Field-driven dynamics of dilute gases, viscous liquids and polymer chains

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007.Includes bibliographical references (p. [131]-136).This thesis is concerned with the exploration of field-induced dynamical phenomena arising in dilute gases, viscous liquids and polymer chains. The problems considered herein pertain to the slip-induced motion of a rigid, spherical or nonspherical particle in a fluid in the presence of an inhomogeneous temperature or concentration field or an electric field, and the dynamics of charged polymers animated by the application of an electric field. The problems studied in this thesis are unified by the existence of a separation of length scales between the macroscopic phenomena of interest and their microscopic underpinnings, and are treated by means of coarse-graining principles that exploit this scale separation. Specifically, the first part of this thesis investigates the dynamics caused by the existence of a slip velocity at a fluid-solid interface. The macroscopic slip boundary condition obtains from the asymptotic matching of the velocity within the microscale layer of fluid adjoining the solid surface, and the velocity in the bulk fluid. In the case of a gas, the microscopic length scale is constituted by the mean free path, and the layer of gas adjoining the solid boundary having a thickness of the order of the mean free path is referred to as the Knudsen layer. The parameter representing the ratio of the mean free path to the macroscopic length scale is the Knudsen number, denoted Kn. The widely-used Navier-Stokes and Fourier equations are valid away from the solid boundary at distances large compared to the mean free path in the limit Kn < 1, and necessitate the imposition of continuum boundary conditions on the gas velocity and temperature at the outer limit of the Knudsen layer. These macroscopic equations are typically solved subject to the no-slip of velocity and the equality of the gas and solid temperatures at the solid boundary.(cont) However, as first pointed out by Maxwell, the no-slip boundary condition fails to explain experimentally observed phenomena when imposed at the surface of a nonuniformly heated solid, and must be replaced by the thermal slip condition obtained via the asymptotic matching of the velocity within the Knudsen layer with that in the bulk gas. Slip has also been proposed to occur at liquid-solid boundaries under conditions of inhomogeneous temperature or concentration. In this thesis, we extend Faxen's laws for the force and torque acting on a spherical particle in a fluid with a prescribed undisturbed flow field to account for the existence of fluid slip at the particle surface. Additionally, we investigate the effect of particle asymmetry by studying the motion of a slightly deformed sphere in a fluid having a uniform unperturbed flow field, and demonstrate that the velocity of a force- and torque-free particle is independent of its size or shape. While the slip-induced motions studied in this thesis are presented in the context of thermally-induced slip arising from the existence of a temperature gradient, the results are equally applicable to more general phoretic transport, encompassing the electrokinetic slip condition employed in the treatment of charged particle dynamics in an electrolytic liquid. Analogous to the thermal slip condition imposed on a gas at the outer limit of the Knudsen layer, the electrokinetic slip condition is imposed at the outer limit of the layer of counterions surrounding a charged surface in an electrolytic liquid. The studies presented in this thesis have potential applications in aerosol and colloid technology, in the nonisothermal transport of particulates in porous media and MEMS devices, and in the electrophoresis of charged bodies. The behavior of a charged polymer molecule in an electric field constitutes the subject of the second part of this thesis.(cont) Motivated by the medical and technological necessity to effect the size-separation of DNA chains in applications ranging from the Human Genome Project to DNA-based criminology, we consider specifically the dynamics of electric-field driven DNA chains in size-based separation devices. The conventional technique of constant-field gel electrophoresis is ineffective in achieving the separation of long DNA chains whose sizes exceed a few tens of kilobase pairs, owing to the fact that the velocity becomes independent of chain size for long chains in a gel. This limitation of gel electrophoresis has spurred the development of alternative separation devices, such as obstacle courses confined to microchannels wherein the obstacles may be either microfabricated or formed from the self-assembly of paramagnetic beads into columns upon the imposition of a magnetic field transverse to the channel plane. Size separation in the latter devices arises from the fact that longer chains, when driven through the channel by an applied electric field, are more likely to collide with the obstacles and take longer to disentangle from the obstacle once a collision has occurred, relative to shorter chains. Consequently, a longer chain requires more time to traverse the array compared to a shorter chain. As a model for the transient chain stretching occurring subsequent to the collision of an electrophoresing DNA molecule with an obstacle, we study the unraveling of a single, tethered polymer molecule in a uniform solvent flow field. In the context of a polymer, the microscopic length scale is associated with the size of a monomer. We, however, employ a coarse-grained representation wherein the polymer is modeled by a chain of entropic springs connected by beads, with each bead representing several monomers, thereby enabling a continuum description of the solvent. We adopt the method of Brownian dynamics applied to the bead-spring model of the polymer chain.(cont) We consider both linear force-extension behavior, representative of chain stretching in a weak field, and the finitely-extensible wormlike chain model of DNA elasticity, which dominates chain stretching under strong fields. The results yield insight into the mechanism of tension propagation during chain unraveling, and are more generally applicable to situations involving transient stretching, such as chain interactions arising in entangled polymer solutions. We next conduct investigations of chain dynamics in obstacle-array based separation devices by means of coarse-grained stochastic modeling and Brownian dynamics simulation of a chain in a self-assembled array of magnetic beads, and predict the separation achievable among different chain sizes. We examine the influence of key parameters, namely, the applied electric field strength and the spacing between obstacles, on the separation resolution effected by the device. Our results elucidate the mechanisms of DNA dynamics in microfluidic separation devices, and are expected to aid in the design of DNA separation devices and the selection of parameters for their optimal operation.by Aruna Mohan.Ph.D

