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This mini review discusses the phenomenon of thermophoresis, also known as the thermophoretic effect. Thermophoretic effect arises from the combination of a temperature gradient and particles of very small dimensions, on the order of magnitude of the mean free path of the molecules of the surrounding gas. Despite being a little-known effect, it is critical to many physical and chemical processes and for characterising the properties of nanostructured materials that could be used in industry for sensing applications. A description and definition of otherwise very similar thermophoresis terms is provided, as well as a brief overview of the literature on this topic, with a focus on research in the twenty-first century.

This mini review discusses the phenomenon of thermophoresis, also known as the thermophoretic effect. Thermophoretic effect arises from the combination of a temperature gradient and particles of very small dimensions, on the order of magnitude of the mean free path of the molecules of the surrounding gas. Despite being a little-known effect, it is critical to many physical and chemical processes and for characterising the properties of nanostructured materials that could be used in industry for sensing applications. A description and definition of otherwise very similar thermophoresis terms is provided, as well as a brief overview of the literature on this topic, with a focus on research in the twenty-first century. [ABSTRACT FROM AUTHOR]

Microplastics are highly persistent emerging pollutants that are widely distributed in the environment. We report the use of MoS 2 @Fe 2 O 3 core-shell micromotors prepared by a hydrothermal approach to explore the degradation of plastic microparticles. Polystyrene was chosen as the model plastic due to its wide distribution and resistance to degradation using current approaches. Micromotors show photophoretic-based motion at speeds of up to 6 mm s -1 and schooling behavior under full solar light spectra irradiation without the need for fuel or surfactants. During this impressive collective behavior, reactive oxygen species (ROS) are generated because of the semiconducting nature of the MoS 2 . Degradation of polystyrene beads is observed after 4 h irradiation because of the synergistic effect of ROS production and localized heat generation. The MoS 2 @Fe 2 O 3 micromotors possess magnetic properties, which allow further cleaning and removal to be carried out after irradiation through magnetic pulling. The new micromotors hold considerable promise for full-scale treatment applications, only limited by our imagination.

Autonomous nanomotors have become the new paradigm for current research as they are expected to shift the momentum in the development of next‐generation technologies. However, there is a grand challenge in gaining control over the nanomotors’ motion, speed, directionality, and using biocompatible fuels to power them. Currently, light is recognized for powering micromotors with advancement in using visible light for driving motion at the nanoscale regime. In this context, micron‐scaled motors are fabricated but they contain metal surfaces and fabrication is quite laborious. Herein, encapsulation of plant organelles into supramolecular assemblies for active motion is conducted to fabricate bio‐nanomotors, utilizing the natural photosynthesis process for powering motion at the nanoscale. The oxygen produced by the water‐splitting reaction by plant organelles in visible light and the photophoresis effect due to the transparent nature of the supramolecular assembly are the main driving forces for bio‐nanomotors. The bio‐nanomotors are observed to have propelled motion with speed reaching up to 120.42 ± 12 μm s−1, together with on‐demand reversible on/off motion and real‐time control over change in directionality at the nanoscale. The observed results shift the momentum toward harnessing energy from natural processes to power nanosystems for varied applications.

Crookes radiometers have been the subject of numerous theoretical, numerical, and experimental studies because of the complicated forces they exhibit as well as their potential applications to light sensing and actuation. The majority of these studies have focused on classical radiometers, which function under low vacuum pressures. In contrast, here we report a radiometer with microengineered vanes that rotates at atmospheric pressure. Its functionality at pressures thousands of times higher than previous light mills is due to unique attributes of the nanocardboard that forms its vanes: 1) the extremely low areal density (0.1 mg/cm2) of nanocardboard reduces the vane masses by two orders of magnitude; 2) its lower thermal conductivity allows a greater cross-vane temperature difference; and 3) its microchannels dramatically increase the thermal transpiration flow that drives the rotation. Intriguingly, the experimentally observed rotation speeds are substantially higher than those theoretically expected. Our device demonstrates new possibilities for micromanipulation, propulsion of aerial vehicles, and light-powered generators. [2020-0165] [ABSTRACT FROM AUTHOR]

