John F. Brady - US grants

This award will support collaborative research between Dr. John Brady, California Institute of Technology, and Dr. George Bossis, Laboratoire de Physique de la Matiere Condensee, University of Nice, France. The objective of this research is to study the transport properties of fractal aggregates and polymers by dynamic simulation. The past several years has seen an increased interest in the transport properties of condensed matter in the form of colloids, ceramics and polymers. Part of this interest arises from the practical uses of such substances, motivated by the potential to design and manufacture materials with novel and desired properties. A large part of the recent theoretical activity in these research areas has been sparked by the recognition that collodial aggregates and polymers have an important geometrical feature in common--they are fractals. A fractal is an object that is not space filling; the mass does not grow as the radius cubed, but rather to some other power called the fractal dimension. Fractals also possess a dilational symmetry; that is, the object looks the same at least statistically, regardless of the magnification used. These two aspects, particularly the dilational symmetry, have allowed researchers to make simple and far reaching predictions about polymer structure and properties. The transport properties of polymers and collodial aggregates depend on the basic fractal microstructure, and an understanding of the relationship between structure and property will aid in predicting and controlling the manufacture of these microstructured materials. In this project, the investigators will use a general molecular-dynamics-like method which they have developed together, called Stokesian dynamics, with the aim of providing a computational bridge betwen analytical theories and laboratory experiments. A major focus of the work will be to assess the importance of hydrodynamic interactions and the restrictions imposed by the preaveraging assumption on aggregate and polymeric behavior. This project will benefit from the specific expertise of Dr. Bossis in statistical mechanics and Dr. Brady's background in fluid mechanics.

This proposal aims to generalize the Stokesian dynamics formulation of fluids with particles, with a fresh theoretical approach and several new computer strategies. The multi- particle simulations will consider a variety of interparticle forces (Brownian, colloidal, external and hydrodynamical forces) and different length scales in the flow. Microstructural and macroscopic (rheology, diffusivity, etc.) properties will be determined under nonequilibrium flow conditions. It is expected that Stokesian simulations of systems with tens of thousands of degrees of freedom will be possible by using sparse matrix systems and parallel processing. This will make feasible quantitative predictions for real systems. The proposed research will provide basic understanding for design in processing of microstructured fluids, such as composite materials, polymers, ceramics, etc. Stokesian dynamics will be developed as a tool for other scientists and engineers.

A graphics workstation will be purchased by the Department of Chemical Engineering to visualize the suspension microstructures and evolving dynamics. The selected system is Stardent ST3020VS. The dynamics of microstructured fluids are simulated using Stokesian dynamics simulation. The microstructured fluids are in the form of particles dispersed in a fluid medium where the particles interact through any combination of Brownian, colloidal, external and hydrodynamics forces. Microstructural (particle distribution functions, etc.) anbd macroscopic (rheology, diffusivities, etc.) properties are determined under highly nonequilibrium conditions typical of those found in the processing of multiphase materials. The goal of the work is to understand the relationship between structure and properties on the one hand and between structure and flow on the other.

9415673 Brady This three-year award will support U.S.-France cooperative research in chemical engineering between John F. Brady of the California Institute of Technology and Georges Bossis of the University of Nice. The objectives of their research are to demonstrate that short-term range repulsion does not change the rheological behavior of hard-sphere suspensions and to determine the minimum size of a particle which can be used in electrorheological fluids without melting the chemical chains. The research involves the use of Stokesian dynamics. The will work on a third, but related project, a monograph on suspension of dynamics. The U.S. and French investigators have a long-standing collaboration and are co-inventors of Stokesian dynamics. The U.S. investigator and his group will carry out the electrorheological fluid simulations. This will be complemented by French investigators work on the simulations of short-term range repulsions. The project will advance our understanding of the physics of suspensions and their role in the engineering of new materials. ***

This three-year award will support U.S.-France cooperative research in chemical engineering between John F. Brady of the California Institute of Technology and Georges Bossis of the University of Nice. The objectives of their research are to demonstrate that short-term range repulsion does not change the rheological behavior of hard-sphere suspensions, and to determine the minimum size of a particle which can be used in electrorheological fluids without melting the chemical chains. The research involves the use of Stokesian dynamics. The will work on a third, but related project, a monograph on suspension of dynamics. The U.S. and French investigators have a long-standing collaboration and are co-inventors of Stokesian dynamics. The U.S. investigator and his group will carry out the electrorheological fluid simulations. This will be complemented by French investigators work on the simulations of short-term range repulsions. The project will advance our understanding of the physics of suspensions and their role in the engineering of new materials.

