models in LBM with a simple amphiphilic pair potential. Thermal fluctuations and close contact corrections are also incorporated along with a novel periodic boundary condition for finite-sized 3D particles. Preliminary results of Janus sphere self-assembly are presented and compared with similar works in the literature. We also comment on the difficulties that LBM faces tackling such problems.

Two Approaches to Solving the Problem of Diffraction on a Janus Sphere

Two Approaches to Solving the Problem of Diffraction on a Janus Sphere

Hydrodynamic interactions between two side-by-side Janus spheres

Janus spheres are heterogeneous amphiphilic particles having partially slippery and partially sticky surface. As a result, hydrodynamic forces such as drag and lift on a Janus sphere are different from those on a homogeneous sphere. Several applications of Janus particles require high particle concentration in a continuous fluid medium making the interactions between them inevitable. Therefore, in this study three-dimensional flow around two Janus spheres held fixed relative to each other with the line connecting their centres normal to a uniform fluid stream, is investigated computationally at Reynolds numbers ranging from 0.1 to 50. The dependence of lift and drag coefficients on the separation distance between the two Janus spheres is studied. Two cases, (A) sticky hemispheres facing each other and (B) slippery hemispheres facing each other are considered. While for Case A, the spheres repel each other for all the separation distances studied, a cross-over from repulsion to attraction regime with the increase in Reynolds number is observed for Case B. Correlations are proposed for drag at various separation distances for both the orientations.

Drag on Janus Sphere in a Channel: Effect of Particle Position

Abstract Potential use of Janus spheres in novel engineering applications is being explored actively in recent years. Hydrodynamics around Janus spheres is different from that around homogeneous sticky or slippery spheres. Instantaneous motion of a sphere in channel flow is governed by hydrodynamic force experienced by the sphere, which in turn depends on the particle to channel size ratio, its instantaneous position, hydrophobicity of its surface, and the particle Reynolds number. We investigate numerically the drag experienced by a Janus sphere located at different off-center positions in a square channel. Two orientations of Janus sphere consisting of a sticky and a slippery hemisphere with the boundary between them parallel to the channel midplane are studied: (1) slippery hemisphere facing the channel centerline and (2) sticky hemisphere facing the channel centerline. The flow field around Janus sphere is found to be steady (for Re ≤ 50 investigated in this work) and asymmetric. Based on the data obtained, a correlation for drag coefficient as a function of particle Reynolds number and dimensionless particle position is also proposed.

Non-Newtonian effects on the slip and mobility of a self-propelling active particle

Abstract

Electromagnetic scattering on Janus spheres

Janus spheres are objects of growing interest in different fields of technology. For example, they can be applied in medicine for a precise drug positioning. They have moreover an enormous potential for the development of new and active materials. Some Janus spheres are already manufactured industrially. Nevertheless, there is a lack of knowledge about the scattering behavior of such objects although it is important not only for diagnostic purposes but also for discovering new applications. In this paper we present the application of the well known T-matrix method to solve the scattering problem of a plane electromagnetic wave on a dielectric sphere whose surface is partially covered with a perfect metal. The application of this approach to a specially oriented Janus sphere reveals an interesting effect for size parameters less than 6.

A numerical study of acoustic scattering by Janus spheres.

Recently Janus particles have become important in several technological fields because they have interesting properties compared with homogeneous particles. The interaction of Janus particles with sound waves is of particular interest for diagnostic purposes, and also in applications in micro- and nanotechnology. In this paper the authors demonstrate that a method of fundamental solution combined with a T-matrix that is computed from far-field information can be applied with benefit to analyse the scattering of sound waves by a particular type of Janus sphere. Moreover, it is shown that this method converges faster than the conventional T-matrix method introduced by Waterman [(1969). J. Acoust. Soc. Am. 45, 1417-1429]. This is of special importance if orientation averaged scattering quantities are required, or if multiple scattering processes on Janus spheres are considered. This method is used to demonstrate the interesting phenomenon of an enhanced side scattering intensity that is larger than the forward scattering intensity, and that this effect can be strengthened using a particular configuration of two identical Janus spheres. Finally, the authors discuss a useful approximation that can be readily applied for two or more Janus spheres.

Influence of cap weight on the motion of a Janus particle very near a wall.

The dynamics of anisotropic nano- to micro scale colloidal particles in confined environments, either near neighboring particles or boundaries, is relevant to a wide range of applications. We utilized Brownian dynamics simulations to predict the translational and rotational fluctuations of a Janus sphere with a cap of nonmatching density near a boundary. The presence of the cap significantly impacted the rotational dynamics of the particle as a consequence of gravitational torque at experimentally relevant conditions. Gravitational torque dominated stochastic torque for a particle >1 μm in diameter and with a 20-nm-thick gold cap. Janus particles at these conditions sampled mostly cap-down or "quenched" orientations. Although the results summarized herein showed that particles of smaller diameter (<1 μm) with a thin gold coating (<5 nm) behave similarly to an isotropic particle, small increases in either particle diameter or coating thickness quenched the polar rotation of the particle. Histogram landscapes of the separation distance from the boundary and orientation observations of particles with larger diameters or thicker gold coatings were mostly populated with quenched configurations. Finally, the histogram landscapes were inverted to obtain the potential energy landscapes, providing a road map for experimental data to be interpreted.

Experimental observation of flow fields around active Janus spheres

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. In particular, the importance of electrokinetic effects on the motility of Pt-PS Janus sphere was recently discussed. Here, we probe the hydrodynamic flow field around a catalytically active colloid using particle tracking velocimetry both in the freely swimming state and when kept stationary with an external force. Our measurements provide information about the fluid velocity in the vicinity of the surface of the colloid, and confirm a mechanism for propulsion that was proposed recently. In addition to offering a unified understanding of the nonequilibrium interfacial transport processes at stake, our results open the way to a thorough description of the hydrodynamic interactions between such active particles and understanding their collective dynamics.

