Colloidal attraction induced by a temperature gradient

Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection

We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of "plasmonic heater" gold nanoparticles and "assembly building block" microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1-10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases-colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light.

Hot crystals of thermo-responsive particles with temperature dependent diameter in the presence of a temperature gradient

Structure formation under non-equilibrium steady state conditions is poorly understood. A non-equilibrium steady state can be achieved in a system by maintaining a temperature gradient. A class of cross-linked microgel particles, such as poly-N-iso-propylacrylamide, is reported to increase in size due to the adsorption of water as the temperature decreases. Here, we study thermo-responsive particles with a temperature sensitive diameter in the presence of a temperature gradient, using molecular dynamics simulations with the Langevin thermostat. We find long-ranged structural order using bond order parameters in both cold and hot regions of the system beyond a certain diameter ratio of the cold and hot particles. This is due to an increase in packing and pressure in both regions. Our observations might be useful in understanding ordered structures under extreme conditions of a non-equilibrium steady state.

Light-Programmed Bistate Colloidal Actuation Based on Photothermal Active Plasmonic Substrate

Active particles have been regarded as the key models to mimic and understand the complex systems of nature. Although chemical and field-powered active particles have received wide attentions, light-programmed actuation with long-range interaction and high throughput remains elusive. Here, we utilize photothermal active plasmonic substrate made of porous anodic aluminum oxide filled with Au nanoparticles and poly( N -isopropylacrylamide) (PNIPAM) to optically oscillate silica beads with robust reversibility. The thermal gradient generated by the laser beam incurs the phase change of PNIPAM, producing gradient of surface forces and large volume changes within the complex system. The dynamic evolution of phase change and water diffusion in PNIPAM films result in bistate locomotion of silica beads, which can be programmed by modulating the laser beam. This light-programmed bistate colloidal actuation provides promising opportunity to control and mimic the natural complex systems.

A long-range order in a thermally driven system with temperature-dependent interactions

Aggregation of macro-molecules under an external force is far from being understood. An important driving situation is achieved by temperature difference. Inter-particle interactions in metallic nanoparticles with ligand capping are reported to be sensitive to temperature and the zeta potential of the particles being reduced in the cold region. Such particles form aggregates in the cold region of the system in the presence of temperature difference. Here we study the aggregation of particles in the presence of temperature difference with temperature-dependent interaction parameters using Brownian dynamics simulation. The particle interaction and particle diffusion are considered to be sensitive to the local temperature. We identify a long-range structural order in the cold region of the system using the Avrami equation for crystal growth kinetics. Our observations might be useful in designing ordered structures with macro-molecules under non-equilibrium steady-state conditions.

Metamachines of pluripotent colloids

Machines enabled the Industrial Revolution and are central to modern technological progress: A machine's parts transmit forces, motion, and energy to one another in a predetermined manner. Today's engineering frontier, building artificial micromachines that emulate the biological machinery of living organisms, requires faithful assembly and energy consumption at the microscale. Here, we demonstrate the programmable assembly of active particles into autonomous metamachines using optical templates. Metamachines, or machines made of machines, are stable, mobile and autonomous architectures, whose dynamics stems from the geometry. We use the interplay between anisotropic force generation of the active colloids with the control of their orientation by local geometry. This allows autonomous reprogramming of active particles of the metamachines to achieve multiple functions. It permits the modular assembly of metamachines by fusion, reconfiguration of metamachines and, we anticipate, a shift in focus of self-assembly towards active matter and reprogrammable materials.

