Photophoretic Force Research Articles

Dynamic Effects of Laser Action on Quasi-Two-Dimensional Dusty Plasma Systems of Charged Particles.

We present the results of an experimental study of the behavior of a colloidal plasma system formed by copper-coated and uncoated polymer particles under the action of laser irradiation. A comparative study of particle velocity distribution profiles depending on the power of the pushing laser was conducted. In the case of uncoated melamine-formaldehyde (MF) particles, we observed the well-known action of light pressure, causing shear stress in the colloidal plasma structure and leading to the occurrence of a laminar flow within the affected area. For the copper-coated MF particles, we revealed some additional patterns of behavior for the dust particles, i.e., kinetic temperature growth due to laser radiation absorption by the copper coating, as well as the appearance of chaotic particle motion. We believe that this happens due to the existence of defects in the coating, causing asymmetric heating of the particles, which in turn leads to chaotic deviations of the photophoretic force pushing the particles in different directions.

Spatial-light-modulator-assisted laser manipulation in air

Spatial light modulators (SLMs) provide a powerful technology to perform high-resolution and high-speed phase modulation of laser radiation for the generation of structured laser beams. The possibility of using SLMs for dynamic laser manipulation of airborne light-absorbing particles due to the action of the photophoretic forces, demonstrating different types of manipulation, including confinement of the trapped particles, two- and three-dimensional guiding of the trapped particles, their controllable revolution, and the transfer of trapped particles between different optical traps, is shown. For this purpose, different phase masks that can be used for the generation of the desired optical traps with the help of SLMs are provided, opening new possibilities for using SLMs for laser manipulation in air and allowing for the realization of all-optical conveyors for the fabrication of microdevices in the future.

Diffusiophoresis and Photophoresis of Heated Large Nonvolatile Aerosol Spherical Particles

This paper describes a theoretical study of the steady motion of a large solid nonvolatile aerosol spherical particle, which contains thermal sources within itself, in a concentration gradient of binary gas mixture components. It is assumed that an average particle surface temperature significantly differs from the temperature of the binary gas mixture surrounding it. Equations of gas dynamics are solved taking into account the power-law dependence of the molecular transfer coefficients (viscosity, thermal conductivity, and diffusion) and the density of the gaseous medium on temperature. Under boundary conditions, diffusion and thermal slip are taken into account. Numerical estimates show that the diffusion and photophoretic forces and velocity substantially depend on the average particle surface temperature.

Optical counting of guided particles in a vortex beam as an optical tube by laser-two-focus method

In this study, microscopic particles such as aerosols were counted by Laser-Two-Focus method, L2F, after being trapped and guided by an optical tube. This prevents particles diffusivity as they pass through the Gaussian beam in the L2F method by optical forces such as radiation pressure and photophoretic forces. In optical tube, particles can guid to the center of the beam where the intensity gets zero. A single-charged Bessel Gaussian beam, BG01, is used as the particle guidance beam in this method, which is generated by passing the first-order Laguerre-Gaussian beam, LG01, from an axicon lens. LG01 beam are also produced by using holographic interference grid mask. The results of the theory and simulations showed that by optical guiding of particles in the L2F-method measurement, their transverse turbulence can be reduced by about 60% and then the probability of measuring all particles to be increased by about 30%. Measurements of in-laboratory aerosols less than 3.5 μm with this method showed a 20% increase in their condensation of them compared to the conventional L2F method.

Laser-induced rotary micromotor with high energy conversion efficiency

Light is a precious resource that nature has given to human beings. Converting green, recyclable light energy into the mechanical energy of a micromotor is undoubtedly an exciting challenge. However, the performance of current light-induced micromotor devices is unsatisfactory, as the light-to-work conversion efficiency is only 10 − 15 – 10 − 12 . In this paper, we propose and demonstrate a laser-induced rotary micromotor operated by Δ α -type photopheresis in pure liquid glycerol, whose energy conversion ratio reaches as high as 10 − 9 , which is 3–6 orders of magnitude higher than that of previous light-induced micromotor devices. In addition, we operate the micromotor neither with a light field carrying angular momentum nor with a rotor with a special rotating symmetrical shape. We just employ an annular-core fiber to configure a conical-shaped light field and select a piece of graphite sheet (with an irregular shape) as the micro-rotor. The Δ α -type photophoretic force introduced by the conical-shaped light field drives the rotation of the graphite sheet. We achieve a rotation rate up to 818.2 r/min, which can be controlled by tuning the incident laser power. This optical rotary micromotor is available for twisting macromolecules or generating vortex and shear force in a medium at the nanoscale.