Toward Understanding of Self-Electrophoretic Propulsion under Realistic Conditions: From Bulk Reactions to Confinement Effects

Active matter concerns itself with the study of particles that convert energy into work, typically motion of the particle itself. This field saw a surge of interest over the past decade, after the first micrometer-sized, man-made chemical motors were created. These particles served as a simple model system for studying in a well-controlled manner complex motion and cooperative behavior as known from biology. In addition, they have stimulated new efforts in understanding out-of-equilibrium statistical physics and started a revolution in microtechnology and robotics. Concentrated effort has gone into realizing these ambitions, and yet much remains unknown about the chemical motors themselves. The original designs for self-propelled particles relied on the conversion of the chemical energy of hydrogen peroxide into motion via catalytic decomposition taking place heterogeneously over the surface of the motor. This sets up gradients of chemical fields around the particle, which allow it to autophorese. That is, the interaction between the motor and the heterogeneously distributed solute species can drive fluid flow and the motor itself. There are two basic designs: the first relies on redox reactions taking place between the two sides of a bimetal, for example, a gold-platinum Janus sphere or nanorod. The second uses a catalytic layer of platinum inhomogeneously vapor-deposited onto a nonreactive particle. For convenience's sake, these can be referred to as redox motors and monometallic half-coated motors, respectively. To date, most researchers continue to rely on variations of these simple, yet elegant designs for their experiments. However, there is ongoing debate on the exact way chemical energy is transduced into motion in these motors. Many of the experimental observations on redox motors were successfully modeled via self-electrophoresis, while for half-coated motors there has been a strong focus on self-diffusiophoresis. Currently, there is mounting evidence that self-electrophoresis provides the dominant contribution to the observed speeds of half-coated motors, even if the vast majority of the reaction products are electroneutral. In this Account, we will summarize the most common electrophoretic propulsion model and discuss its strengths and weaknesses in relation to recent experiments. We will comment on the possible need to go beyond surface reactions and consider the entire medium as an "active fluid" that can create and annihilate charged species. This, together with confinement and collective effects, makes it difficult to gain a detailed understanding of these swimmers. The potentially dominant effect of confinement is highlighted on the basis of a recent study of an electro-osmotic pump that drives fluid along a substrate. Detailed analysis of this system allows for identification of the electro-osmotic driving mechanism, which is powered by micromolar salt concentrations. We will discuss how our latest numerical solver developments, based on the lattice Boltzmann method, should enable us to study collective behavior in systems comprised of these and other electrochemical motors in realistic environments. We conclude with an outlook on the future of modeling chemical motors that may facilitate the community's microtechnological ambitions.

Orientational dynamics of a heated Janus particle.

Using large scale molecular dynamics simulations, we study the orientational dynamics of a heated Janus particle which exhibits self-propulsion. The asymmetry in the microscopic interaction of the colloid with the solvent is implemented by choosing different wetting parameters for the two halves of the sphere. This choice leads to a different microscopic Kapitza resistance across the solid-fluid boundary of the two halves of the sphere, and consequently a gradient in temperature is created across the poles of the sphere. It is this self-created temperature gradient which leads to a self-propulsion along the direction of the symmetry axis. In this article, we look at the orientational dynamics of such a system, as well as the subsequent enhancement of the translational diffusivity of the heated Janus colloid at late times. The orientational correlation of the symmetry axis is measured from the simulation and provides a direct access to the rotational diffusion constant. The heating leads to an increase in the rotational diffusivity of the colloid. We quantify this increase in rotational diffusion D r against the temperature difference δT ≡ T(R, 0) - T(R, π) across the poles of the Janus sphere as well as the average surface temperature difference ΔT ≡ T(R) - T(∞) from the ambient fluid. Since the rotational diffusion is determined by the complete flow field in the solvent, we illustrate that comparing D r against δT is misleading and is better quantified when compared against ΔT. The later quantification results in a data collapse for different choices of the microscopic interaction. The average propulsion velocity is also measured for different choices of the wetting parameter. The directionality of self-propulsion changes depending on the microscopic interaction. We show that whenever the attractive interaction of the colloid with the solvent is switched off, the phoretic mobility changes sign. Furthermore, the propulsion velocity is zero for heating below a certain threshold value. This is also corroborated by the probability distribution of the angle between the displacement vector Δr(t) ≡ r(t) - r(0) and the symmetry axis. Finally, we combine the measured propulsion velocity and the rotational diffusion time τ r = 1/2D r to estimate the enhancement in the long time diffusion coefficient of the particle.

Artificial chemotaxis of phoretic swimmers: instantaneous and long-time behaviour

Phoretic swimmers are a class of artificial active particles that has received significant attention in recent years. By making use of self-generated gradients (e.g. in temperature, electric potential or some chemical product) phoretic swimmers are capable of self-propulsion without the complications of mobile body parts or a controlled external field. Focusing on diffusiophoresis, we quantify in this paper the mechanisms through which phoretic particles may achieve chemotaxis, both at the individual and the non-interacting population level. We first derive a fully analytical law for the instantaneous propulsion and orientation of a phoretic swimmer with general axisymmetric surface properties, in the limit of zero Péclet number and small Damköhler number. We then apply our results to the case of a Janus sphere, one of the most common designs of phoretic swimmers used in experimental studies. We next put forward a novel application of generalised Taylor dispersion theory in order to characterise the long-time behaviour of a population of non-interacting phoretic swimmers. We compare our theoretical results with numerical simulations for the mean drift and anisotropic diffusion of phoretic swimmers in chemical gradients. Our results will help inform the design of phoretic swimmers in future experimental applications.