Autonomous nanomotors have become the new paradigm for current research as they are expected to shift the momentum in the development of next‐generation technologies. However, there is a grand challenge in gaining control over the nanomotors' motion, speed, directionality, and using biocompatible fuels to power them. Currently, light is recognized for powering micromotors with advancement in using visible light for driving motion at the nanoscale regime. In this context, micron‐scaled motors are fabricated but they contain metal surfaces and fabrication is quite laborious. Herein, encapsulation of plant organelles into supramolecular assemblies for active motion is conducted to fabricate bio‐nanomotors, utilizing the natural photosynthesis process for powering motion at the nanoscale. The oxygen produced by the water‐splitting reaction by plant organelles in visible light and the photophoresis effect due to the transparent nature of the supramolecular assembly are the main driving forces for bio‐nanomotors. The bio‐nanomotors are observed to have propelled motion with speed reaching up to 120.42 ± 12 μm s−1, together with on‐demand reversible on/off motion and real‐time control over change in directionality at the nanoscale. The observed results shift the momentum toward harnessing energy from natural processes to power nanosystems for varied applications. [ABSTRACT FROM AUTHOR]

We report a novel, dry, and clean method to directly deposit single-walled carbon nanotubes (SWCNTs) onto a desired substrate with the photophoresis phenomenon. We demonstrate the photophoresis method to be convenient and reproducible to deposit low-bundled nanotubes to form either non-percolating or continuous networks with controlled SWCNT density and surface distribution. Employing the structure-dependent optical features of the nanotubes, we show the correlation between the light source spectrum and the configuration of the deposited SWCNTs due to the selective light interaction with SWCNTs of certain chiralities. Thus, photophoretic deposition is a promising method for the fabrication of various advanced devices, such as electrically driven single-photon emitters or field-effect transistors from semiconducting SWCNTs with a desired electronic structure. [Display omitted] • Single-walled carbon nanotubes were synthesized by the aerosol CVD method. • Novel method for the photophoretic deposition of aerosol SWCNTs was proposed. • Photophoresis can be used to deposit both individual and bundled SWCNTs. • Chirality selective photophoretic deposition was observed for the first time. [ABSTRACT FROM AUTHOR]

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. • A new approximation is presented for the photophoretic force using the perturbation theory. • Four contributions to the photophoretic force are described, two of which are dominant. • The ratio between contributions strongly depends on the particle size, due to IR radiation. • Micron-sized aerosol particles can levitate in the Earth's atmosphere. [ABSTRACT FROM AUTHOR]

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. • The role of the molecule-surface interaction in the photophoresis has been studied. • Calculations were carried out using several known models of the scattering kernel. • The use of the different models of the kernel leads to qualitative agreement of the results. [ABSTRACT FROM AUTHOR]

The two thermal effects, thermophoresis and photophoresis phenomena that cause particle movements due to thermal gradient through the liquid and thermal gradient through the particle, respectively, have been widely studied over the past years because of their wide range of applications. This thermal gradient can be made by laser beam. There are a few studies concerning these two effects, especially photophoresis, in liquid media. In this paper, these two effects and their induced velocity to particles are studied in liquid media. The affecting parameters on these effects are studied and their effect on particles are determined. Effect of laser parameters like laser power and wavelength in the channel are discussed and the maximum velocity and temperature inside the channel are calculated. Also in the photophoresis part, the effect of parameters like laser power, particle and laser beam diameter is calculated. By considering the existing models for calculation of thermophoretic velocity, Brenner model is chosen as the most accurate model and will be used in calculations. It is also found that the effect of laser wavelength on thermophoretic velocity is more than changing laser power. In the photophoresis part, photophoretic velocity is calculated by using existing analytical models. The calculated velocities of thermophoresis and photophoresis are compared with the experimental values and there is an acceptable matching between them. The results of this paper will be used for designing and making a particle separator tool. [ABSTRACT FROM AUTHOR]

