Photophoretic Force Research Articles

Thermal and photophoretic properties of dust mantled chondrules and sorting in the solar nebula

Many chondrules are found to be surrounded by a fine grained rim. Supposedly, these rims were acquired from dust accreted to the chondrules in a protoplanetary disk. In numerical simulations we study the heat transfer in illuminated bare and dust mantled chondrules. The calculations consider the chondrule size, the dust mantle size, and the thermal conductivities of both components as parameters. We calculate the photophoretic force and compare the numerical results to analytical approximations. We give an expression to quantify the photophoretic force on a spherical particle in the free molecular regime to better than 2%. We describe the influence of a dust mantle on the photophoretic strength by an effective thermal conductivity of the core-mantle particle. The effective thermal conductivity significantly depends on the size ratio between mantle and chondrule but not on the absolute sizes. It also strongly depends on the thermal conductivity of the mantle with minor influence of the thermal conductivity of the chondrule. The size ratio between rim and chondrule in meteorites is found to vary systematically with overall size by other authors. Based on this, our calculations show that a photophoretic size sorting can occur for dust mantled chondrules in optically thin disks or at the evolving inner edge of the solar nebula.

Polarization-sensitive photophoresis

We photophoretically trap spherical airborne particles using a single radially or azimuthally polarized laser beam and show that the trapping efficiency is significantly higher for the radial polarization. The demonstrated polarization sensitivity of the photophoretic force, which is caused by polarization-dependent reflection from the particles, adds additional flexibility to the optical micromanipulation of light absorbing particles in gaseous media.

Effects of thermal stress slip on thermophoresis and photophoresis

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.

Photophoretic trapping of absorbing particles in air and measurement of their single-particle Raman spectra

A new method is demonstrated for optically trapping micron-sized absorbing particles in air and obtaining their single-particle Raman spectra. A 488-nm Gaussian beam from an Argon ion laser is transformed by conical lenses (axicons) and other optics into two counter-propagating hollow beams, which are then focused tightly to form hollow conical beams near the trapping region. The combination of the two coaxial conical beams, with focal points shifted relative to each other along the axis of the beams, generates a low-light-intensity biconical region totally enclosed by the high-intensity light at the surface of the bicone, which is a type of bottle beam. Particles within this region are trapped by the photophoretic forces that push particles toward the low-intensity center of this region. Raman spectra from individual trapped particles made from carbon nanotubes are measured. This trapping technique could lead to the development of an on-line real-time single-particle Raman spectrometer for characterization of absorbing aerosol particles.

Predictions and detection of the “accommodation” forces on Janus particles subjected to directed radiation in a rarefied gas

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.

Particle separation in fluidic flow by optical fiber

We report a separation of two different size particles in fluidic flow by an optical fiber. With a light of 1.55 μm launched into the fiber, particles in stationary water were massively trapped and assembled around the fiber by a negative photophoretic force. By introducing a fluidic flow, the assembled particles were separated into two different downstream positions according to their sizes by the negative photophoretic force and the dragging force acted on the particles. The intensity distribution of light leaked from the fiber and the asymmetry factor of energy distribution have been analysed as crucial factors in this separation. Poly(methyl methacrylate) particles (5-/10-μm diameter), SiO(2) particles (2.08-/5.65-μm diameter), and SiO(2) particles (2.08-μm diameter) mixed with yeast cells were used to demonstrate the effectiveness of the separation. The separation mechanism has also been numerical simulated and theoretical interpreted.

Microbe removal using a micrometre-sized optical fiber

The growing global shortage of fresh water has lead to the need for technological innovations for water purification and reuse. The removal of pathogenic microbes from urban, laboratory or industrial wastewater is one of the most challenging and critical issues due to the potential risk of microbe outbreaks. In addition, microbe removal in human blood or tissues has also inspired novel techniques for extracting and collecting different cells in fluidic channels or vessels. Recently, efficient removal of microbes from flowing water running under gravity feed has been achieved using filters in nanotubes or nanofibers. Here we report a highly efficient removal of microbes from flowing water in a fluidic channel using a reusable micrometre-sized optical fiber. Our technique is based on photophoresis of the microbes induced by the radiation of 1.55 μm wavelength injected into the fiber. Yeast cell suspensions, as a sample of microbe-contaminated water, are flown through a fluidic channel and the suspended cells are collected by the photophoretic forces, leading to a consistent accumulation of the yeast cells. The experiments indicate that a removal efficiency of 99.9% can be obtained when the flow velocity of the suspensions is less than the peak photophoretic velocity of the yeast cells.