Optically induced forces on anisotropic plasmonic nanoparticles

Die Verleihung des Nobelpreises für Physik im Jahr 2018 für Arthur Ashkins bahnbrechende Arbeit über optische Pinzetten hat dem Forschungsfeld der optischen Manipulation zu breiter Anerkennung verholfen. Im Laufe der letzten vier Jahrzehnte hat dieses Forschungsgebiet Anwendungen auf vielen Gebieten ermöglicht. Die Bandbreite erstreckt sich dabei von Einzelzellmikroskopie bis zu Nanolithographie. In jüngerer Vergangenheit war vor allem die Manipulation plasmonischer Nanopartikel von besonderem Interesse, da diese als optische Sensoren auf der Nanoskala verwendet werden können. Das genaue Verhalten derartiger Partikel ist jedoch ein komplexes Zusammenspiel vieler Parameter, die von der Geometrie und Beschaffenheit der Partikel und deren umgebendes Medium sowie der Laserstrahlung zur resonanten Anregung der plasmonischen Eigenschaften abhängen. Der Fokus dieser Arbeit liegt insbesondere auf nichtsphärischen, also anisotropen, Goldnanopartikeln und dem Einfluss dieser Anisotropie auf die resultierenden optisch-induzierten Kräfte. Zunächst wurden optische Streukräfte dazu benutzt, einzelne plasmonische Nanopartikel nach ihrer Form und damit auch ihrer Plasmonenresonanz zu sortieren, indem sie auf ein Substrat gedruckt wurden. Dabei wurde für jede Partikelspezies ein Laser verwendet, der resonant zur jeweiligen Plasmonenresonanz war. Dieser neuentwickelte Ansatz nutzt die Abhängigkeit der Plasmonenresonanz und damit auch der Streukräfte von der Form des Partikels aus. Als erste Anwendung wurde die Dynamik der Nanopartikelsynthese durch die Reduktion von Au(III) durch Natriumsulfid aufgeklärt, die Gegenstand einer langanhaltenden Debatte in der Literatur war. Es war möglich einen spektralen Peak im Nahinfrarotbereich der Bildung dreieckiger Nanopartikel zuzuschreiben, was im Gegensatz zu früheren Studien steht, die dies Kern-Schale Partikel oder Partikelaggregate zurückgeführt hatten. Durch Erhöhung der Laserintensität nimmt plasmonisches Heizen derart zu, dass das Deformieren von Partikeln möglich wird. Normalerweise verformen sich anisotrope Partikel wie Nanostäbe zu sphärischen Partikeln um ihre Oberflächenenergie zu verringern. In dieser Arbeit wurde jedoch gezeigt, dass das Anlegen sehr starker Laserintensitäten zu einer Aufspaltung des Nanostäbchens in ein Dimer aus zwei sphärischen Nanopartikeln gleicher Größe führt. Mittels einer Analyse der optischen Eigenschaften konnte ein Partikelabstand im Subnanometerbereich abgeschätzt werden. Durch computergestützte Modellierung wurde ein Model entwickelt, das die Aufspaltung einer Kombination von oberflächenspannungsinduzierter Deformation sowie inhomogen wirkender optischer und hydrodynamischer Kräfte zuschreibt. All diese Beiträge sind optisch induziert. Die Herstellung von Dimeren mit derart kleinen Partikelabständen ist üblicherweise herausfordernd. Daher kann dieser neu entwickelte Ansatz in Zukunft für Anwendungen wie oberflächenverstärkte Raman Streuung oder das induzieren chemischer Reaktionen mittels heißer Elektronen Bedeutung erlangen. Dehnt man die Anisotropie der Partikel auf die Materialzusammensetzung aus, indem man plasmonisch-dielektrische Janus Nanopartikel erzeugt, tritt eine weitere Kraft unter Laserbestrahlung, Thermophorese, auf. Wird die Laserintensität erhöht wird dadurch der Partikel in vertikaler Richtung aus der optischen Ebene gedrückt. Im Rahmen dieser Dissertation wurde dieses Verhalten zum ersten Mal für Nanopartikel berichtet. Dies wurde angewandt, um DNS-funktionalisierte Janus Nanopartikel auf lebende Zellen zu heben und anschließend durch die Zellmembran zu injizieren. Es wurde gezeigt, dass die DNS diesen Vorgang übersteht, da die Wärme ausschließlich an der plasmonischen Spitze des Partikels erzeugt wird. Dies bereitet den Weg für biotechnische Anwendungen wie Zelltransfektion. Diese Arbeit trägt zu einem