A microphysical model describing the photophoretic motion of soot aggregates with allowance for their fractal structure is presented. The comparison with known experimental data has been performed and their good qualitative and quantitative agreement has been found. The calculated characteristics of the motion of fractal-like soot aggregates in the atmospheric radiation field are presented. It is shown that photophoretic effects for soot aerosol under conditions of a steady-state atmosphere in the model for fractal-like particles manifest themselves most significantly at heights of the upper troposphere and middle stratosphere. [ABSTRACT FROM AUTHOR]

Small, illuminated aerosol particles embedded in a gas experience a photophoretic force. Most approximations assume the mean particle surface temperature to be effectively the gas temperature. This might not always be the case. If the particle temperature or the thermal radiation field strongly differs from the gas temperature (optically thin gases), given approximations for the free molecule regime overestimate the photophoretic force by an order of magnitude on average and for individual configurations up to three magnitudes. We apply the radiative equilibrium condition from the previous paper (Paper 1) – where photophoresis in the free molecular flow regime was treated – to the slip flow regime. The slip-flow model accounts for thermal creep, frictional and thermal stress gas slippage and temperature jump at the gas–particle interface. In the limiting case for vanishing Knudsen numbers – the continuum limit – our derived formula has a mean error of only 4% compared to numerical values. Eventually, we propose an equation for photophoretic forces for all Knudsen numbers following the basic idea from Rohatschek by interpolating between the free molecular flow and the continuum limit. [ABSTRACT FROM AUTHOR]

• We show that certain classes of integrals with products of Bessel functions of the first kind and their derivatives can be solved analytically. • Such integrals appear in photophoresis problems when introducing partial wave expansions for the fields internal to the scatterers. • Specific numerical integrations in the field of photophoresis can now be replaced by closed-form analytical expressions. • Computational time can be reduced by two or even three orders of magnitude. • Examples of computations are given for asymmetry factors. In certain physical problems of light scattering, classes of integrals appear which involve particular products of Bessel functions of the first kind with complex argument and integer orders n and n ± 1 (− ∞ ≤ n ≤ ∞), and also products of derivatives of such Bessel functions. Due to the lack of available analytical solutions in the literature, numerical calculations of these integrals have been recently carried out for the evaluation of photophoretic asymmetry factors (PAFs) in problems involving the illumination of lossy infinite cylinders, either in isolation or close to conducting corner spaces or planar boundaries, by plane waves or light-sheets. Here, we show that these integrals can actually be resolved analytically, therefore allowing for faster computation of physical quantities of interest in light scattering by small particles. [ABSTRACT FROM AUTHOR]

Aerosol particles experience significant photophoretic forces at low pressure. Previous work assumed the average particle temperature to be very close to the gas temperature. This might not always be the case. If the particle temperature or the thermal radiation field differs significantly from the gas temperature (optically thin gases), given approximations overestimate the photophoretic force by an order of magnitude on average with maximum errors up to more than three magnitudes. We therefore developed a new general approximation which on average only differs by 1% from the true value. [ABSTRACT FROM AUTHOR]