A numerical study on the closed packed array of gold discs as an efficient dual mode plasmonic tweezers

In this report, we propose the closed pack array of gold discs on glass, as a dual mode plasmonic tweezers that benefits from two trapping modes. The first trapping mode is based on leaky surface plasmon mode (LSPM) on the gold discs with a longer penetration depth in the water and a longer spatial trapping range, so that target nanoparticles with a radius of 100 nm can be attracted toward the gold surface from a vertical distance of about 2 µm. This trapping mode can help to overcome the inherent short range trapping challenge in the plasmonic tweezers. The second trapping mode is based on the dimer surface plasmonic mode (DSPM) in the nano-slits between the neighboring gold discs, leading to isolated and strong trapping sites for nanoparticles smaller than 34 nm. The proposed plasmonic tweezers can be excited in both LSPM and DSPM modes by switching the incident wavelength, resulting in promising and complementary functionalities. In the proposed plasmonic tweezers, we can attract the target particles towards the gold surface by LSPM gradient force, and trap them within a wide half width half maximum (HWHM) that allows studying the interactions between the trapped particles, due to their spatial proximity. Then, by switching to the DSPM trapping mode, we can rearrange the particles in a periodic pattern of isolated and stiff traps. The proposed plasmonic structure and the presented study opens a new insight for realizing efficient, dual-mode tweezers with complementary characteristics, suitable for manipulation of nanoparticles. Our thermal simulations demonstrate that the thermal-induced forces does not interefe with the proposed plasmonic tweezing.

Controlled Optofluidic Crystallization of Colloids Tethered at Interfaces

We report experiments that show rapid crystallization of colloids tethered to an oil-water interface in response to laser illumination. This light-induced transition is due to a combination of long-ranged thermophoretic pumping and local optical binding. We show that the flow-induced force on the colloids can be described as the gradient of a potential. The nonequilibrium steady state due to local heating thus admits an effective equilibrium description. The optofluidic manipulation explored in this work opens novel ways to manipulate and assemble colloidal particles.

Thermophoresis of colloids in nematic liquid crystal

Thermophoresis, or the directional motion of colloidal particles in liquids driven by a temperature gradient, is of both fundamental interest and practical use. In this work we explore the thermophoresis of colloids suspended in nematic liquid crystals (LCs). We observe that the motion of these colloids is fundamentally different from that in isotropic systems as a result of elastic distortions in the director fields caused by the colloidal inclusions. In the case of a sufficiently large local temperature and gradient, the elastic energy drives negative thermophoresis of immersed particles, which has a strongly nonlinear dependence on temperature. We develop a theory that incorporates elastic energy minimization into the traditional thermophoretic formulation and demonstrated a good agreement with experimental observations. We also examine the temperature dependence of the effective viscosity of the colloids and highlight the large magnitude of the Soret coefficient (|ST| > 5000), which results from the inherent enhancement in thermophoresis due to elastophoretic considerations and suppression of Brownian diffusion in LC media.

Clustering of Janus particles in an optical potential driven by hydrodynamic fluxes

Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologically-inspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.

Self-assembly and complex manipulation of colloidal mesoscopic particles by active thermocapillary stress

We demonstrate that the active thermocapillary stresses induced by multiple microbubbles offer simple routes to directed self-assembly and complex but controllable micromanipulation of mesoscopic colloidal particles embedded in a liquid. The microbubbles are nucleated on a liquid-glass interface using optical tweezers. The flow around a single bubble causes self-assembly of the particles in rings at the bubble-base, while an asymmetric temperature profile generated across the bubble interface breaks the azimuthal symmetry of the flow, and induces simultaneous accumulation and repulsion of particles at different axial planes with respect to the bubble. The flow due to two adjacent bubbles leads to more diverse effects including the sorting of particles, and to local vorticity that causes radial and axial rotation of the particles - the latter being obtained for the first time using optical tweezers. The sorting is enabled by nucleating the bubbles on spatially discrete temperature profiles, while the vorticity is generated by nucleating them in the presence of a temperature gradient which once again causes a strong symmetry-breaking in the azimuthal flow. The flow profiles obtained in the experiments are explained by analytical solutions or qualitative explanations of the associated thermocapillary problem employing the Stokes and heat equations.

