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Elementary Mass Transfer
Published in Anthony F. Mills, Heat and Mass Transfer, 2018
Very small particles of 10−3−10−1 μm size-for example, smoke, soot, and mist—behave much like large molecules. Ordinary diffusion of such particles is called Brownian motion and is described in most elementary physics texts. Diffusion of particles due to a temperature gradient is called thermophore sis and plays an important role for larger particles, typically in the size range 10−1−1 μm. Diffusion of particles in a gas mixture due to concentration gradients of molecular species is called diffusiophoresis. Forced diffusion of a charged particle in an electrical field is similar to that for an ionized molecular species. Thermal and electrostatic precipitators are used to remove particles from power plant and incinerator stack gases, and depend on thermophoresis and forced diffusion, respectively, for their operation. Diffusion phenomena are unimportant for particles of size greater than about 1 μm in air at I atm; the motion of such particles is governed by the laws of Newtonian mechanics. Brownian motion is dealt with in Section 9.7, and in Chapter 11 electrostatic precipitators and particle filters are analyzed as examples of mass exchangers.
Heat and Mass Transfer
Published in Raj P. Chhabra, CRC Handbook of Thermal Engineering Second Edition, 2017
Robert F. Boehm, Swati A. Patel, Raj P. Chhabra, George D. Raithby, K.G. Terry Hollands, Anoop K. Gupta, N.V. Suryanarayana, Thomas F. Irvine, Massimo Capobianchi, Michael F. Modest, Van P. Carey, John C. Chen, Vasilios Alexiades, Jan Kośny, Anthony F. Mills
Very small particles of 10−3 to 10−1 μm size—for example, smoke, soot, and mist—behave much like large molecules. Ordinary diffusion of such particles is called Brownian motion and is described in most elementary physics texts. Diffusion of particles due to a temperature gradient is called thermophoresis and plays an important role for larger particles, typically in the size range 10−1 to 1 μm. Diffusion of particles in a gas mixture due to concentration gradients of molecular species is called diffusiophoresis. Forced diffusion of a charged particle in an electrical field is similar to that for an ionized molecular species. Thermal and electrostatic precipitators are used to remove particles from power plant and incinerator stack gases, and depend on thermophoresis and forced diffusion, respectively, for their operation. Diffusion phenomena are unimportant for particles of size greater than about 1 μm in air at 1 atm; the motion of such particles is governed by the laws of Newtonian mechanics. Transport of particles is dealt with in the aerosol science literature.
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Published in Luis Liz-Marzán, Colloidal Synthesis of Plasmonic Nanometals, 2020
Denis Rodríguez-Fernández, Luis M. Liz-Marzán
The catalytic activity of metallic Janus particles has also been recently used by several groups for different purposes, including the design of swimmers or nanoengines[162–168] and nanotransporters.[169] A nanoengine is a nanoparticle that makes use of a chemical reaction to obtain the necessary self-propulsion to overcome Brownian motion, while a nanotransporter is a swimmer that additionally loads a cargo. Two mechanisms have been proposed to explain the motion of these particles in solution using H2O2 as fuel, generating H2O and O2. The first model proposes that oxygen nanobubbles detach from the surface of the catalyst due to the amount of liberated oxygen, which makes it impossible to be dissolved and thus provides the device with a momentum. These bubbles have been observed with large catalysts, but it is unclear whether they are responsible for the motion of smaller swimmers. The second mechanism suggests that the motion is caused by the concentration gradient generated during the reaction, called diffusiophoresis. The first model only explains the movement in the direction opposite to the metal, whereas in the second the direction depends on the interactions between the solvent and the catalytic and non-catalytic parts of the particle.[166] Actually, there is no single mechanism explaining the behavior of swimmers in all systems, and other aspects like geometry of nanoengines do affect the propulsion.[170] An important contribution was provided by Howse et al., who observed that Pt-PS semishells only display Brownian motion in the absence of fuel, but when fuel is added the particles self-propel and if the concentration is increased the velocity of the particles also becames higher.[162] The aim of these devices involves the transport of a cargo within a liquid medium, with potential applications in drug delivery or biosensing.[169]
Accelerated mass transfer enhancement by density-driven natural convection
Published in Indian Chemical Engineer, 2022
In the context of transport phenomena, the presence of a concentration gradient of the solute in a solvent significantly influences the nanoparticles migration. In this process, the chemical energy of concentration gradients is converted into the mechanical energy of colloids, and this phenomenon is known as diffusiophoresis [1]. Numerous experimental and theoretical studies have been performed for ionic as well as nonionic solutes [2–10]. In contrast to ionic solute, colloids migration in nonionic solute gradients is less studied experimentally. Depending on the types of particle interactions to the solute, that is, attractive or repulsive, the particle migrates towards higher or lower solute concentration, respectively [11].
Control volume finite element method for entropy generation minimization in mixed convection of nanofluids
Published in Numerical Heat Transfer, Part B: Fundamentals, 2019
Thermophoresis (also known as the Soret effect) occurs due to the drifting of nanoparticles against a temperature gradient, from a high-temperature region to the region of low temperature. This phenomenon is most significant in a free convection process, in which the flow is driven by temperature and buoyancy. A reduction in bulk density increases the rate of heat transfer. In contrast, diffusiophoresis (or osmophoresis) is a transport mechanism which involves drifting of suspended nanoparticles from a zone of lower concentration to a zone of higher concentration. This is generally not a preferred method due to the agglomeration of the nanofluids [41].