This award supports summer research by undergraduates at the Whitney Laboratory of the University of Florida. Students pursue research related to ongoing faculty research programs that study the physiology, anatomy, biochemistry and molecular biology of various marine invertebrates and fishes. Students also participate in weekly seminars and informal research presentations, as well as discussions of scientific ethics and lectures and field trips that expose students to the ecology of nearby marine ecosystems.

Project Summary
The increased demand for knowledge of small-scale behavior is making microrheology a key
step in the understanding of biological systems, the design and use of advanced materials,
and the novel applications of already existing materials. Most microrheological experiments
and analyses to date have focused on linear viscoelastic properties, by correlating the
random thermally-driven displacements of small tracers to the complex modulus through a
generalized Stokes-Einstein relation, a process which is relatively well understood but which
is limited in its scope to equilibrium systems. Many systems of practical interest are driven
out of equilibrium and display nonlinear behaviors, but the microrheology work in this
area has been scarce (conventional macroscale rheometers are used to measure both linear
and nonlinear regimes), and the connection between micro and macroscale measurements
is unclear. The need and interest in microscale measurements, whether due to the scarcity
of the material or the size of system, makes microrheology an important technology, but
unfortunately one which currently lacks fundamental understanding in many aspects.
The proposed research examines the relation between micro and macrorheology through
theoretical studies, with a particular and strong emphasis on the `active' (driven) and nonlinear regime.
This research is best described in terms of applications to colloidal dispersions, and will focus on such systems
because they offer very well-defined and well-characterized materials, allowing for comparisons
to macroscale measurements. However, the impact of this research extends beyond
colloidal systems as the theoretical foundation and general conclusions are extendable to
many complex materials, especially biomaterials. Specific issues such as shear thickening
in microrheology, the effect of the size ratio (tracer size to typical medium length scale) on
the `continuum approximation' and on microscale velocity fluctuations, and the interactions
between pairs of moving particles leading to structure formation are addressed. This is the first attempt to analyze the nonlinear behavior of materials within the context of microrheology
and provides a fundamental validation of microrheology as a sound technique, critical for
its continued application and future growth.
Broader Impact: In a broader context, this research will engage PhD students who will
become experts in colloidal physics and rheology, and who will go on to positions of leadership in industry and/or academia.
To aid in the education of future generations of scientists and engineers, a microrheology section for the undergraduate
chemical engineering laboratory at Caltech will be developed. To disseminate the research
as widely as possible, in addition to publication in conventional technical journals, a website
will be maintained with research results that are accessible to the general public. Since this
research provides the theoretical foundation for a new experimental technique that has
widespread application in science and technology, its impact is both very broad and deep.

CBET-0754967
Brady

Intellectual Merit: The design of nanoengines that can convert stored chemical energy
into motion is a key transformative challenge of nanotechnology, especially for nano-engines that can operate autonomously. Recent experiments have demonstrated that it is possible to power the motion of nanoscale and microscale objects by using surface catalytic reactions so-called catalytic nanomotors. The precise mechanism responsible for this motion is not known, although a number of ideas have been put forth. This project involves a very simple mechanism is proposed: osmotic propulsion. A surface chemical reaction creates local concentration gradients of the reactive and product species which generate a net osmotic force on the motor. The motor is able to harness the ever present random thermal motion via a chemical reaction to achieve directed autonomous motion. This research demonstrates that such an 'osmotic' motor is possible and addresses such questions as: How fast can the motor move? How large of a force can it generate? How much 'cargo' can it carry? How much fluid can it pump? How can its motion be controlled and directed? What chemistry can be used? What is the efficiency of such an osmotic motor?