Rapid Assembly of Colloidal Crystals under Laser Illumination on a GeSbTe Substrate

Optical techniques have been actively studied for manipulating nano- to microsized objects. However, long-range attraction and rapid transport of particles within thin quasi-two-dimensional systems are difficult because of the weak thermophoretic forces. Here, we introduce an experimental system that can rapidly generate quasi-two-dimensional colloidal crystals in deionized water, sandwiched between two hard plates. When a pulsed laser is irradiated on a chalcogenide phase-change material spattered on one side of the plates, the induced Marangoni-like flow causes a colloidal self-assembly in the order of tens of micrometers within the laser spot, with a transport velocity of a few tens of micrometers per second. This is due to the large thermal gradient induced by chalcogenide characteristics of high laser absorption and low thermal conductivity, and a strong hydrodynamic slip flow at the hydrophobic chalcogenide interface. Moreover, the colloidal crystals exhibit various lattice structures, depending on the laser intensity and chamber distance. For a certain range of the chamber distance, the colloidal crystal phases can be alternated by tuning the laser intensity in real time. Our system forms and deforms quasi-two-dimensional colloidal crystals at an on-demand location on a GeSbTe substrate.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Light-induced thermoosmosis about conducting ellipsoidal nanoparticles

We consider the central problem of a non-spherical (ellipsoidal) polarizable (metallic) nanoparticle freely suspended in a conducting liquid phase which is irradiated (heated) by a laser under the Rayleigh (electrostatic) approximation. It is shown that, unlike the case of perfectly symmetric (spherical) particles, the surface temperature of general orthotropic particles exposed to continuous laser irradiation is not uniform! Thus, the induced surface slip (Soret type) velocity may lead to a self-induced thermoosmotic flow (sTOF) about the particle, in a similar manner to the electroosmotic flow driven by the Helmholtz—Smoluchowski slippage. Using the recent advancement in the theory of Lamé functions and ellipsoidal harmonics, we analytically present new solutions for two key physical problems. (i) Heat conduction and temperature distribution inside and outside a conducting laser-irradiated homogeneous tri-axial ellipsoid which is subjected to uniform Joule heating. (ii) Creeping (Stokes) sTOF around a fixed impermeable metallic ellipsoidal nanoparticle driven by a Soret-type surface slip velocity (i.e. proportional to the surface-temperature gradient).

Osmosis, from molecular insights to large-scale applications

Osmosis is a universal phenomenon occurring in a broad variety of processes and fields. It is the archetype of entropic forces, both trivial in its fundamental expression - the van 't Hoff perfect gas law - and highly subtle in its physical roots. While osmosis is intimately linked with transport across membranes, it also manifests itself as an interfacial transport phenomenon: the so-called diffusio-osmosis and -phoresis, whose consequences are presently actively explored for example for the manipulation of colloidal suspensions or the development of active colloidal swimmers. Here we give a global and unifying view of the phenomenon of osmosis and its consequences with a multi-disciplinary perspective. Pushing the fundamental understanding of osmosis allows one to propose new perspectives for different fields and we highlight a number of examples along these lines, for example introducing the concepts of osmotic diodes, active separation and far from equilibrium osmosis, raising in turn fundamental questions in the thermodynamics of separation. The applications of osmosis are also obviously considerable and span very diverse fields. Here we discuss a selection of phenomena and applications where osmosis shows great promises: osmotic phenomena in membrane science (with recent developments in separation, desalination, reverse osmosis for water purification thanks in particular to the emergence of new nanomaterials); applications in biology and health (in particular discussing the kidney filtration process); osmosis and energy harvesting (in particular, osmotic power and blue energy as well as capacitive mixing); applications in detergency and cleaning, as well as for oil recovery in porous media.

