Anthony Dinsmore - US grants

This IMR grant will enable the purchase of an ultrafast, high-resolution, video-imaging system for advanced imaging of fast motions in soft-condensed matter systems from a scale of microns to several centimetres. The physical systems worked on by the group of PIs include gels, colloids, crumpled membranes, granular materials, and multiphase flow in porous media. The unifying theme in this work is the study of non-equilibrium systems in which the elementary units are large compared to molecular scales. The underlying theoretical framework to deal with such non-thermal statistical systems is still in its infancy, however, with the appropriate instrumentation it is possible to perform detailed imaging of the dynamics of the individual macroscopic "atoms and molecules" of these systems in a way that is not possible in conventional materials. The video-imaging system will enhance the frame rate, spatial resolution and the length of data streams we are able to acquire. The increased frame rate will allow capturing fast dynamics, such as ballistic motions in highly excited granular media; the improved resolution will help resolve small motions such as thermal motions in caged systems and longer data streams will enable a search for rare but important dynamical events such as intermittent crack jumps and buckling events in stressed materials. The imaging system will also be available to other local users by scheduling time via a webpage. The users will be aided by graduate students, thus broadening their exposure to a broad spectrum of scientific problems. Many of the projects involve considerable involvement by undergraduate students and the new capabilities will afford them exposure to advanced techniques in a very visual branch of materials physics.



This IMR grant will enable the purchase of an ultrafast, high-resolution, video-imaging system for advanced imaging of fast motions in soft-condensed matter systems from a scale of microns to several centimetres. The physical systems worked on by the group of investigators include gels, colloids, crumpled membranes, granular materials, and multiphase flow in porous media. These are all systems of immense industrial importance, however, they also present new basic scientific challenges in that they are statistically-sized systems with elementary units that are very large compared to molecular scales. The underlying theoretical framework to deal with such non-thermal systems is still in its infancy, however, with the appropriate instrumentation it is possible to perform detailed imaging of the dynamics of the individual macroscopic "atoms and molecules" of these systems in a way that is not possible in conventional materials. The video-imaging system will enhance the frame rate, spatial resolution and the length of data streams making it possible to study new phenomena. Apart from projects scheduled by the investigators themselves, the imaging system will be available to other local users by scheduling time via a webpage. The users will be aided by graduate students, thus broadening their exposure to a broad spectrum of scientific problems. Many of the projects involve considerable involvement by undergraduate students and the new capabilities will afford them exposure to advanced techniques in a very visual branch of materials physics.

This soft condensed matter physics project deals with disordered solids, which include glassy materials, polymer gels, particle gels, and sand piles. Although the microscopic details differ tremendously, there is increasing evidence that these materials are examples of a more general class known as jammed solids. From a fundamental point of view, the great challenge of these materials is that they are far from thermal equilibrium and continuum elasticity might not apply at microscopic or mesoscopic length scales. This project will use optical methods to obtain direct and quantitative imaging of forces, their spatial correlations, and the topology of inter-particle connections in three dimensional samples. Results are to be compared to recent theories, simulations and measurements on surfaces or on two-dimensional materials. This focus is on confocal optical microscopy of monodisperse liquid droplets with fluorescent surfaces, a model system that allows imaging of forces in the interior of three-dimensional samples. By varying the sizes of the droplets used, the systems studied will be varied between those solidified by attractive inter-particle forces and subject to strong thermal fluctuations, to those solidified by gravitational stresses at, effectively, zero temperature. The project also investigates how these force maps are changed by external stresses or by point forces applied inside the material. The project provides in-depth training of students for research in applied or fundamental programs.