• We introduce optical forces from thermal origins with the aid of the generalized Lorenz-Mie theory (GLMT) • It is, to the best of the authors' knowledge, the first formal analysis to describe photophoresis with the GLMT stricto sensu • Analytical formulas are presented for the cartesian components of the asymmetry vector, which is proportional to photophoretic forces • Results are valid both in the slip-flow and in the continuous regime and for any arbitrary shaped beam Based on the adjoint boundary value problem proposed decades ago by Zulehner and Rohatschek [1] , analytic and closed-form expressions for the photophoretic forces exerted by arbitrary-shaped beams on homogeneous and low-loss spherical particles is derived in both the free molecular and slip flow regimes. To do so, the asymmetry vector for arbitrary refractive index particles is explicitly calculated by expanding the internal electromagnetic fields with the aid of the generalized Lorenz-Mie theory (GLMT). The approach here proposed is, to the best of the authors' knowledge, the first systematic attempt to incorporate the GLMT stricto sensu into the field of photophoresis and might as well be extended, e.g. to spheroids and find important applications, among others, in optical trapping and manipulation of microparticles, in geoengineering, particle levitation, optical trap displays and so on. [ABSTRACT FROM AUTHOR]

Photophoresis is a force which can dominate the motion of illuminated aerosols in low pressure environments of laboratory experiments, planetary atmospheres or protoplanetary disks. In drop tower experiments we quantified the photophoretic force on a sample of micrometer-sized basalt grains and aggregates thereof. The particle motions are systematic (linear, helical, in one direction), with most particles moving along the direction of light. Our results are consistent with analytical estimates and numerical simulations of photophoretic forces for small dust aggregates. It implies that the forces are dominated by Δ T - photophoresis with little evidence for Δ α - photophoresis in the micrometer size range. [ABSTRACT FROM AUTHOR]

In low pressure environments macroscopic bodies can be subject to photo- and thermophoretic motion if temperature gradients are present. Porosity can increase the efficiency of this propulsion. We developed a setup to generate a temperature field and measure phoretic accelerations in a parabolic flight. In a first campaign we studied the pressure dependence of the acceleration for thin plates (1.7 mm thickness, 2.2 and 3.5 cm diameter) consisting of sintered glass spheres of a size range of 150 to 250 μm and 40 to 70 μm. We find evidence for two characteristic propulsion maxima at pressures related to the overall size of the plate as well as the bead (pore) size. The increase of the magnitude of acceleration due to the porosity is on the order of 10 g for the specific samples. This force is comparable to the phoretic force attributed to the overall size of the plate. [ABSTRACT FROM AUTHOR]

Abstract: By computations we demonstrate a new form of interaction between aerosol particles—a photophoretic interaction of a gas-kinetic nature that emerges when the particles are illuminated in a rarefied gas medium. Under the effect of light, the temperatures of particles differ from the temperature of the ambient gas and, under the effect of other particles in the gas medium, upsets the uniformity of accommodation of molecular energy and momentum over the surface of these particles to bring forth forces affecting the particles. Photophoretic forces were calculated on the basis of a free molecular flow regime approximation and a previously developed Monte Carlo algorithm. Calculations showed that when submicron particles that are soot-like in their optical properties are irradiated with solar light, significant repulsive forces arise between them that are by orders of magnitude higher than gravity, and these forces decrease approximately as the inverse square of the distance between the particles. Forces calculated for the free-molecule approximation are fairly accurate for a rarefied gas when the Knudsen number of the aerosol system is greater than one. Molecular collisions essentially decrease the photophoretic interaction forces for low Knudsen number. Photophoretic repulsion can be increased manyfold as the intensity of the incident radiation increases. Photophoretic interaction results not only in the repulsion of particles but can also be the cause of their attraction. For example, at night in the Earth's atmosphere at altitudes close to the stratopause the particles do not repulse but are attracted to each other. A study of individual aerosol particles interacting with clusters demonstrates noticeable spatial asymmetry of this interaction. The photophoretic interaction is shown to decrease the coagulation constant of submicron particles strongly absorbing radiation by orders of magnitude compared to the constant in darkness, even at an intensity of the external radiation equal to the solar light intensity. The significant effect of the photophoretic interaction on coagulation has been demonstrated. [Copyright &y& Elsevier]

Highlights: [•] RBCs were migrated faster than WBCs and blood pellets by laser photophoresis. [•] Photophoretic efficiency of RBC and WBC was simulated by the Mie scattering theory. [•] Spontaneous orientation of RBC parallel to the migration direction was elucidated. [•] Laser photophoretic separation of RBC and WBC was possible in a tip flow system. [Copyright &y& Elsevier]