Diffusiophoretic design of self-spinning microgears from colloidal microswimmers

The development of strategies to assemble microscopic machines from dissipative building blocks are essential on the route to novel active materials. We recently demonstrated the hierarchical self-assembly of phoretic microswimmers into self-spinning microgears and their synchronization by diffusiophoretic interactions [Aubret et al., Nat. Phys., 2018]. In this paper, we adopt a pedagogical approach and expose our strategy to control self-assembly and build machines using phoretic phenomena. We notably introduce Highly Inclined Laminated Optical sheets microscopy (HILO) to image and characterize anisotropic and dynamic diffusiophoretic interactions, which cannot be performed by conventional fluorescence microscopy. The dynamics of a (haematite) photocatalytic material immersed in (hydrogen peroxide) fuel under various illumination patterns is first described and quantitatively rationalized by a model of diffusiophoresis, the migration of a colloidal particle in a concentration gradient. It is further exploited to design phototactic microswimmers that direct towards the high intensity of light, as a result of the reorientation of the haematite in a light gradient. We finally show the assembly of self-spinning microgears from colloidal microswimmers and carefully characterize the interactions using HILO techniques. The results are compared with analytical and numerical predictions and agree quantitatively, stressing the important role played by concentration gradients induced by chemical activity to control and design interactions. Because the approach described hereby is generic, this works paves the way for the rational design of machines by controlling phoretic phenomena.

Dynamics of Binary Active Clusters Driven by Ion-Exchange Particles

We present a framework to quantitatively predict the linear and rotational directed motion of synthetic modular microswimmers. To this end, we study binary dimers and characterize their approach motion as effective interactions within a minimal model. We apply this framework to the assembly of small aggregates composed of a cationic ion-exchange particle with up to five passive particles or anionic ion-exchange particles at dilute conditions. Particles sediment and move close to a substrate, above which the ion-exchange particles generate flow. This flow mediates long-range attractions leading to a slow collapse during which long-lived clusters of a few particles assemble. The effective interactions between unlike particles break Newton's third law. Depending on their symmetry, assemblies thus can become linear or circle swimmers, or remain inert (no directed motion).

Dynamics of a colloidal particle near a thermoosmotic wall under illumination

The effects of light on colloidal particles in solution are multiple, including transfer of photonic linear/angular momentum and heating of the particles or their surroundings. The temperature increase around colloidal particles due to light heating can drive a thermoosmotic flow along a nearby boundary wall, which significantly influences the motion of the particles. Here we perform mesoscopic dynamics simulations to study two typical scenarios, where thermoosmosis is critical. In the first scenario, we consider a light-absorbing colloidal particle heated by uniform illumination. Depending on the fluid-wall interactions, the thermoosmotic flow generated by the wall can exert a long-ranged hydrodynamic attraction or repulsion on the "hot" Brownian particle, which leads to a strong accumulation/depletion of the particle to/from the boundary. In the second scenario, we investigate the motion of a colloidal particle confined by an optical tweezer in a light-absorbing solution. In this case, thermoosmosis can induce a unidirectional rotation of the trapped particle, whose direction is determined by thermoosmotic properties of the wall. We show that colloidal particles near a thermoosmotic wall exhibit rich dynamics. Our results can be applied for the manipulation of colloidal particles in microfluidics.

Numerical Investigation of Tunable Plasmonic Tweezers based on Graphene Stripes

We are proposing tunable plasmonic tweezers, consisting two parallel graphene stripes, which can be utilized to effectively trap and sort nanoparticles. We show that by electrostatically tuning the chemical potential of a graphene stripe by about 100 meV (equivalent to ΔV_G ≈ 4.4 V), the plasmonic force can be switched efficiently, without a need to switch the laser intensity. This enables high speed and low power switching with a large number of switching cycles. By applying two independent and appropriate gate bias voltages to the stripes, the direction of the plasmonic force can be reversed, which leads to separation of nanoparticles that satisfy the trapping conditions. Numerical simulations show that the potential depths obtained for polystyrene nanoparticles of refractive index n = 1.5717 and radii r ≥ 50 nm is deeper than −10 k_BT , confirming the ability of the proposed system to effectively separate such nanoparticles. This capability holds for smaller nanoparticles with larger refractive indices. Finally, performing thermal simulations, we have demonstrated that the heat induced by the illumination increases the fluid temperature by at most 9 °C, having negligible effect on the trapping mechanism. The proposed system opens up new possibilities in developing tunable on-chip manipulation devices, suitable for biological applications.