besseren Verständnis der Vielzahl an Kräften bei, die auf plasmonische Nanopartikel in einem fokussierten Laserstrahl wirken. Insgesamt konnten für alle Grundlagenexperimente potentielle Anwendungen gezeigt werden, was das große Potential dieses Forschungsfeldes für zukünftige Technologien demonstriert.With the award of the noble prize in physics in 2018 for Arthur Ashkin’s seminal work on optical tweezers, great honor has been brought to the field of optical manipulation. Over the past four decades, this field has developed applications in numerous fields ranging from single cell microscopy to nanolithography. Recently, the manipulation of plasmonic nanoparticles has been the subject of particular interest, since they offer an all optical handle at the nanoscale. However, the exact behavior of such particles is a complex interplay of many parameters of the the incident laser light, the particle itself and its surrounding. This thesis puts its focus especially on nonspherical, hence anisotropic, gold nanoparticles and the impact of this anisotropism on the optically induced forces. First, optical scattering forces were used to sort single plasmonic nanoparticles according to their shape and therefore plasmon resonance by printing them on a substrate using a laser tuned to this particular resonance. This newly developed approach makes use of the shape dependence of the plasmon resonance and therefore the optical force excerted on the particles. It was first applied to shed light on the temporal dynamics of the nanoparticle synthesis via the reduction of Au(III) with sodium sulfide that has been a longstanding matter of debate. It was possible to assign a spectral near infrared peak to the formation of triangular nanoparticles, which is in contrast to previous reports that claimed core-shell particles or particle clusters. When increasing the incident laser intensity, plasmonic heating contributes in a way that particle deformation becomes possible. Anisotropic particles such as nanorods usually converge to spherical particles upon heating to decrease surface energy. However, here it was found that applying very strong laser power densities on single gold nanorods lead to a split-up of the particle and the formation of a dimer consisting of two equally sized spheres. Optical analysis revealed the particles to have subnanometer gap distances. A model was conceived through computational modelling attributing the split-up to a combination of surface tension driven deformation, imhomogeneous optical forces and hydrodynamic forces. All those forces are in the end optically induced. Dimers with such small gap distances are usually challenging to produce. Therefore, this newly developed approach could be important for applications such as surface enhanced Raman scattering or hot electron driven chemical reactions. Upon extending the anisotropy of the particles to their material composition thus creating plasmonic dielectric Janus nanoparticles, another force, namely thermophoresis, occurs when increasing the laser intensity thus pushing the particle out of the focal plane in vertical direction. Here, this behavior was found for the first time for a particle on the nanoscale. This was applied by lifting DNA functionalized Janus nanoparticles on top of living cells and injecting them through the cell membrane. It was shown that the DNA survives this treatment as the heat generation is concentrated at the plasmonic side of the particle, thus paving the way for biotechnical applications such as transfection. This work helps to further understand the multitude of forces acting on plasmonic nanoparticles when subject to a focused laser beam. Overall, all fundamental experiments could be brought to applications, hence showcasing the great potential of the field for future technology

Investigation of single drop particle scavenging using an ultrasonically levitated drop