Abstract: An analysis is presented for the effects of thermal stress slip on the thermophoresis and photophoresis of a spherical aerosol particle. The Knudsen number is assumed to be moderately small so that the fluid motion is described by a near-continuum slip-flow model with a temperature jump, a thermal creep, a frictional slip, and a thermal stress slip at the particle surface. In the limit of small Peclet and Reynolds numbers, the energy and momentum equations governing the systems are solved analytically at steady state. Expressions for the migration velocities of the particle are obtained in simple closed forms for the cases of thermophoresis in a uniformly prescribed bulk-gas temperature gradient and of photophoresis subject to an intense light beam. Our results indicate that the effects of thermal stress slip can be significant when the Knudsen number is not too small or the particle is not too large. The effects of thermal stress slip can increase or decrease the thermophoretic mobility of the particle, depending on some properties of the particle and surrounding gas, but always augment the photophoretic mobility of the particle. The analytical predictions with the effects of thermal stress slip in general fit with available experimental data of thermophoretic and photophoretic forces on aerosol spheres better than those without the effects do. [Copyright &y& Elsevier]

Abstract: The results of theoretical and experimental study of so-called “accommodation” forces acting on particles with artificial asymmetry of surface properties in rarefied gas in the directed radiation field are presented. The gas-kinetic theory of phenomenon in the free-molecular regime for some different types of boundary conditions of gas-surface interactions is developed. The theory predicts appearance of the unusual force caused by the difference of the normal momentum accommodation coefficients on hemispheres of considered model Janus particle. The measurements conducted by the experimental technique with macroparticles have confirmed the existence of the accommodation forces affecting a particle together with the forces of radiometric photophoresis. The experimental results for the system “steel particle–helium” are in good agreement with theoretical predictions, and the ratio of the “accommodation” forces to the photophoretic force does not exceed 3%. The estimations show that the “accommodation” forces can be comparable in some cases with the radiometric photophoretic forces for micron-sized particles in stratosphere, but do not exceed the gravity force. The action of “accommodation” forces, in our opinion, is not capable to provide effective vertical transport of aerosol particles against gravity at stratospheric altitudes. [Copyright &y& Elsevier]

Abstract: The present study is concerned with photophoresis of a microsized long cylinder-shaped particle suspended in an absorbing gas medium. To facilitate the analysis, an infinite cylinder subjected to an intensive light beam is considered as the physical model. The electromagnetic energy can be absorbed by the particle and turned into thermal energy heating surface unevenly, which results in a net momentum transfer between gas molecules and the particle to drive particle in photophoretic motion. Effects of the governing parameters on the absorbed energy distribution in the cylindrical particle are investigated. The results demonstrate that increasing either the radius or absorptivity of the cylinder enhances the energy absorbed on the illuminated side and tends to generate positive photophoresis; while an increase in the refractivity of the particle tends to enhance the internal electric field intensity and shift the absorption peak on the shaded side toward the particle center. Increase in medium absorptivity reduces the energy reaching at the particle, which significantly degrades the level of energy absorption and therefore weakens the photophoretic mobility of the particle. It is also found that, at the same conditions, the source function peaks in long cylinder-shaped particles are generally lower than those in spheres due to the weaker light refraction of the cylindrical shape. [Copyright &y& Elsevier]