Broader Impact: Osmotic propulsion provides a very simple and general means to convert chemical energy into mechanical motion and work. Exploiting the random thermal motion in colloidal systems via osmotic propulsion can revolutionize the design and operation of microfluidic and nanodevices, with applications in directed self-assembly of materials, thermal management of micro- and nanoprocessors, and the design and operation of chemical and biological sensors. This research will provide explicit prescriptions for the construction and operation of colloidal particles that can be used as osmotic motors. This fundamental and transformative study must be undertaken if we wish to enable many of the nano-scale technologies envisioned for the future: tiny medical 'nanobots' that can access human illness inside the body, at the cellular level, and repair it. Or devices that can sense their way through micro channels in 'lab on a chip' devices, stirring or separating nano-liters of chemicals. Or even a nano-motor that senses intrusion of a specific molecule, swims toward it, and closes a channel in the process triggering an alarm switch for biological contaminants. Any of these types of devices is possible provided the physics of motion at that scale is correctly understood and utilized. And finally, studies of autonomous motors may help to understand more generally chemomechanical transduction as occurs in biological systems, and also create novel artificial motors that mimic living organisms and which can be harnessed to perform useful tasks.

CBET-0828563
Brady

Project Summary
Intellectual Merit: Suspensions and granular media are found widely in nature and industry. And although the two fields have developed independently, recently it has been noted that viscous suspensions and dry granular materials often display similar flow phenomena. Despite the similarities, the two fields nevertheless remain separate. Here we propose that this separation is unnecessary and that suspensions and granular media - wet and dry - actually correspond to different limiting behaviors of one common system. The linking parameter is the Stokes number - the ratio of the inertial to shear forces: small Stokes numbers correspond to viscous suspensions and high Stokes numbers to dry granular media. The proposed research is a simulation study covering the entire range of Stokes numbers (and concentrations) and thus explores the flow behavior and rheological connection between these two fields. Not only will we learn about this connection, we may also advance our understanding of both suspensions and granular media. This study also provides vital rheological data (shear and normal stresses, shear-induced diffusivities, microstructures, etc.) for inertial suspensions, of which there appears to be surprisingly little.

Broader Impact: In a broader context, this research will help elucidate the behavior of an often called new state of matter - granular matter. Granular flow forms part of the larger area of multiphase flow with applications ranging from sediment transport in rivers and bays, to pneumatic conveying of minerals, to the production of pharmaceutical powders. The work will also engage PhD students who will become experts in computational methods, multiphase fluid physics and rheology. This research provides the foundation for modeling multiphase flows that has widespread application in science and technology.

0931418
Brady


Intellectual Merit: The increased demand for knowledge of small scale behavior has made microrheology a key step in the understanding of biological systems and the design and use of advanced materials and nano scale devices. Most microrheological work to date has focused on linear viscoelastic properties, by correlating the random thermally driven displacements of tracers to the complex modulus through a generalized Stokes Einstein relation, a process which is well understood but which is limited in its scope to equilibrium systems. But many systems of practical interest are driven out of equilibrium and display (indeed, rely upon) nonlinear behaviors. Recently a body of work has emerged focusing on this active, nonlinear microrheology. In such a system, tracer particles undergo displacements due not only to random thermal fluctuations, but also due to the application of an external force applied directly to the tracer. The dispersion is driven out of equilibrium, and as with macrorheology, dynamic responses such as viscosity can be measured. Since the tracer probes the material at its own (micro)scale, much smaller samples are required compared to macrorheology, and localized heterogeneity can be explored. Recent experiments confirm the theory; but in both theory and experiment, the focus thus far has remained on the mean response of the material the viscosity and little focus has been devoted to particle uctuations. Just as the shear flow in macrorheology enhances particle diffusion, an analogous `force induced' diffusivity arises due to the single particle forcing of active microrheology. This diffusive motion is fundamental to the motion of an active microscale particle important both for scientific and technology considerations. The proposed research extends the theory of active microrheology to the force induced diffusive motion of individual particles, as well as normal microstress differences. This work will combine theoretical and computational studies, focusing on colloidal systems because they offer very well characterized materials, allowing for comparisons to macroscale measurements. But the impact of this research extends beyond colloids, as the theoretical foundation and general conclusions are extendable to many complex materials, especially biomaterials. Other issues such as the effect of tracer size on the `continuum approximation', and hydrodynamic interactions between pairs of moving particles leading to structure formation, will be addressed. This work will expose new material capabilities and ultimately provide a validation of microrheology as a sound technique, critical for its continued application and future growth.