Sustained Self-Starting Orbital Motion of a Glass-Fiber “Nanoengine” Driven by Photophoretic Forces

Controllable optically driven rotation of microscopic objects is desirable in many applications, but is difficult to achieve. Here we report a sustained self-starting orbital motion of a clamped elongated nanostructure, a glass-fiber nanospike, when a CW laser beam is focused axially onto its tip. Analysis shows that photophoretic antitrapping forces, acting on the nanospike with a delayed response, introduce optomechanical gain into the mechanical motion, overcoming the intrinsic mechanical dissipation and resulting in growth from noise of oscillations at the resonant frequency of the nanospike. These photophoretic forces further enable phase-locking of the orthogonal fast and slow vibrations of the nanospike (induced by slight mechanical anisotropy), giving rise to a self-sustained orbital motion. The locked phase of orbital motion can be changed by tuning the gas pressure and adjusting the geometrical asymmetry of the system. This light-driven nanoengine opens up a new degree of freedom for controlling the rotational motion of elongated nano-objects.

“Optical mill”—A tool for the massive transfer of airborne light-absorbing particles

We present an all-optical tool for the massive transfer of airborne light-absorbing particles. A generated light sheet trap can be used as an “optical mill” for guiding particles via photophoretic forces. We show the possibility of transferring hundreds to thousands of trapped particles from one cuvette to another in a controllable manner. Two different types of particles were used for demonstration—nonspherical agglomerations of carbon nanoparticles and printer toner particles with a more regular shape. The proposed tool can be used for the transportation of light-absorbing particles, such as biological nano- and micro-objects, or for the touch-free sampling of airborne particles being measured.

X-typed curvilinear transport of strongly absorbing particle in a dual-beam fiber optical trap.

We propose and demonstrate a novel approach to transport a strongly absorbing particle in an X-typed trajectory reciprocally in pure liquid glycerol based on a dual-beam optical fiber trap. We perform the X-typed light field by integrating a glass microsphere on the tip of a two-core fiber. The motion of the absorbing particle in pure liquid glycerol is dominated by the Δα-type photophoretic forces (FΔα). The incident laser power determines the direction of FΔα. Therefore, we may perform the reciprocating transport of the absorbing particle by changing and controlling the laser power. It is simple to manufacture the fiber probe and convenient to operate the transport of the microparticle. Our research expands the applications of absorbing particles in targeted drug delivery, biological sampling, and optically mediated particle clearing.

Thermophoretic and photophoretic velocities and forces of a spherical particle embedded in Brinkman medium

An analytical investigation is considered for the thermophoresis and photophoresis of a spherical aerosol particle embedded in a porous medium. The Knudsen number is assumed to be in the slip-flow regime. The porous medium is assumed to be homogenous, isotropic and the solid matrix is in thermal equilibrium with the fluid through the voids of the medium. At the surface of the particle, a temperature jump, a thermal creep, a viscous slip and thermal stress slip are considered in the analysis of motion. Expressions for thermophoretic and photophoretic velocities and forces are obtained. The effect of the Brinkman number characterizing the permeability of the medium is investigated as functions of the thermal properties of the porous medium and particle. The limiting cases of Stokes and Darcy's flows and no-slip case are discussed.

Simulation of motion of charged particle within a fluid in an optical tube

In this study, the behavior of charged particles within a fluid is investigated when exposed to the radiation of a Laguerre-Gaussian beam as an optical tube. Optical forces (e.g., Photophoretic, Radiation pressure and Lorentz forces) and non-optical forces (e.g., Drag and gravitational forces) determine the trajectories of ions movement within these beams. Trapping of charged particles in the direction of the beam axis in an optical tubes depends on the superiority of the radial component of the photophoretic force on the electromagnetic force. The factors affecting this, such as the amount and type of electrostatic charge and the radius of the particle, and the shape and power of the beam are studied. As a result, the charged particle can be guided to various paths based on their electrostatic charge and particle diameter. This could be done in the study of ions within a fluid by optical methods. One of the applications proposed in this regard is that we can counted charged and uncharged particles within a fluid (e.g., liquid ions or aerosols) by separating them in an optical tube.