Airborne particulate, known as aerosols, produced by both natural and anthropogenic means, have significant health and environmental impacts. Therefore understanding the produc-tion and removal of these particles is of critical importance. The main thrust of this thesis research is concerned with improving the understanding of removal of particulates via interaction with falling liquid drops, known as wet deposition. This process occurs naturally within rain and can be imposed in industrial applications with wet scrubbers. Therefore improved models for wet scavenging have applications in both climatology and pollution control. To perform this study, first the performance of existing models for wet deposition was investigated. Models for drop scavenging of aerosols via inertial impaction proposed by Slinn and by Calvert were compared with published experimental measurements. A parametric study was performed on the residual of the model predictions from the measurements to identify dimensionless groups not included in these models, which might increase model performance. The study found that two dimensionless groups, the relative Stokes number, Stkr and the drop Reynolds number Re, are both well correlated with the residual of these models. They are included in modified versions of both of these models to provide better performance. That these two dimensionless groups improve model performance suggests that an inertial mechanism and an advective mechanism not accounted for in the existing models play some role in aerosol scavenging in the inertial regime. These findings were experimentally investigated to identify more specifically these mecha-nisms. To do this, single drop particle scavenging was experimentally measured using an ultrasonic levitation technique. This technique enabled measurements of scavenging efficiency, E, for individ-ual drops, and allowed for control of drop axis ratio, α, drop shape oscillations, and Re independently from drop diameter. This allowed for more controlled manipulation of the drop wakes in both at-tached and vortex shedding regimes. Non-evaporating drops were used which resulted in essentially zero temperature and vapor concentration difference between the drop surface and the surrounding air, virtually eliminating the possibility of confounding phoretic effects. Plots of E versus Stokes number, Stk, were found to depend on α. These plots became independent of α when Stk was calculated using the Sauter mean diameter (as opposed to the equivolume diameter). Furthermore, E was shown to be insensitive to both Re and drop shape oscillations, suggesting that wake effects do not have a measurable impact on E. Finally, a method was developed to relate models of E for spherical drops (the assumed shape in existing scavenging model predictions) to E for arbitrarily deformed drops, such as those occurring in rain. Of note, these are the first measurements of droplet scavenging obtained using ultrasonic levitation. Finally, as drop scavenging is heavily dependent on particle size, a novel technique was identified and explored for improving aerosol sizing measurements. To do this, experiments were carried out in an impactor where the distance between the impactor nozzle and the impactor plate was small, much less than the typically used one nozzle diameter separation. The aerosol deposition patterns in this impactor were investigated for aerosols in the 3µm to 15µm diameter range. Ring-shaped deposition patterns were observed where the internal diameter and thickness of the rings were a function of the particle diameter. Specifically, the inner diameter and ring thickness were correlated to the Stokes number, Stk; the ring diameter decreased with Stk, and the ring thickness increased with Stk. At Stk ∼ 0.4 the ring closed up, leaving a mostly uniform disk deposition pattern. These ring patterns do not appear to correspond to patterns previously described in the literature, and an order of magnitude analysis shows that this is an inertially dominated process. Though this method was not used for particle sizing in this thesis research, it is possible that further development of this approach will result in a more advanced particle sizing tool for aerosol science research

Towards long term colloid suspension in a vertically rotated system.

Within a colloidal suspension gravity may compromise the observation of governing physical interactions, especially those that are weak and/or take significant time to develop. Conducting the experiment in a long-term microgravity environment is a viable option to negate gravitational effects, though significant resources are required to do so. While it may not be possible to simulate long-term microgravity terrestrially, particles can resist quick sedimentation in a confined suspension system rotating vertically with appropriate rotation speed. The goal of the investigation is to demonstrate the existence of long-term particle suspension regime for a certain colloidal suspension while characterizing colloidal behavior due to hydrodynamic interactions. First, to understand the colloidal suspension in a rotational system, I studied the colloidal behavior in such a system where colloidal particles and underlying surfaces interact to each other hydrodynamically. Therefore, I studied the collective behavior of colloidal particles (4.0 µm PMMA), located near the solid surface in a fluid medium confined in a cylindrical cell (3.0 mm diameter, 0.25 mm height) which was rotated vertically at a low rotational speed (20 rpm). The observed colloidal behavior was then validated through a Stokesian dynamics simulation where the concept of hydrodynamic contact force or lubrication interactions were avoided which is not physically intuitive and mathematically cumbersome. Rather, I adopted hard-sphere like colloidal collision or mobility model. I found that colloidal agglomeration is a function of the applied rotation scheme, either forming colloidal clusters or lanes. While evolving into dynamic structures, colloids also laterally migrate away from the underlying surface. While forming colloidal structures due to hydrodynamic interactions among particles and nearby solid surface, particles migrate away from the surface and eventually redistribute throughout the sample cell. After redistribution, I demonstrated long term colloidal stability within the sample cell. When particles are redistributed with relatively equal spacing and not concentrated near a solid surface, structure formation is minimized and does not evolve any further which can be considered as long-term suspension