Abstract: When irradiated sideways, by visible light, a particle can perform different kinds of motion, (e.g. in direction of irradiation, opposite to irradiation, vertical movement, helicoidally, etc.). This phenomenon is called photophoresis. Photophoresis is based on momentum transfer between the aerosol particle and surrounding gas molecules. Photophoresis strongly depends on the pressure of the surrounding gas. Particles mostly influenced by photophoresis are those of μm size. Two main types of forces describe photophoretic motion:ΔT force: The thermal accommodation coefficient α is constant over the particle surface. As a result of the thermal accommodation, gas molecules on the warm side of a particle leave the surface faster than gas molecules on the cold side. This leads to ΔT force on the particle towards the colder side. Typical motion of the particle will be either away from light irradiation (positive photophoresis), or in direction of light irradiation (negative photophoresis). In the case of negative photophoresis, the back side of the particle, due the nature of light absorption, will be heated more than front side of the particle.Δα force: If the particle is at a constant temperature, which is different from the temperature of the surrounding gas, and the thermal accommodation coefficient α varies over the particle surface, the net momentum between gas molecules and particle will be transferred. In this case, the result will be body fixed Δα force. Depending on the particle surface properties, Δα force can direct the particle in any possible photophoresis could also play important role in planet formation and astrophysics. [Copyright &y& Elsevier]

Abstract: Types of photophoresis oriented to a field are induced by a body-fixed photophoretic force in combination with a restoring torque, related to gravitational, magnetic or electric field. The motion of a particle generally consists of the superposition of a systematic helical with a random, light-dependent component. This paper is concentrated on the stochastic part. Estimates of Brownian motion contributions to irregular as well as field photophoresis (in particular for gravito-photophoresis) are presented. [Copyright &y& Elsevier]

Aerosols inserted high into the atmosphere might be subject to a significant photophoretic force. At lower altitudes the air pressure is too high for photophoresis to be effective but at stratospheric and mesospheric pressure photophoresis can compensate a particle''s gravity under certain conditions. The application of photophoretic particle transport requires fundamental knowledge of photophoresis for complex particles. With regard to this, we started to carry out experiments on photophoresis. We detected particle lift by negative radiometric photophoresis on graphite aggregates of about 10μm in size at a light flux comparable to solar in an electrodynamic trap. In another set-up we could levitate large aggregates of graphite and other materials by negative photophoresis in a slightly focused downward directed laser beam at higher light intensities. The points of stability could be moved up or down in non-predictable direction but reversible by changing the intensity of the laser beam. A third experiment describes the lift-off of surface particles invoked by a combination of photophoresis and solid-state greenhouse effects. Together, the pressure dependence of the forces, particle rotation, and the stability and direction of motion in directed laser beams allow us to distinguish between the different kinds of photophoretic forces suggested for atmospheric particle transport especially for gravito-photophoresis. [Copyright &y& Elsevier]

The photophoretic motion of a solid spherical particle in a viscous fluid is described theoretically in the Stokes approximation for small Péclet and Reynolds numbers and large temperature differences near the particle. In solving the hydrodynamic equations, an exponential-power law is used for the temperature dependence of the viscosity. The heat transfer equations are solved using the method of matched asymptotic expansions. The possibility of the experimental observation of photophoresis in liquids is discussed. [ABSTRACT FROM AUTHOR]

Abstract: A photon as a particle has an energy and a momentum. In a matter–photon interaction, the matter and photons may exchange their momenta observing the momentum conservation law. The consequence of the momentum transfer from a photon to a matter particle is a mechanical force exerted on the particle. Several separation methods based on this force of light are reviewed. Photophoresis separations for micron-sized particles and optical force chromatography for chemical-sized molecules are discussed. [Copyright &y& Elsevier]

Abstract: The formation of aerosol layers in the earth''s stratosphere and mesosphere under the influence of gravito-photophoretic forces is considered. The minimum set of effective particle parameters for describing gravito-photophoretic forces, and their critical values for vertical transport and levitation of particles are discussed. The behaviour of two types of particles are investigated specifically: (1) soot-like particles strongly absorbing both solar (visible) and IR radiation, and (2) sulfate-like particles which are transparent in the visible, but strongly absorb the terrestrial IR radiation. Only particles larger than one micrometer can levitate under the influence of gravito-photophoresis. Particles larger than can be suspended in the atmosphere. They need to have a variation in accommodation coefficient over the surface. As a rule transparent particles should have a density smaller than the one of water. Both black particles and white particles can be levitated. Gravito-photophoresis explains the existence of narrow aerosol layers at altitudes of approximately 20 and , and also at approx. 70 and 80–. The thickness of the layers can be less than if the accomodation coefficient exhibits only small differences over the surface. Transparent particles may form thick layers above rather easily. [Copyright &y& Elsevier]