Photophoresis of aerosol particles with nonuniform gas–surface accommodation in the free molecular regime

The problem of calculating the photophoretic force acting on an aerosol particle, with nonuniform distributions of accommodation coefficients over the particle's surface in a rarified equilibrium gas medium in the free molecular regime, is considered within the framework of perturbation theory, using the assumption that the variations of accommodation coefficients are sufficiently small. In general, the expression for the force acting on a convex particle involves four contributions. The energy accommodation coefficient and the radiometric accommodation coefficient for normal momentum are used.The formulae for calculating all four contributions to the photophoretic force are presented for a uniform spherical particle with finite thermal conductivity and a nonuniform axisymmetric distribution of accommodation coefficients over its surface. According to our estimates, the photophoretic force is defined with good accuracy by only two contributions: the “delta-alpha” force and “delta-T” force. The other contributions are small corrections to these two prevailing forces of order 1% or less.The ratio between the “delta-alpha” and “delta-T” forces depends on the particle size, Knudsen number, and the differences in the accommodation coefficients. For micron-sized spherical particles, the photophoretic “delta-alpha” force exceeds the “delta-T” force, and also exceeds gravity if the variation in the radiometric accommodation coefficient over the particle's surface is more than 0.055. Thus, levitation of particles in the Earth's stratosphere is possible due to gravito-photophoresis. For centimetre-sized particles, the “delta-T” force is a few orders of magnitude larger than the “delta-alpha” force due to the influence of IR radiation on the particles' temperature.

Investigation on the laser trapping mechanism of light-absorbing particles in air

We report on the micron-sized light-absorbing particle trapping in two configurations (horizontal and vertical), in order to elucidate the laser trapping mechanism based on the photophoretic force. Two types of carbon particles (irregular graphite particles and carbon microspheres) were tested in both Gaussian and hollow beam traps. By comparing the trapping efficiency and stability under various circumstances, we confirmed that there are two types of photophoretic forces: and forces on the laser irradiating particle. Furthermore, the forces and moments exerting the particles in various traps were analyzed, which explained the experimental phenomena very well. It was found that the force due to the thermal accommodation difference among the irregular particles helps the irregular particles to be balanced more easily and of higher trapping efficiency and stability. This work provides important references for people to choose a suitable trapping scheme according to the particles, which would be of significance in the applications of single-particle analysis.

The role of molecule-surface interaction in the photophoresis of an aerosol particle

The role of the interaction of gas molecules with a surface on the photophoresis of a spherical aerosol particle is studied under the conditions of a free molecule regime. An expression for the photophoretic force is obtained for an arbitrary scattering kernel. Calculations are carried out using various scattering-kernel models.

Optical attraction of strongly absorbing particles in liquids.

Although optical tweezers function well for the majority of transparent particles, the absorbing particles experience a considerably high absorption force that can destroy the stable optical traps. Photophoretic force is an alternative mechanism that can be used to trap the absorbing particles. The major difficulty that is associated with the utilization of photophoretic forces for trapping strongly absorbing particles in liquids is the presence of considerable absorption on the illuminated side; a positive photophoretic force is usually induced, thereby pushing away the absorbing particles from the high-intensity region of the laser source. Here, we demonstrate a novel principle for the optical trapping and manipulation of strongly absorbing particles by harnessing strong Δα-type photophoretic forces while suppressing their stochastic nature in pure liquid glycerol using a normal divergent Gaussian beam and a Bessel-like beam. Further, our approach expands the optical manipulation of strong absorbing particles to liquid media and provides position control over the trapped particles, including the optical transportation and pinpoint positioning of the 3-μm objects over a distance of a millimeter.

Photonic tractor beams: a review

Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object’s optical parameters. The third one is designing the structured material background, in which the light–matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.

