The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
Gerhard Wurm - One of the best experts on this subject based on the ideXlab platform.
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Self-sustained Recycling in the Inner Dust Ring of Pre-transitional Disks
The Astrophysical Journal, 2016Co-Authors: Tim Husmann, Christoph Loesche, Gerhard WurmAbstract:Observations of pre-transitional disks show a narrow inner dust ring and a larger outer one. They are separated by a cavity with no or only little dust. We propose an efficient recycling mechanism for the inner dust ring which keeps it in a steady state. No major particle sources are needed for replenishment. Dust particles and pebbles drift outwards by radiation pressure and Photophoresis. The pebbles grow during outward drift until they reach a balanced position where residual gravity compensates Photophoresis. While still growing larger they reverse their motion and drift inward. Eventually, their speed is fast enough for them to be destroyed in collisions with other pebbles and drift outward again. We quantify the force balance and drift velocities for the disks LkCa15 and HD 135344B. We simulate single-particle evolution and show that this scenario is viable. Growth and drift timescales are on the same order and a steady state can be established in the inner dust ring.
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Photophoresis on polydisperse basalt microparticles under microgravity
Journal of Aerosol Science, 2014Co-Authors: Markus Küpper, Gerhard Wurm, Caroline De Beule, Lorin Matthews, Jesse B. Kimery, Truell HydeAbstract:Abstract 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.
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Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments
2014Co-Authors: Christoph Duermann, Gerhard WurmAbstract:In optically thin parts of protoplanetary disks Photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces down to 100 nN on plates at a light flux of 100 W/m2. We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2 mm thickness and circular shape with diameters of 10 mm, 30 mm and 50 mm. The influence of channels was probed on rectangular-shaped circuit boards of 35 mm × 35 mm area and 1.5 mm thickness. The number of channels was 169 and 352. The pressure was varied over three decades between 0.001 and 1 mbar. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by Photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r1, the channel radius r2, and the thickness of the plate which equals the length of the channel d as l = r0.35 · d0.65. The highest force is found at a pressure pmax = 15 · l−1 Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: Photophoresis due to the overall size and cross section of the plate and Photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosit
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Photophoretic Strength on Chondrules 1: Modeling
The Astrophysical Journal, 2013Co-Authors: Christoph Loesche, Gerhard Wurm, Jens Teiser, Jon M. Friedrich, Addi BischoffAbstract:Photophoresis is a physical process that transports particles in optical thin parts of protoplanetary disks, especially at the inner edge and at the optically surface. To model the transport and resulting effects in detail, it is necessary to quantify the strength of Photophoresis for different particle classes as a fundamental input. Here, we explore Photophoresis for a set of chondrules. The composition and surface morphology of these chondrules was measured by X-ray tomography. Based on the three-dimensional models, heat transfer through illuminated chondrules was calculated. The resulting surface temperature map was then used to calculate the photophoretic strength. We found that irregularities in particle shape and variations in composition induce variations in the photophoretic force. These depend on the orientation of a particle with respect to the light source. The variations of the absolute value of the photophoretic force on average over all chondrules is $4.17\%$. The deviation between the direction of the photophoretic force and illumination is $3.0^\circ \pm 1.5^\circ$. The average photophoretic force can be well approximated and calculated analytically assuming a homogeneous sphere with a volume equivalent mean radius and an effective thermal conductivity. We found an analytic expression for the effective thermal conductivity. The expression depends on the two main phases of a chondrule, and decreases with the amount of fine-grained devitrified, plagioclase-normative mesostasis up to factor of three. For the chondrule sample studied (Bjurb\"ole chondrite), we found a dependence of the photophoretic force on chondrule size.
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Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments
Astronomy & Astrophysics, 2013Co-Authors: Christoph Duermann, Gerhard Wurm, Markus KuepperAbstract:In optically thin parts of protoplanetary disks Photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces. We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2mm thickness and circular shape with diameters of 10mm, 30mm and 50mm. The influence of channels was probed on rectangular-shaped circuit boards of 35mm x 35mm area and 1.5mm thickness. The number of channels was 169 and 352. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by Photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r_1, the channel radius r_2, and the thickness of the plate which equals the length of the channel d as l=r^{0.35} x d^{0.65}. The highest force is found at a pressure p_max = 15 x l^{-1}Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: Photophoresis due to the overall size and cross section of the plate and Photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosity on the photophoretic force can reverse the inward drift of large solids, for instance meter-sized bodies, and push them outward within the optically thin parts of the disk.