Broader Impact: Motion control for active microscale particles is a major focus in many fields from biophysics to alternative energy to nanomedicine and it begins with understanding the fluctuations in particle motion. Since this research provides the theoretical foundation for new experimental techniques that have widespread application in science and technology, its impact is both very broad and deep. This research will develop PhD students into experts in colloid physics, rheology, and computational methods, who will become leaders in industry and academia. To aid in the education of future scientists and engineers, a microrheology section for the Caltech chemical engineering laboratory will be created. To disseminate the research widely, a publicly accessible website showcasing research results will accompany publication in technical journals.

With this award from the Major Research Instrumentation (MRI) program and the Chemistry Division, Professor Thomas F. Miller and colleagues John F. Brady, William A. Goddard, Zhen-Gang Wang and Niles A. Pierce from the California Institute of Technology will acquire a computer cluster with graphical processing units. The proposal will enhance research in a variety of areas characterized as soft matter behavior/simulations. The projects include investigations aimed at the rational design of nucleic acid, protein and enzyme systems, conformational dynamics of proteins and molecular motors, enzyme-catalyzed electron-transfer and hydrogen-transfer dynamics, trans-membrane signaling and transport processes, the nucleation of membrane adhesion, protein secretion across a cellular membrane, the formation of gels, the dynamics of ring-polymer mixtures, and polymer-based tissue engineering.

A computer cluster is a group of linked processors that work in concert to achieve vastly more computational power that individual computers. These are employed to investigate complex problems using computational methods based on theoretical models and programs. Such calculations, often used in conjunction with experimental data, allow chemists and biochemists to better understand many types of complex chemical and biological phenomenon. This resource will be used by students and faculty to develop the use of computer clusters based on graphical processing units (GPUs) rather than CPUs. This approach can speed up calculations and simulations enabling larger, more complex systems to be investigated.

1235955 / 1236242
PI: Furst / Brady

Why and how do colloidal gels form? What is the relationship between gels and glasses, states that occur when particles crowd each other to the point of dynamic arrest? There is long-running debate regarding the mechanism of colloidal gelation, particularly at moderate to high particle volume fractions and relatively weak attractive interactions. The aim of this work is to investigate the microrheology of colloidal glasses and gels to determine whether there is a direct relation between these states. The work is accomplished through the development of optical trapping experiments and Stokesian dynamics simulations that measure the response of individual probe particles in colloidal suspensions. Since theories of colloidal glasses and gels are microrheological, but few studies to date have examined and attempted to validate these theories on microscopic length scales, such work is timely.

Colloidal gels are an arrested, non-equilibrium state of matter that impact shelf life of consumer care products, agrochemicals, coatings, pigments and inks?potentially any product or material in which particles are suspended at high concentration in a fluid phase. Moreover, glasses and gels are ubiquitous states of matter in molecular, macromolecular and colloidal materials. The insights gained under this project have broad implications for our fundamental understanding of many materials. Through an integrated education and professional training of graduate students from underrepresented backgrounds in STEM disciplines and outreach activities involving undergraduate research experiences and research experiences for teachers, this work enhances the capacity for US innovation in the chemical and advanced materials industries.