Photophoretic forces on chondrules in drop tower experiments

Studies of meteoritic and cometary materials give evidence for radial mixing of solid material within the early solar system. Among other transport mechanisms, photophoresis is a proposed mechanism for radial material transport in protoplanetary disks. Within this study, the photophoretic motion of chondrules in microgravity is investigated in drop tower studies. Chondrules form the meteorite Bjurböle were exposed to an intense laser beam. The acceleration due to photophoresis was observed and tracked with two cameras. The experimental results give evidence that photophoresis indeed is an efficient transport mechanism for chondrules and other small particles in the solar nebula. The measured values for the photophoretic force correspond well to the results of earlier studies and show clearly, that the models for particle transport by photophoresis are in agreement with the experimental data.

The global picture of aerosol layers formation in the stratosphere and in the mesosphere under the influence of gravito-photophoretic and magneto-photophoretic forces

[1] Photophoretic forces act on a particle which has an inhomogeneous surface, especially with respect to the accommodation coefficient. Gas molecules incident on it leave the surface after more or less accommodation, resulting in a force and torque on the particle. Due to Brownian rotation of the particle the net force is zero. If the particle is also exposed to an orientating torque, the photophoretic force points in a preferred direction resulting in net photoporetic force. The orienting torque can be caused by gravity or by the earth magnetic field. Photophoretic forces are important if the mean free path of the gas molecules is larger than the size of the particles. This is the case for all particles found in the stratosphere and mesosphere. The direction of the photophoretic force can be opposite to gravity and it can exceed the weight of the particle. In that case the particle is lifted upwards, until it reaches conditions where the two forces are equal. The stratification of the stratospheric and mesospheric aerosol is given fact. The temperature profile of the atmosphere, the incoming solar flux density, and the emitted terrestrial infrared radiation, are used to investigate systematically the role of photophoresis in the formation of aerosol layers at specific altitudes for various types of particles around the globe. Photophoresis can be responsible for aerosol layer formation at altitudes of 20, 30, 50, 70 and 85 km. The layers form in specific geographic regions and at certain seasons.

Photophoretic assembly and migration of dielectric particles and Escherichia coli in liquids using a subwavelength diameter optical fiber

We demonstrate a photophoretic assembly and migration of dielectric (SiO(2) and TiO(2)) particles and bacteria (Escherichia coli) in liquids by using a subwavelength diameter fiber. With a lightwave at 1.55 μm launched into the fiber, the objects are radiated by the leaking light of the fiber to yield negative photophoretic forces which drive the objects to move toward the fiber, with an average assembling/migrating speed of 5-15 individuals per second (ind/s). The influences of laser-on duration, optical power, and size of particles on the photophoretic velocities are also investigated.

Magneto‐photophoresis and mesospheric particles

So far, levitation of solid particles in the atmosphere by photophoretic forces has been attributed exclusively to gravito‐photophoresis. In this paper attention is drawn to magneto‐photophoresis, a motion of ferromagnetic particles related to a magnetic field during their irradiation by light. In regions showing an approximately vertically directed magnetic field, magneto‐photophoretic levitation is likewise possible. Laboratory experiments with particles produced by an arc discharge in air between iron electrodes have quantitatively shown magneto‐photophoretic levitation. Using these results, we show that magneto‐photophoresis can be an essential factor for the formation and persistence of particle layers. Under conditions of illumination by the sun, magneto‐photophoretic levitation takes place with particles having sizes around and below 0.1 μm whereas gravito‐photophoresis occurs for particle sizes around 1 μm.

Photophoretic levitation of engineered aerosols for geoengineering

Aerosols could be injected into the upper atmosphere to engineer the climate by scattering incident sunlight so as to produce a cooling tendency that may mitigate the risks posed by the accumulation of greenhouse gases. Analysis of climate engineering has focused on sulfate aerosols. Here I examine the possibility that engineered nanoparticles could exploit photophoretic forces, enabling more control over particle distribution and lifetime than is possible with sulfates, perhaps allowing climate engineering to be accomplished with fewer side effects. The use of electrostatic or magnetic materials enables a class of photophoretic forces not found in nature. Photophoretic levitation could loft particles above the stratosphere, reducing their capacity to interfere with ozone chemistry; and, by increasing particle lifetimes, it would reduce the need for continual replenishment of the aerosol. Moreover, particles might be engineered to drift poleward enabling albedo modification to be tailored to counter polar warming while minimizing the impact on equatorial climates.

Photo-thermophoresis as a new tool for aerosol characterization

Aerosols are particles in the size range of nanometers to some micrometers, suspended in air or other gases. Their characterization and separation according to different physical and chemical properties is of great interest for environmental analysis and certain industrial applications. Photo-thermophoresis is an optothermal effect, which occurs when aerosol particles are illuminated by intense light. Locally inhomogeneous heating, resulting in locally increased impingement rates of the gas molecules, generates driving forces. These photophoretic forces can move fine particles either towards or away from the light source. We employ this effect for characterization and separation according to optical and thermal properties of aerosols. We hope to establish this new approach as a helpful supplement to the existing separation methods based on electrical, thermal or flow field forces.

Experiments on the photophoretic motion of chondrules and dust aggregates—Indications for the transport of matter in protoplanetary disks

In a set of 16 drop tower experiments the motion of sub-millimeter to millimeter-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of millimeter-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.