C. Y. Soong - One of the best experts on this subject based on the ideXlab platform.
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Theoretical Analysis of Cylindrical Microparticle Photophoresis in a Perpendicular Optical Field with Thermal Stress Slip Model
Journal of Mechanics, 2012Co-Authors: P.-y. Tzeng, C.-h. Liu, C. Y. SoongAbstract:ABSTRACTThe present study is concerned with a theoretical analysis of the Photophoresis of a microsized long cylinder in a perpendicular optical field. Different from previous studies of Photophoresis, thermal stress slip usually neglected is taken into account in the analysis. The gaseous fluid relative to the microparticle in photophoretic motion falls into slip-flow regime. Asymmetric distribution of the absorbed heat energy within the particle becomes the driving force for photophoretic motion of the cylinder-shaped particle. By evaluating heat source function distributions at various conditions, the study focuses on the effects of particle size and optical properties on the energy distribution and the resultant influences on the Photophoresis. The photophoretic mobility is developed by the slip flow model with consideration of thermal stress slip. The results reveal that the photophoretic mobility decreases with the increase of particle thermal conductivity (k*) and increases with Knudsen number (Kn). The thermal stress slip effect on photophoretic velocity is more noticeable at high Kn, but disappears at the continuum limit. A long cylinder-shaped particle has higher photophoretic velocity than a spherical particle at low k*, while the situation reverses at high k*. With thermal stress slip considered, the critical condition for crossing of the photophoretic velocity curves of cylindrical and spherical particles is mildly influenced by Kn.
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Analysis of transition and mobility of microparticle Photophoresis with slip-flow model
Microfluidics and Nanofluidics, 2010Co-Authors: C. Y. Soong, Pei-yuan Tzeng, Chung-ho LiuAbstract:The objective of the present study is to investigate photophoretic motion of a spherical microparticle in slip-flow regime of gaseous medium. Energy from incident light absorbed by the particle is calculated by employing Mie scattering theory. Temperature and relative velocity distributions of the gaseous flow around the microparticle are developed using a slip-flow model with consideration of thermal stress slip effect. It is demonstrated that the present results agree well with previous measurements and theoretical predictions. Heat source function and asymmetry factor indicating, respectively, the level and the uneven characteristics of the energy distribution within the particle are evaluated. At low, intermediate, and high absorptivities, three different patterns of asymmetry factor versus size parameter are found and named negative Photophoresis prevailing, normal switching of Photophoresis, and positive Photophoresis dominant. Influences of particle optical properties on the critical size for transition of negative–positive Photophoresis are analyzed. The results demonstrate that increasing absorptivity or refractivity of the particle leads to a reduction in critical size for Photophoresis transition. Both increase in Knudsen number and reduction in particle-to-gas thermal conductivity ratio enhance the photophoretic mobility.
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Parametric Analysis of Energy Absorption in Micro-particle Photophoresis in Absorbing Gaseous Media
Defence Science Journal, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:The study deals with Photophoresis of a spherical micro-particle suspended in absorbing gaseous media. Photophoretic motion of the particle stems from the asymmetric distribution of absorbed energy within the particle. By evaluating the so-called heat source function at various conditions, the study focuses on the effects of governing parameters on the energy distribution within the particle and their potential influences to the Photophoresis. The results reveal that the increase in either particle size or absorptivity enhances the energy intensity on the illuminated (leading) side and tends to generate positive Photophoresis. For a particle of low absorptivity, the energy distribution is dominated by particle refraction. Enhancing particle refractivity, the energy tends to be focused onto a certain spot area on the shaded (trailing) side and leads to a tendency of negative Photophoresis. Increasing medium absorptivity significantly degrades the level of energy absorbed by the particle and in turn weakens the driving force of the particle Photophoresis. Defence Science Journal, 2010, 60(3), pp.233-237 , DOI:http://dx.doi.org/10.14429/dsj.60.347
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Effect of thermal stress slip on microparticle Photophoresis in gaseous media
Optics letters, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:This Letter presents a study on Photophoresis of a microparticle in gaseous media with focus on the effect of thermal stress slip, which is deemed as one of factors causing deviations of previous theoretical predictions from measurements. The present modified theory agrees well with the measurements and, combining with 1 order-of-magnitude analysis, demonstrates the significance of the thermal stress slip in Photophoresis of a particle, especially, of small radius. With the physical mechanisms addressed, the parametric analysis reveals that this interfacial thermal effect becomes more pronounced with reducing thermal conductivity of the particle and increasing Knudsen number as well.