1337097
PI: Brady

Colloidal suspensions are widely used in industry, medicine and in natural environments, and encompass systems as diverse as toothpaste, paints, the interior of a cell and sprayable solar panels. Understanding the rheological properties of suspensions is critical to their processing, dispensing, durability and performance. Most studies of suspension rheology have been at fixed volume (or fixed volume fraction). While this may be adequate for many applications, often suspension flows are not at fixed volume but rather at fixed stress (or fixed pressure or pressure drop). Is the flow behavior the same at fixed volume and fixed pressure? If the volume fraction of suspended particles is low enough it should be possible to covert one measurement into the other. But as the maximum flowing fraction is approached, it is no longer clear that the two conditions will lead to the same flow behavior. A simulation study of colloidal suspensions at fixed pressure, allowing the system to dilate or contract and the volume fraction fluctuate as necessary, is proposed. The Accelerated Stokesian dynamics simulation methodology will be adapted to permit the simulation volume to change and used to study the flow behavior of Brownian hard-sphere suspensions as the strength of the shearing forces compared to thermal Brownian forces is varied over a wide range. Complete microscale detail is available from simulation, including particle distribution functions, order parameters, short- and long-time particle displacements, etc., and will connect the observed macroscopic behavior to the underlying particle dynamics. Particular attention will be focused on the flow behavior as the maximum flowing fraction is approached and the scaling of the flow properties near this point.

Understanding suspension rheology is an important subject in its own right, but examining the flow behavior as the maximum flowing fraction is approached may have important implications for glassy and jammed systems. Colloidal dispersions at rest are known to form a glass at volume fractions near 0.58, well below random close packing (0.64 for monodisperse spheres). Experiment on both rapid granular flows and viscous non-Brownian suspensions at fixed pressure and shear stress have shown very similar behaviors: the ratio of shear to normal stress - the friction coefficient - is the same in the two systems, as is the maximum flowing volume fraction, despite the very different microscale physics - inertial dynamics versus viscous forces. It is quite possible that Brownian colloidal dispersions will display a similar behavior, which would then make an important link between jammed granular media and colloidal glasses. If demonstrated, such a connection would transform our understanding of glasses and jammed systems, and possibly provide a universal understanding of jamming.

This research will enable the design, at the particle scale, of colloidal dispersions to meet the flow requirements of specific applications in, for example, the paints and coatings industry, thus reducing energy consumption and product waste. Contributing to the understanding of glasses and glass-forming systems, and particular their dynamic properties, would have broad impact across disciplines from fundamental physics and chemistry to biology - the motion of proteins and protein complexes in the crowded interior of a cell has strong similarities with the hindered and heterogeneous motion in colloidal glasses. Finally, the graduate student supported by this research will be well-trained in continuum and statistical mechanics, colloidal physics and computational science, and will join the scientific workforce of the nation.

CBET 1437570

Suspensions containing particles that can propel themselves through a liquid are interesting examples of a class of materials called active matter. The self-propelled particles, which are sometimes thought of as swimmers, can contribute to the properties of the suspension as a whole. This project will explore a novel concept called the swim pressure that can help characterize the state of such a suspension. The swim pressure is similar to the usual pressure that molecules in a gas exert on the walls of a container, but in this case the pressure is exerted by self-propelled particles that could be microorganisms, chemically reactive particles, or micro-motors moving through liquid. Preliminary data suggest that the swim pressure and its dependence on particle concentration can be used to predict changes in the suspension, including phase changes, deformation and motion, that cannot be predicted by other theories for active matter. The project will investigate the swim pressure and its utility in predicting suspension behavior by carrying out a series of numerical simulations. The results will be useful to scientists and engineers who process active matter suspensions in pharmaceutical, medicinal, food and similar industries.

The micromechanical origin of spontaneous self-assembly in suspensions of self-propelled particles will be investigated by focusing on the swim pressure exerted by the particles. Numerical simulations based on Accelerated Stokesian Dynamics and other methods will be used to determine the variation of swim pressure with particle concentration, activity, etc., which can be used to formulate a nonequilibrium equation of state and pressure-volume phase diagrams for the suspension. Hydrodynamic interactions among the particles will be included in the computations. The swim pressure will then be correlated with detailed motions of the active particles that can lead to spontaneous formation of clusters, aggregates and other patterns, as well as the overall deformation and motion of the suspension.