Abstract: A combined analytical–numerical study is presented for the quasisteady photophoretic motion of a spherical aerosol particle of arbitrary thermal conductivity and surface properties exposed to a radiative flux perpendicular to a large plane wall. The Knudsen number is assumed to be so small that the fluid flow is described by a continuum model with a temperature jump, a thermal slip, and a frictional slip at the surface of the radiation-absorbing particle. In the limit of small Peclet and Reynolds numbers, the appropriate equations of conservation of energy and momentum for the system are solved using a boundary collocation method and numerical results for the photophoretic velocity of the particle are obtained for various cases. The presence of the neighboring wall causes two basic effects on the particle velocity: first, the local temperature gradient on the particle surface is enhanced or reduced by the wall, thereby speeding up or slowing down the particle; second, the wall increases viscous retardation of the moving particle. The net effect of the wall can decrease or increase the particle velocity, depending upon the relative conductivity and surface properties of the particle as well as the relative particle–wall separation distance. In general, the boundary effect of a plane wall on the photophoresis of an aerosol particle can be quite significant in some situations. In most aerosol systems, the boundary effect on photophoresis is weaker than that on the motion driven by a gravitational field. [Copyright &y& Elsevier]

Abstract: The photophoretic motion of a freely suspended aerosol spheroid exposed to a radiative heat flux that is oriented arbitrarily with respect to its axis of revolution is analytically studied. The Knudsen number is assumed to be so small that the fluid flow can be described by a continuum model with a thermal slip at the particle surface. In the limit of small Peclet and Reynolds numbers, the appropriate energy and momentum equations are solved using the bifocal-coordinate transformations. Expressions for the photophoretic velocity and force are obtained in closed form for various cases of prolate and oblate spheroidal particles. The average photophoretic velocity and force for an ensemble of identical, noninteracting spheroids with random orientation distribution are also determined. The results indicate that the aspect ratio and relative thermal conductivity of a spheroidal particle and its orientation with respect to the incident light can have significant effects on its photophoretic behavior. [Copyright &y& Elsevier]

Soot aggregates formed by propane combustion in a diffusion flame and benzene pyrolysis in a flow reactor are studied by a microscopic video system (at standard temperature and pressure) and transmission electron microscope. The radius of soot aggregates is in the range 0.1–1.0 μm. The size of primary particles in aggregates is 10–30 nm for the combustion aggregates and 40–200 nm for the pyrolysis aggregates. By video observations of aggregate movement in the electric field it is determined that soot aggregates are charged. The typical aggregate charge is a few elementary units. The soot charge distribution is roughly bipolar and symmetric. This charge distribution is governed by the Boltzmann law with the charging equivalent diameter dE being larger than the aggregate mean mobility diameter by a factor of 1.4, 3.0 and 1.8 for soots sampled from the flame axis, region over the flame and from the flow reactor, respectively.Photophoretic movement of soot aggregates driven by a helium–neon laser beam (3 W/cm2) is observed by the video system. The aggregate photophoretic velocity is determined to be increasing from 15 to 160 μm/s with the equivalent radius increasing from 0.1 to 2.9 μm. Estimations of the sun photophoretic velocity VPhSun (in the Earth''s atmosphere at ground level) indicate that it is about 20% of the settling velocity for the range of aggregate radii studied in this work. The velocities of soot aggregate photophoresis in the Earth thermal irradiation were estimated to be negligible with regard to the settling velocity. [Copyright &y& Elsevier]