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Theoretical analysis for Photophoresis of a microscale hydrophobic particle in liquids
Optics express, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:In the present study, combining the conventional photothermal analysis and the concept of interaction of solvent molecules in interfacial layer used for thermophoresis in liquid, a theory for Photophoresis of a hydrophobic particle suspended in liquids is developed. To characterize hydrophobicity of the micro-particle, slip length in Navier’s formula is used as an index. Analytical expressions are derived and a parametric analysis for photophoretic velocity is performed with emphasis on the influences of particle characteristics such as size, optical properties, hydrophobicity, and thermal conductivity. Heat source function and the corresponding asymmetry factor at various conditions are evaluated to interpret the mechanisms of negative and positive Photophoresis and the conditions for transition between them. The present theory discloses that the particle surface hydrophobicity or fluid slippage at particle-liquid interface may lead to a remarkable enhancement in the particle photophoretic velocity in liquids. Higher particle thermal conductivity and larger size of liquid molecules both result in weaker photophoretic motion.
Pei-yuan Tzeng - One of the best experts on this subject based on the ideXlab platform.
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Analysis of transition and mobility of microparticle Photophoresis with slip-flow model
Microfluidics and Nanofluidics, 2010Co-Authors: C. Y. Soong, Pei-yuan Tzeng, Chung-ho LiuAbstract:The objective of the present study is to investigate photophoretic motion of a spherical microparticle in slip-flow regime of gaseous medium. Energy from incident light absorbed by the particle is calculated by employing Mie scattering theory. Temperature and relative velocity distributions of the gaseous flow around the microparticle are developed using a slip-flow model with consideration of thermal stress slip effect. It is demonstrated that the present results agree well with previous measurements and theoretical predictions. Heat source function and asymmetry factor indicating, respectively, the level and the uneven characteristics of the energy distribution within the particle are evaluated. At low, intermediate, and high absorptivities, three different patterns of asymmetry factor versus size parameter are found and named negative Photophoresis prevailing, normal switching of Photophoresis, and positive Photophoresis dominant. Influences of particle optical properties on the critical size for transition of negative–positive Photophoresis are analyzed. The results demonstrate that increasing absorptivity or refractivity of the particle leads to a reduction in critical size for Photophoresis transition. Both increase in Knudsen number and reduction in particle-to-gas thermal conductivity ratio enhance the photophoretic mobility.
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Parametric Analysis of Energy Absorption in Micro-particle Photophoresis in Absorbing Gaseous Media
Defence Science Journal, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:The study deals with Photophoresis of a spherical micro-particle suspended in absorbing gaseous media. Photophoretic motion of the particle stems from the asymmetric distribution of absorbed energy within the particle. By evaluating the so-called heat source function at various conditions, the study focuses on the effects of governing parameters on the energy distribution within the particle and their potential influences to the Photophoresis. The results reveal that the increase in either particle size or absorptivity enhances the energy intensity on the illuminated (leading) side and tends to generate positive Photophoresis. For a particle of low absorptivity, the energy distribution is dominated by particle refraction. Enhancing particle refractivity, the energy tends to be focused onto a certain spot area on the shaded (trailing) side and leads to a tendency of negative Photophoresis. Increasing medium absorptivity significantly degrades the level of energy absorbed by the particle and in turn weakens the driving force of the particle Photophoresis. Defence Science Journal, 2010, 60(3), pp.233-237 , DOI:http://dx.doi.org/10.14429/dsj.60.347
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Effect of thermal stress slip on microparticle Photophoresis in gaseous media
Optics letters, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:This Letter presents a study on Photophoresis of a microparticle in gaseous media with focus on the effect of thermal stress slip, which is deemed as one of factors causing deviations of previous theoretical predictions from measurements. The present modified theory agrees well with the measurements and, combining with 1 order-of-magnitude analysis, demonstrates the significance of the thermal stress slip in Photophoresis of a particle, especially, of small radius. With the physical mechanisms addressed, the parametric analysis reveals that this interfacial thermal effect becomes more pronounced with reducing thermal conductivity of the particle and increasing Knudsen number as well.
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Theoretical analysis for Photophoresis of a microscale hydrophobic particle in liquids
Optics express, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:In the present study, combining the conventional photothermal analysis and the concept of interaction of solvent molecules in interfacial layer used for thermophoresis in liquid, a theory for Photophoresis of a hydrophobic particle suspended in liquids is developed. To characterize hydrophobicity of the micro-particle, slip length in Navier’s formula is used as an index. Analytical expressions are derived and a parametric analysis for photophoretic velocity is performed with emphasis on the influences of particle characteristics such as size, optical properties, hydrophobicity, and thermal conductivity. Heat source function and the corresponding asymmetry factor at various conditions are evaluated to interpret the mechanisms of negative and positive Photophoresis and the conditions for transition between them. The present theory discloses that the particle surface hydrophobicity or fluid slippage at particle-liquid interface may lead to a remarkable enhancement in the particle photophoretic velocity in liquids. Higher particle thermal conductivity and larger size of liquid molecules both result in weaker photophoretic motion.