Thermophoretic Transport in Dispersions of Asymmetric Colloids and Microchannels

Temperature gradients trigged force on colloid (or fluid) is referred to as thermophoretic (thermoosmotic) force. This driven mechanism offers rich transport phenomena out of thermodynamic equilibrium. With a mesoscale hydrodynamic simulations method, this thesis focuses on thermophoretic response of colloids with geometric and compositional asymmetries, and their resulting net flows, from both fundamental mechanism and application viewpoints. Firstly, combined analytical theory and simulation, we study thermophoretic / diffusiophoretic flows and forces, and related finite size effects for spherical colloids. Local quantities such as slip flow and associated local pressure at the solid-liquid boundary layer are obtained which explicitly explain the microscopic mechanisms of thermophoresis. Then, we exploit how the particle shape influences thermophoresis. The elongated colloids exhibit an orientation dependent thermophoretic response, i.e., the anisotropic thermophoresis. We introduce a linear decomposition scheme to show and understand this anisotropic phenomenon. Quite contradictory from anisotropic friction, we realize that the thermophoretic force of a rod oriented with the temperature gradient can be larger or smaller than when oriented perpendicular to it. This transition depends not only on the geometric details of the surface, also on the colloid-solvent interaction. Then the dependence on the rod aspect ratio is studied. Later, we move our attention to the heterodimer composed of two beads with different thermophoretic properties. The resulting alignment is linearly dependent on temperature gradient and strongly relies on the size ratio. Additionally, the interacting heterodimers in a confined slit with walls are investigated in the presence of a temperature gradient. The colloids first align to the gradient due to thermophoretic torques, then accumulate at the wall. We observed the exponential decay of both positional and orientation order as the distance to the accumulation walls increases. This is reminiscent of "sedimentation-diffusion equilibrium" phenomenon. Hydrodynamic interaction in the case of phoretic heterodimers seems to be of importance when colloids are close to the wall. With an application perspective, we propose two types of micropumps which use thermophoresis as surface forcing mechanism, but with different symmetry breaking by incorporating obstacles in the middle of the microchannel. In the first micropump, the temperature gradient is applied perpendicular to the channel walls; and elongated obstacles are fixed and tilted to the gradient. This geometric asymmetry and thermophoresis enable fluid to flow along the channel. The resulting flow patterns, the magnitude, and direction of the net flux density rely on the channel geometric parameters. The flow strength, path, and direction can be tunned by the length, rugosity, and thermophobic/thermophilic properties of the obstacles. The net flow flux for obstacles with various interfacial properties can be captured by anisotropic thermophoresis. The second micropump uses fixed, metallic / non-metallic compositional obstacles aligned with the channel walls. By laser illumination, temperature gradient can be established due to the higher heat absorption in the metallic composition, which consequently leads to a net flow flux. The resultant far field flow resembles Poiseuille flow. Its pumping capability strongly depends on the length of the non-metallic part as well as the inter-separation distance of the obstacles but is only slightly dependent on the channel width. Finally, a comparison of the pumping capability between different phoretic pumps is made

Second Microgravity Fluid Physics Conference

The conference's purpose was to inform the fluid physics community of research opportunities in reduced-gravity fluid physics, present the status of the existing and planned reduced gravity fluid physics research programs, and inform participants of the upcoming NASA Research Announcement in this area. The plenary sessions provided an overview of the Microgravity Fluid Physics Program information on NASA's ground-based and space-based flight research facilities. An international forum offered participants an opportunity to hear from French, German, and Russian speakers about the microgravity research programs in their respective countries. Two keynote speakers provided broad technical overviews on multiphase flow and complex fluids research. Presenters briefed their peers on the scientific results of their ground-based and flight research. Fifty-eight of the sixty-two technical papers are included here