Light-harvesting synthetic nano- and micromotors: a review

Nano- and micromotors are machines that can be made to perform specialized tasks as they propel themselves in response to certain stimuli. While the design of these self-propelling nano- and micromotors remains challenging, they have nevertheless attracted considerable research due to their many promising applications. Most self-propelled nano- and micromotors are based on the conversion of chemical energy into mechanical movement. Recently, however, the development of motors that can be propelled by light as an external stimulus has received much attention. The reason being that light is a renewable energy source that does not require any physical connection to the motor, does not usually lead to any waste products, and is easy to control. This review highlights recent progress in the development of light-harvesting synthetic motors that can be efficiently propelled and accurately controlled by exposure to light, and gives an overview of their fabrication methods, propulsion mechanisms, and practical applications.

Thermal gradient induced tweezers for the manipulation of particles and cells

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Thermoosmotic microfluidics

Microchannels with asymmetrically ratcheted walls are here shown to behave as effective and versatile microfluidic pumps if locally heated. When the boundary walls have different temperatures, the confined liquid experiences a temperature gradient along the sawtooth edges, which can induce a thermoosmotic flow. A mesoscale molecular simulation approach is here employed to investigate the flows which are contrasted using an analytical approach. Microchannels can be composed by one or two ratcheted walls which can be straight or cylindrical. Varying the channel geometry can not only change the overall fluid flux, but also vary the flow patters from shear to capillary type, or even to extensional type flows. This scheme does not require multiphase fluids or any movable channel parts, although they are possible to be implemented. The proposed principle is then very versatile to locally manipulate complex fluids, and a promising tool to recover waste heat, to facilitate cooling of microchips, and to manufacture portable lab-on-a-chip devices.

Self-assisted optothermal trapping of gold nanorods under two-photon excitation

We report a self-assisted optothermal trapping and patterning of gold nanorods (GNRs) on glass surfaces with a femtosecond laser. We show that GNRs are not only the trapping targets, but also can enhance the optothermal trapping of other particles. This trapping phenomenon is the net result of thermophoresis and a convective flow caused by localized heating. The heating is due to the conversion of absorbed photons into heat at GNR's longitudinal surface plasmon resonance (LSPR) wavelength. First, we investigated the optothermal trapping of GNRs at their LSPR wavelength on the glass surface with as low as 0.5 mW laser power. The trapping range was observed to be larger than a typical field of view, e.g. 210 µm  ×  210 µm here. Second, by adjusting the distance between the laser focus and the glass surface, ring patterns of GNRs on the glass surface were obtained. These patterns could be controlled by the laser power and the numerical aperture of the microscope objective. Moreover, we examined the spectral emission of GNRs under different trapping conditions using the spectral phasor approach to reveal the temperature and association status of GNRs. Our study will help understanding manipulation of flows in solution and in biological systems that can be applied in future investigations of GNR-induced heating and flows.

Thermo-Osmotic Flow in Thin Films

We report on the first microscale observation of the velocity field imposed by a nonuniform heat content along the solid-liquid boundary. We determine both radial and vertical velocity components of this thermo-osmotic flow field by tracking single tracer nanoparticles. The measured flow profiles are compared to an approximate analytical theory and to numerical calculations. From the measured slip velocity we deduce the thermo-osmotic coefficient for both bare glass and Pluronic F-127 covered surfaces. The value for Pluronic F-127 agrees well with Soret data for polyethylene glycol, whereas that for glass differs from literature values and indicates the complex boundary layer thermodynamics of glass-water interfaces.