Oliver Krauss - One of the best experts on this subject based on the ideXlab platform.
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Photophoretic Structuring of Circumstellar Dust Disks
The Astrophysical Journal, 2008Co-Authors: Taku Takeuchi, Oliver KraussAbstract:We study dust accumulation by Photophoresis in optically thin gas disks. Using formulae of 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 the 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 the particle size, and forms an inner disk. Radiation pressure pushes out small particles (< 1 mm) further and forms 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 equals the maximum size of the particles that Photophoresis effectively works on (100 micron - 10 cm, depending on the dust property). The dust disk structure formed by Photophoresis can be distinguished from the structure of gas-free dust disk models, because the particle sizes of the outer disks are larger, and the inner hole radius depends on the gas density.
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Experiments on negative Photophoresis and application to the atmosphere
Atmospheric Environment, 2008Co-Authors: Gerhard Wurm, Oliver KraussAbstract: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 10 μm 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.
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Concentration and sorting of chondrules and CAIs in the late Solar Nebula
Icarus, 2006Co-Authors: Gerhard Wurm, Oliver KraussAbstract:Abstract The high concentration and sorting of chondrules, sub-mm sized spherules found in undifferentiated meteorites, is one of the great unsolved mysteries in planetology. Here we present a unifying explanation for these phenomena based on the assumption that chondrules were present when the Solar Nebula was optically thin but had a significant amount of gas. An immediate consequence is that chondrules feel a force known as Photophoresis. Photophoresis is based on a temperature gradient over the surface of a particle resulting from absorption of radiation and non-uniform interaction with its gaseous environment. In comparison to well-known forces originating from starlight, i.e. radiation pressure, Poynting–Robertson drag, or the Yarkovski effect, Photophoresis can be stronger by many orders of magnitude in gaseous environments. In the application discussed here Photophoresis concentrates chondrules and CAIs, which are both found in chondrites, in the region of the asteroid belt. Chondrules from any place in the Solar Nebula will be dragged to the asteroid belt region, while smaller dust particles and their aggregates will be removed from this region at the same time. This leads to a high relative concentration of chondrules, sorted with respect to their thermal conductivity, density, and size, for building chondrite parent bodies. Furthermore, Photophoresis prevents CAIs from being lost to the Sun.
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Photophoresis and the pile-up of dust in young circumstellar disks
The Astrophysical Journal, 2005Co-Authors: Oliver Krauss, Gerhard WurmAbstract:A rapidly growing number of observations reveal ever more structure in young circumstellar disks that are presumed to be forming planetary systems. Prominent features observed are ring-shaped dust distributions with sharp inner edges around stars like the young, main-sequence star HR 4796A. Models aiming to explain the formation of these dust rings by grain migration incorporate radiation pressure of the central star as one shaping force in radial direction. However, the radiometric effect of Photophoresis has been ignored, so far, in this context. This effect is based on a radiation-induced temperature gradient on the surface of a particle and the consequential nonuniform interaction with surrounding gas. The resulting force is able to effectively influence the motion of particles in gaseous environments, but so far Photophoresis has been limited to applications in the field of aerosol science. Here we present calculations that underline the relevance of the photophoretic force for the dynamics of particles in gas-rich, optically thin circumstellar disks. Depending on the gas pressure, Photophoresis can be stronger than radiation pressure, gas drag, and gravity by orders of magnitude. Then the motion of particles ranging in size from 1 μm to 10 m will be dominated by Photophoresis. Since the photophoretic force is a function of the gas density, it provides an efficient mechanism for fast radial migration of particles to a definite distance from the star where the gas density reaches a value at which Photophoresis is in equilibrium with all other forces at work. By this effect, material is swept out from the inner region of the disk and piled-up in a more or less confined belt around the star. Thus, the formation of ringlike structures of the dust distribution can most naturally be explained without any further assumptions. Since photophoretic pile-up also works for larger bodies, it might even trigger the formation of Kuiper belts.