Theory of thermophoresis of a spherical particle embedded in a micropolar fluid

Analytical investigations are considered for the related problems of thermophoresis, photophoresis and thermophoresis with chemical reactions of a spherical aerosol particle embedded in a gaseous micropolar medium. The Knudsen number is assumed to be in the slip-flow regime. Viscous and gyrational dissipations are ignored in the energy equation. At the surface of the particle, a thermal stress slip and the creep slip are considered in the analysis of motion. Expressions for thermophoretic and photophoretic velocities and forces are obtained as functions of the micropolarity parameter characterizing the micropolar medium and the thermal properties of the medium and particle. Results of the effect of the chemical reaction within the particle on its thermophoresis velocity are presented for various distributions of the composition-dependent factor or heat generation parameter of the chemical reaction as well as the relative thermal and surface properties of the aerosol spherical particle. The corresponding limiting cases of the classical viscous fluids are obtained.

Absorbing particle 3D trap based on annular core fiber tweezers

We proposed and demonstrated a new method to trap and manipulate a strongly absorbing particle by harnessing strong Δα-type photophoretic forces in pure liquid glycerol with a focused annular beam, which was introduced by an annular core fiber optical tweezers. We performed the focused annular light field by integrating a high refractive index silica microsphere on the annular core fiber end facet. The silica microsphere, which acted as a lens, focused laser beam for trapping. We tested and calculated Δα-type photophoretic forces tendency by using the fluorescence dye method, and the simulated results were in agreement with the experimental results. The proposed optical fiber tweezers were small in size, large in capture range, simple to manipulate, and inexpensive in cost. It provided a new approach for absorbing particles trapping. Tailoring the trapping and manipulation of absorbing particles in liquid environments will open avenues for studying the special, yet-unexplored characteristics of absorbing particles.

Photophoretic force measurement on microparticles in binary complex plasmas

This article presents a method to measure the photophoretic force on microparticles in complex plasmas. A laser heating setup is used to increase the kinetic temperatures of binary mixtures composed of melamine-formaldehyde (MF) and silica (SiO2) particles. The temperature gain of both particle species due to the additional thermal reservoir is compared to a laser force model. Contributions of radiation pressure and photophoretic forces are determined through a variation of neutral gas pressure and application of Lorenz-Mie theory. The photophoretic force is found to be significant for MF particles at typical conditions in complex plasma experiments.

Reynolds number and diffusion coefficient of micro- and nano-aerosols in optical pipelines

In this study, the microscopic particle motion inside an optical pipeline, such as particle motion through a mechanical tube, is investigated. The photons in an optical tube guide the particles towards the center of the light beam by inducing photophoretic and radiation pressure forces. Laguerre–Gaussian- and Bessel-like beams are examples of such optical tubes. The Reynolds number of particle motion in optical tubes is investigated. The power of the light beam and the ratio of the particle radius to the light beam ring radius influence the turbulence of the particle flow and the value of the Reynolds number. The diffusion coefficient of particle movement in such pipelines is derived, which indicates that an optical tube is a good tool for guiding and trapping particles in micron- and nanometer-scale dimensions.

Temporal Dependence of Photophoretic Force Optically Induced on Absorbing Airborne Particles by a Power-Modulated Laser

Photophoretic force due to the optically-induced thermal effect provides an effective way to manipulate the light-absorbing particles suspended in ambient gases. However, how this force temporally responds to the intensity modulation of the illumination light is unclear. Here, by vertically trapping a micron-sized absorbing particle with a negative photophoretic force generated by a focused Gaussian beam, we demonstrate that the temporal change in the photophoretic force in response to the intensity modulation is remarkably slow (with a time constant up to ~1 s) due to the slow change in the particle's temperature. When the trapping beam is turned off for a few tens or hundreds of milliseconds, the trapped particle is found to be pulled up towards the light source by the remained photophoretic force, whereas when the intensity of the trapping beam is increased for a short duration, the particle is pushed off by the radiation pressure. The instantaneous position of the trapped particles following the intensity modulation of the trapping laser is tracked and theoretically modeled. The understanding of the temporal behavior of the photophoretic force would be useful for the control of photophoretic-based optical pulling, transportation, and manipulation of atmospheric particles.