Transfer matrices and solution of the heat-mass transfer problem for aerosol clusters in a rarefied gas medium by the Monte Carlo method

The form of the decomposition of summands in Neumann series and the general solution to a linear integral equation based on separation of variables are considered under some assumptions on the properties of the integral transform. It is shown that one can reduce the order of the problem. Special transfer matrices are proposed for estimation of functionals. Based on weighted Monte Carlo methods and the use of transfer matrices, an algorithm for the calculation of molecular heat-mass transfer in an aerosol cluster is constructed in the approximation of free molecular gas kinetics. The cluster exists in a rarefied gas medium and, in the general case, absorbs radiation in the visible and infrared wave ranges. The algorithm allows one to compute the molecular heat transfer, photophoretic forces and their momenta, the four tensors describing the viscous forces and their momenta, and also the relationship between the translatory and rotatory motion of the cluster under an approximation linear with respect to the translatory and rotatory velocities of the system. The results of the application of the developed algorithms to the analysis of the influence of gravito-photophoresis on aerosol stratification in the stratosphere and mesosphere of the Earth are discussed as well.

Self-Sustained Levitation of Dust Aggregate Ensembles by Temperature-Gradient-Induced Overpressures

In laboratory experiments we observe dust aggregates from 100 mum to 1 cm in size composed of micrometer-sized grains levitating over a hot surface. Depending on the dust sample aggregates start to levitate at a temperature of 400 K. Levitation of dust aggregates is restricted to a pressure range between 1-40 mbar. The levitating is caused by a Knudsen compressor effect. Based on thermal transpiration through the dust aggregates the pressure increases between surface and aggregates. Dust aggregates are typically balanced approximately 100 microm over the surface. On a slightly concave surface individual aggregates are trapped at the center. Ensembles of aggregates are confined in a 2D plane. Aggregates are subject to systematic and random translational and rotational motion. The levitated aggregates are well suited to study photophoretic or thermophoretic forces or the mutual interaction between dust aggregates.

Photophoretic manipulation of absorbing aerosol particles with vortex beams: theory versus experiment

We develop a theoretical approach for describing the optical trapping and manipulation of carbon nanoclusters in air with a dual-vortex optical trap, as realized recently in experiment [V. Shvedov et al., Opt. Express 17, 5743 (2009)]. We calculate both longitudinal and transverse photophoretic forces acting on a spherical absorbing particle, and then compare our theoretical predictions with the experimental data.

Optical guiding of absorbing nanoclusters in air

We suggest a novel approach in all-optical trapping employing a photophoretic force for manipulation of absorbing particles in open air. We demonstrate experimentally the robust three-dimensional guiding, over the distances of a few millimeters, of agglomerates of carbon nanoparticles with the size spanned from 100 nm to 10 microm, as well as their acceleration up to velocities of 1 cm/sec. We achieve stable positioning and guiding of particles as well as simultaneous trapping of a large number of particles in a dual-beam optical trap created by two counter-propagating and co-rotating optical vortex beams.

Resonant tuning fork detector for electromagnetic radiation

A mechanical quartz microresonator (tuning fork) is used to detect electromagnetic radiation. The detection scheme is based on forces created due to the incident electromagnetic radiation on the piezoelectric tuning fork. A force can be created due to the transfer of the photon momentum of the incident electromagnetic radiation. If the surfaces of the tuning fork are nonuniformly heated, a second force acts on it, the so-called photophoretic force. These processes occur for all wavelengths of the incident radiation, making the detector suitable for sensing of ultraviolet, visible, and mid-infrared light, even THz-radiation. Here the detector is characterized in the visible range; noise analysis is performed for 650 nm and 5.26 microm. A linear power characteristic and the dependence on pulse lengths of the incoming light are shown. Examples for applications for the visible and mid-infrared spectral region are given by 2f and absorption spectroscopy of oxygen and nitric oxide, respectively.

Photophoretic Structuring of Circumstellar Dust Disks

We study dust accumulation by photophoresis in optically thin gas disks. Using formulae for the photophoretic force that are applicable for the free molecular regime and for the slip-flow regime, we calculate dust accumulation distances as a function of particle size. It is found that photophoresis pushes particles (smaller than 10 cm) outward. For a Sun-like star, these particles are transported to 0.1-100 AU, depending on their size, and form an inner disk. Radiation pressure pushes small particles (≲1 mm) out further to form an extended outer disk. Consequently, an inner hole opens inside ~0.1 AU. The radius of the inner hole is determined by the condition that the mean free path of the gas molecules equal the maximum size of the particles that photophoresis effectively works on (100 μm-10 cm, depending on the dust properties). The dust disk structure formed by photophoresis can be distinguished from the structure of model gas-free dust disks, because the particle sizes in the outer disk are larger, and the inner hole radius depends on the gas density.