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Accepted by the Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 10/09/06 PHOTOPHORETIC STRUCTURING OF CIRCUMSTELLAR DUST DISKS
1Co-Authors: Taku Takeuchi, Oliver KraussAbstract:We study dust accumulation by Photophoresis in optically thin gas disks. Using formulae of 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 the 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 the particle size, and forms an inner disk. Radiation pressure pushes out small particles ( � 1 mm) further and forms 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 equals the maximum size of the particles that Photophoresis effectively works on (100 µm − 10 cm, depending on the dust property). The dust disk structure formed by Photophoresis can be distinguished from the structure of gas-free dust disk models, because the particle sizes of the outer disks are larger, and the inner hole radius depends on the gas density. Subject headings: circumstellar matter — planetary systems: formation — solar system: formation 1
Chung-ho Liu - One of the best experts on this subject based on the ideXlab platform.
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Analysis of transition and mobility of microparticle Photophoresis with slip-flow model
Microfluidics and Nanofluidics, 2010Co-Authors: C. Y. Soong, Pei-yuan Tzeng, Chung-ho LiuAbstract:The objective of the present study is to investigate photophoretic motion of a spherical microparticle in slip-flow regime of gaseous medium. Energy from incident light absorbed by the particle is calculated by employing Mie scattering theory. Temperature and relative velocity distributions of the gaseous flow around the microparticle are developed using a slip-flow model with consideration of thermal stress slip effect. It is demonstrated that the present results agree well with previous measurements and theoretical predictions. Heat source function and asymmetry factor indicating, respectively, the level and the uneven characteristics of the energy distribution within the particle are evaluated. At low, intermediate, and high absorptivities, three different patterns of asymmetry factor versus size parameter are found and named negative Photophoresis prevailing, normal switching of Photophoresis, and positive Photophoresis dominant. Influences of particle optical properties on the critical size for transition of negative–positive Photophoresis are analyzed. The results demonstrate that increasing absorptivity or refractivity of the particle leads to a reduction in critical size for Photophoresis transition. Both increase in Knudsen number and reduction in particle-to-gas thermal conductivity ratio enhance the photophoretic mobility.
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Parametric Analysis of Energy Absorption in Micro-particle Photophoresis in Absorbing Gaseous Media
Defence Science Journal, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:The study deals with Photophoresis of a spherical micro-particle suspended in absorbing gaseous media. Photophoretic motion of the particle stems from the asymmetric distribution of absorbed energy within the particle. By evaluating the so-called heat source function at various conditions, the study focuses on the effects of governing parameters on the energy distribution within the particle and their potential influences to the Photophoresis. The results reveal that the increase in either particle size or absorptivity enhances the energy intensity on the illuminated (leading) side and tends to generate positive Photophoresis. For a particle of low absorptivity, the energy distribution is dominated by particle refraction. Enhancing particle refractivity, the energy tends to be focused onto a certain spot area on the shaded (trailing) side and leads to a tendency of negative Photophoresis. Increasing medium absorptivity significantly degrades the level of energy absorbed by the particle and in turn weakens the driving force of the particle Photophoresis. Defence Science Journal, 2010, 60(3), pp.233-237 , DOI:http://dx.doi.org/10.14429/dsj.60.347
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Effect of thermal stress slip on microparticle Photophoresis in gaseous media
Optics letters, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:This Letter presents a study on Photophoresis of a microparticle in gaseous media with focus on the effect of thermal stress slip, which is deemed as one of factors causing deviations of previous theoretical predictions from measurements. The present modified theory agrees well with the measurements and, combining with 1 order-of-magnitude analysis, demonstrates the significance of the thermal stress slip in Photophoresis of a particle, especially, of small radius. With the physical mechanisms addressed, the parametric analysis reveals that this interfacial thermal effect becomes more pronounced with reducing thermal conductivity of the particle and increasing Knudsen number as well.
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Theoretical analysis for Photophoresis of a microscale hydrophobic particle in liquids
Optics express, 2010Co-Authors: C. Y. Soong, Chung-ho Liu, Pei-yuan TzengAbstract:In the present study, combining the conventional photothermal analysis and the concept of interaction of solvent molecules in interfacial layer used for thermophoresis in liquid, a theory for Photophoresis of a hydrophobic particle suspended in liquids is developed. To characterize hydrophobicity of the micro-particle, slip length in Navier’s formula is used as an index. Analytical expressions are derived and a parametric analysis for photophoretic velocity is performed with emphasis on the influences of particle characteristics such as size, optical properties, hydrophobicity, and thermal conductivity. Heat source function and the corresponding asymmetry factor at various conditions are evaluated to interpret the mechanisms of negative and positive Photophoresis and the conditions for transition between them. The present theory discloses that the particle surface hydrophobicity or fluid slippage at particle-liquid interface may lead to a remarkable enhancement in the particle photophoretic velocity in liquids. Higher particle thermal conductivity and larger size of liquid molecules both result in weaker photophoretic motion.