Hydrodynamic Boundary Effects on Thermophoresis of Confined Colloids

We study hydrodynamic slowing down of a particle moving in a temperature gradient perpendicular to a wall. At distances much smaller than the particle radius, h≪a, the lubrication approximation leads to the reduced velocity u/u_{0}=3(h/a)[ln(a/h)-9/4], where u_{0} is the velocity in the bulk. With Brenner's result for confined diffusion, we find that the trapping efficiency, or effective Soret coefficient, increases logarithmically as the particle gets very close to the wall. Our results provide a quantitative explanation for the recently observed enhancement of thermophoretic trapping at short distances. Our discussion of parallel and perpendicular thermophoresis in a capillary reveals a good agreement with experiments on charged polystyrene particles, and sheds some light on a controversy concerning the size dependence and the nonequilibrium nature of the Soret effect.

Effect of angular momentum conservation on hydrodynamic simulations of colloids

In contrast to most real fluids, angular momentum is not a locally conserved quantity in some mesoscopic simulation methods. Here we quantify the importance of this conservation in the flow fields associated with different colloidal systems. The flow field is analytically calculated with and without angular momentum conservation for the multiparticle collision dynamics (MPC) method, and simulations are performed to verify the predictions. The flow field generated around a colloidal particle moving under an external force with slip boundary conditions depends on the conservation of angular momentum, and the amplitude of the friction force is substantially affected. Interestingly, no dependence on the angular momentum conservation is found for the flow fields generated around colloids under the influence of phoretic forces. Moreover, circular Couette flow between a no-slip and a slip cylinder is investigated, which allows us to validate one of the two existing expressions for the MPC stress tensor.

Fast-moving bacteria self-organize into active two-dimensional crystals of rotating cells

We investigate a new form of collective dynamics displayed by Thiovulum majus, one of the fastest-swimming bacteria known. Cells spontaneously organize on a surface into a visually striking two-dimensional hexagonal lattice of rotating cells. As each constituent cell rotates its flagella, it creates a tornadolike flow that pulls neighboring cells towards and around it. As cells rotate against their neighbors, they exert forces on one another, causing the crystal to rotate and cells to reorganize. We show how these dynamics arise from hydrodynamic and steric interactions between cells. We derive the equations of motion for a crystal, show that this model explains several aspects of the observed dynamics, and discuss the stability of these active crystals.

Melting of a finite-sized two-dimensional colloidal crystal

We have studied the melting process of a finite-sized two-dimensional colloidal crystal by video microscopy. The local area fraction ϕ and the local hexatic orientational order parameter ψ(6) have been evaluated for respective Voronoi cells in the crystal. The histogram of ϕ exhibits a peak and the peak ϕ continuously decreases with the time elapsed. The histogram of |ψ(6)| shows an abrupt broadening for ϕ < 0.65. This critical value of ϕ is the transition point between the hexatic and dense liquid phases in finite crystal. We have also evaluated ϕ and |ψ(6)| as a function of the distance from the center of the crystal r. ϕ(r) is almost constant within the crystal and monotonously decreases with the time elapsed. |ψ(6)(r)| gradually decreases with r but there is the core with |ψ(6) = 1 at earlier time stage. The temporal change of the average ϕ within the crystal is qualitatively explained by the slow diffusion of the particles situated at the crystal edge. The steric repulsion between the particles within the crystal enhances the expansion rate of the crystal edge. Overall melting behavior is same in the crystals with different sizes. We have also studied the melting of a finite-sized crystal composed of soft-core particles by Brownian dynamics simulation and verified the finite-size effect on the melting process. The simulated behavior is qualitatively in good agreement with the experimental results.

Thermophoresis of colloids by mesoscale simulations

The motion of a colloid induced by a temperature gradient is simulated by means of multiparticle collision dynamics, a mesoscale simulation technique. Two algorithms to quantify the thermophoretic behavior are employed and contrasted. The validity of the methods is verified as a function of the temperature gradient, system size, and algorithm parameters. The variation of the solvent-colloid interaction from attractive to purely repulsive interestingly results in the change of the colloid behavior from thermophobic to thermophilic.