A distinguishing feature of many living organisms is their ability to move, to self-propel, to be active. Constituents of "active matter" systems are capable of independent self-propulsion by converting fuel into mechanical motion. Examples of active matter include both microscopic entities like microorganisms and motor proteins within our cells and large bodies like fishes and birds. Inanimate, nonliving bodies can also achieve self-propulsion using mechanisms that are different than living organisms. The outcome of the collective behavior of these nonliving active systems is not necessarily different from living active systems. Indeed, active matter systems of all scales have the tendency to associate together and move collectively, from colonies of bacteria, swarms of insects, flocks of birds, schools of fish, to herds of cattle. The question addressed in this research is the micromechanical, hydrodynamic, origin for living (and nonliving) organisms to exhibit collective and coherent motion and how it can be explained and modeled using simple physical principles. Such insight will enable the prediction, design, and control of active soft matter systems and their exploitation in nature and in industry.

The intrinsic activity imparts new behaviors to active matter that distinguish it from equilibrium condensed matter systems. Active matter systems generate their own internal stress, which drives them far from equilibrium and thus frees them from conventional thermodynamic constraints, and by so doing can control and direct their own behavior and that of their surrounding environment. Active matter is always at least a two-component system - the active body and the embedding medium off of which the active body self-propels. In this research fluid-mediated hydrodynamic interactions among self-propelled bodies are incorporated for the first time. Hydrodynamics significantly affect the forces active particles exert on boundaries or other objects and can profoundly affect the phase separation in active systems by modifying the mechanical "swim pressure." The swim pressure provides a pressure-concentration relation for active matter that can quantitatively predict condensation and phase separation in active systems and provides a route for determining the amount of work that can be harvested from the often random motion of active systems. We also show that, in general, the swim stress has off-diagonal or deviatoric contributions, especially when an active system is subject to shearing motion. The swim stress predicts that, under very general conditions, active particles can reduce the suspension effective viscosity to zero, enabling spontaneous flow of active matter. The mechanical swim stress perspective allows one to understand, analyze and exploit a wide class of active soft matter systems, from swimming bacteria to catalytic nanobots to molecular motors that activate the cellular cytoskeleton.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

This award is a collaboration between an experimental group at the University of Potsdam in Germany and a theoretical group at the California Institute of Technology to study the light-activated chemically induced flow of small particles in confined geometries. By applying simple optical stimuli, colloidal particles, both individually and in bulk, can be manipulated with unprecedented levels of control and precision. The research aims to establish the foundation for ‘virtual’ microfluidic devices that utilize soft boundaries generated from light-activated flow patterns whose strength and location are tunable and allow the manipulation and control of colloidal-scale objects. The research has potential technological applications in, for example, cell sorting, DNA manipulation, and the assembly of colloidal-based materials. The collaborative nature of the award will help broaden the horizon of research students as they work with others from different countries and backgrounds, which is increasingly important in this globally connected but fragile world.<br/><br/>This award builds on and extends the recently discovered phenomenon of light-driven diffusion-osmosis in which a chemical surfactant’s hydrophobicity is altered by illumination and generates an osmotic pressure gradient that drives fluid and particle motion. The study is aimed at a fundamental understanding of the diffusiophoretic flow; that is, how the motion depends on the basic physical properties of chemical concentration, ionic strength, light intensity, etc., as well as the hydrodynamic interactions among particles and with the confining substrate. Three different processes to manipulate ensembles of particles adjacent to a boundary are investigated: (i) global spatial patterns of light intensity that cause particles to accumulate in (vacate from) regions of low (high) solute concentration, allowing one to ‘paint’ with colloids; (ii) self-generated repulsion between porous (source) particles that crystalize and enhance motion of trapped passive colloids; and (iii) self-propelled Janus particles whose speed and duration can be dynamically controlled. Theory suggests that the light-induced flow profiles, despite being non-equilibrium phenomena, can be expressed in terms of an equilibrium-like chemical ‘solute potential,’ which suggests intriguing analogies to crystallization, phase separation, etc. The award is a close alignment of theory and experiment regarding a unique non-equilibrium system that touches upon many cutting-edge problems of phoretically-driven particle dynamics and hydrodynamics, such as segregation dynamics in mixtures of particles and the motion of active self-propelled particles in dynamically fluctuating confined geometries.<br/><br/>This project was awarded through the “Chemistry and Transport in Confined Spaces (NSF-DFG Confine)" opportunity, a collaborative solicitation that involves the National Science Foundation and Deutsche Forschungsgemeinschaft (DFG).<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.