Reconfigurable optothermal microparticle trap in air-filled hollow-core photonic crystal fiber

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as "thermal creep flow," this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

Dynamic clustering in active colloidal suspensions with chemical signaling

In this Letter, we explore experimentally the phase behavior of a dense active suspension of self-propelled colloids. In addition to a solidlike and gaslike phase observed for high and low densities, a novel cluster phase is reported at intermediate densities. This takes the form of a stationary assembly of dense aggregates-resulting from a permanent dynamical merging and separation of active colloids-whose average size grows with activity as a linear function of the self-propelling velocity. While different possible scenarios can be considered to account for these observations-such as a generic velocity weakening instability recently put forward-we show that the experimental results are reproduced mathematically by a chemotactic aggregation mechanism, originally introduced to account for bacterial aggregation and accounting here for diffusiophoretic chemical interaction between colloidal swimmers.

Optical funneling and trapping of gold colloids in convergent laser beams

The simultaneous trapping of a large number of sedimenting Au colloids by optical radiation forces has been studied in detail. The particles are collected by a convergent laser beam and compressed against gravity and osmotic pressure at the upper window of the cell, thereby forming a dense colloidal gas. A minimum critical laser power is required to transport colloids into the trap. In contrast to conventional optical tweezers, the trap cannot be described by a 3D potential. Once the trap is sufficiently filled, the laser power can be reduced below the critical value, thereby stabilizing the trap population. Some characteristic properties of the trap, like the critical laser power and the transit time, are readily understood from a simple deterministic model. A detailed description that is capable of quantitatively accounting for the time dependence of the trap population, the finite leak rate at low power levels, or hysteresis effects requires the incorporation of fluctuations by means of a proper Langevin equation. Multiple independent traps are realized by time multiplexing of the laser beam, which allows for splitting up, independent manipulation, and subsequent recombination of a trapped colloidal cloud.

Driving forces and polymer hydrodynamics in the Soret effect

A temperature gradient induces different driving forces on the components of a mixture which translates into their segregation. We show that these driving forces constitute the physical picture behind the thermodiffusion effect, and provide an alternative expression of the Soret coefficient which can be applied to both colloidal suspensions and molecular mixtures. To verify the validity of the formalism, we quantify the related forces in an Eulerian reference frame by non-equilibrium molecular simulations. Furthermore, we present an analytical argument to show that the hydrodynamic interactions need to be accounted for to obtain the proper scaling of the thermophoretic force. This result combined with the presented expression satisfactorily explains the experimentally known size dependence of the thermodiffusion coefficient in dilute polymer solutions.

Accumulating microparticles and direct-writing micropatterns using a continuous-wave laser-induced vapor bubble

Through the enhanced photothermal effect, which was achieved using a silver film, a low power weakly focused continuous-wave laser (532 nm) was applied to create a vapor bubble. A convective flow was formed around the bubble. Microparticles dispersed in water were carried by the convective flow to the vapor bubble and accumulated on the silver film. By moving the laser spot, we easily manipulated the location of the bubble, allowing us to direct-write micropatterns on the silver film with accumulated particles. The reported simple controllable accumulation method can be applied to bimolecular detection, medical diagnosis, and other related biochip techniques.

Fabrication and placement of a ring structure of nanoparticles by a laser-induced micronanobubble on a gold surface

We have developed a new fabrication method for a ring structure of assembled nanoparticles on a gold surface by the use of continuous Nd:YAG laser light. A micronanobubble on a gold surface, created by laser local heating, acts as a template for the formation of the ring structure. Both Marangoni convection flow and capillary flow around the micronanobubble are responsible for the driving force to assemble nanoparticles such as CdSe Q-dots into the ring structure from the solution. Because a single micronanobubble was generated by the Nd:YAG laser focusing point, the precise positioning of the ring structure was feasible directly on the gold surface, which makes it possible to fabricate various patterns of rings such as arrays and letters and even a double-ring structure without any photomasks or any templates.

Laser-induced thermophoresis of individual particles in a viscous liquid

This paper presents a detailed investigation of the motion of individual micro-particles in a moderately-viscous liquid in direct response to a local, laser-induced temperature gradient. By measuring particle trajectories in 3D, and comparing them to a simulated temperature profile, it is confirmed that the thermally-induced particle motion is the direct result of thermophoresis. The elevated viscosity of the liquid provides for substantial differences in the behavior predicted by various models of thermophoresis, which in turn allows measured data to be most appropriately matched to a model proposed by Brenner. This model is then used to predict the effective force resulting from thermophoresis in an optical trap. Based on these results, we predict when thermophoresis will strongly inhibit the ability of radiation pressure to trap nano-scale particles. The model also predicts that the thermophoretic force scales linearly with the viscosity of the liquid, such that choice of liquid plays a key role in the relative strength of the thermophoretic and radiation forces.

Holographic optical tweezers and their relevance to lab on chip devices

During the last decade, optical tweezers have been transformed by the combined availability of spatial light modulators and the speed of low-cost computing to drive them. Holographic optical tweezers can trap and move many objects simultaneously and their compatibility with other optical techniques, particularly microscopy, means that they are highly appropriate to lab-on-chip systems to enable optical manipulation, actuation and sensing.

Direct observation of grain rotation-induced grain coalescence in two-dimensional colloidal crystals

A spatially modulated laser is used to produce multiple localized thermal gradients in a colloidal sample placed above a gold surface. We use an optical microscope to observe real time dynamics of the resulting two-dimensional colloidal crystal grains and find that grain rotation-induced grain coalescence (GRIGC) occurs with the rotation of both grains before coalescence. Control over the grain size shows that the time scale for grain boundary annealing in our system is in good agreement with theoretical expressions formulated for nanocrystal growth.

Flow-assisted single-beam optothermal manipulation of microparticles

An optothermal tweezer was developed with a single-beam laser at 1550 nm for manipulation of colloidal microparticles. Strong absorption in water can thermally induce a localized flow, which exerts a Stokes' drag on the particles that complements the gradient force. Long-range capturing of 6 microm polystyrene particles over approximately 176 microm was observed with a tweezing power of approximately 7 mW. Transportation and levitation, targeted deposition and selective levitation of particles were explored to experimentally demonstrate the versatility of the optothermal tweezer as a multipurpose particle manipulation tool.

Drift-diffusion kinetics of a confined colloid

The drift-diffusion equation on a finite interval with reflecting boundary conditions is solved by Laplace transformation. The Green function is obtained as a series in powers of e(-hu/D), where u is the drift velocity, D the diffusion coefficient and h the width of the interval. In the drift-dominated regime hu/D >> 1, the first terms provide an exact solution in the limit of short and long times, and a good approximation in the intermediate regime. As a possible application, we discuss confined colloidal suspensions subjected to an external field.

Hydrodynamic attraction of immobile particles due to interfacial forces

Applying the method of reflections, we derive the flow pattern around a confined colloidal particle with quasislip conditions at its surface, in powers of the ratio a/h of particle radius and wall distance. The lowest order corresponds to a single reflection at the confining wall. Significant corrections occur at higher order: the linear term in a/h modifies the amplitudes of the well-known one-reflection approximation, whereas new features arise in quadratic order. Our results agree with recent experiments where thermo-osmosis drives hydrodynamic attractive forces in confined colloids.

Biomimetic behavior of synthetic particles: from microscopic randomness to macroscopic control

Randomness is an inherent property of biological systems. In contrast, randomness has been mostly avoided in designing synthetic or artificial systems. Particularly, in designing micro/nano-motors, some researchers have successfully used external fields to gain deterministic control over the directionality of the objects, which otherwise move in completely random directions due to Brownian motion. However, a partial control that preserves a certain degree of randomness can be very useful in certain applications of micro/nano-motors. In this Perspective we review the current progress in establishing autonomous motion of micro/nano-particles that possess controlled randomness, provide insight into the phenomena where macroscopic order originates from microscopic disorder and discuss the resemblance between these artificial systems and biological emergent/collective behaviors.