Disordered solids are very common in everyday life, with examples including window glass, yogurt, soot, and sand piles. Although the microscopic details of these examples differ tremendously - molecules in one case and millimeter-sized rough sand grains in another - there is compelling evidence that they may be understandable within a single theoretical framework. From a fundamental point of view, the great challenge of these materials is that they never approach thermal equilibrium and do not form ordered (crystalline) structures. Moreover, continuum elasticity theory might be incorrect at length scales comparable to several times the size of the basic particle. This work will provide the first experimental maps of forces, structure, and connectivity inside these materials. The major experimental challenge is, somehow, to peer inside a sand pile and measure forces between adjacent particles. Here this is accomplished using liquid droplets (instead of sand grains) whose deformation is quantified in three dimensions using optical microscopy. In addition to allowing investigation of a number of new questions, the measurements will provide the first tests of theoretical predictions and computer simulations, and will connect with earlier experiments on two-dimensional materials or on the surface of three-dimensional materials. The project also provides training for students at all levels in soft condensed matter physics and advanced research techniques suitable for academic or industrial research positions.

Proposal No. CTS-0421043
Principal Investigator: H. Winter, University of Massachusetts Amherst

This grant is for the development of two novel instruments. An optical micro-rheometer and a filament stretching rheometer will be designed to simultaneously measure the evolution of stress and material structure as a function of time and accumulated strain in shear and in uniaxial extension. The optical micro-rheometer combines stress measurement with simultaneous observation of light scattering, microscopy, fluorescence, and birefringence from the sample. The filament stretching rheometer is capable of measuring the response of complex fluids to a transient extensional flow starting from an imposed initial microstructural deformation, alignment and morphology. Both instruments are designed for studies on very small samples that are required for collaboration with polymer chemists who typically prepare their most advanced materials as small samples only. Experiments with the proposed instruments will generate a deeper understanding of the behavior of complex materials such as liquid crystalline polymers under shear, crystallizing polymers as a function of molecular topology, phase separating polymer blends in shear, self assembling micellar systems in shear and extension, and particle topology in particle gels. The properties of these complex materials are strongly affected by the deformation and alignment of their microstructure in addition to their molecular composition. The study of rheology attempts to relate the local state of stress in such complex materials to the local deformation rate, elapsed time, and accumulated strain through a series of carefully designed experiments. Specifically for the new experiments, simultaneous measurement of stress and structure are expected to lead to conclusive information about flow-induced transitional states. Among the broader impacts of this work is to facilitate the development of new products, which depends on the availability of suitable materials. Therefore it is essential to develop new instruments that further our ability to create and understand new materials. In addition, the availability of the proposed instruments as a multi-user facility will have great impact on the education of all researchers involved. Once the instruments are completed, broader interaction and cross-fertilization of ideas will be enabled. Regular interdisciplinary research meetings will involve both graduate and undergraduate students. While the new instruments allow graduate students to perform their advanced research, undergraduate students will particularly profit from learning the proposed optical methods that will allow them to perform very advanced materials research on complicated topics through visual observation before their analytical skills have been developed to a comparable level. This will introduce students at an early state of their education to the excitement of discovery.

Technical Abstract:

Optical microscopy will be used to measure structure, topology, forces and elastic constants inside disordered solids in three dimensions. The great scientific challenge of these materials is that they are both random and far from thermal equilibrium and thus not readily described by thermodynamics. The basic approach of this project is to measure the positions of particles in two or three dimensions, along with their thermal fluctuations or the distortions of their shape by contact forces. The particles used will range from micron-size neutrally buoyant colloidal spheres -- whose motions are thermal -- to 20-micrometers heavy droplets, which form static piles with zero effective temperature. Attractive interactions induced among the small particles lead to gelation and this process will also be studied in reverse. The data allow unusually explicit comparisons of forces, elastic constants, and material structure, which are essential to develop and test theoretical models. High-school, undergraduate, and graduate students participate in the research. The project also informs development of a course in soft matter physics, which will be shared with instructors at other institutions.

Non-technical Abstract:

Disordered solids are very common in everyday life, with examples including window glass, polymer gels, particle gels, and sand piles. Although the microscopic details of these examples differ tremendously -- molecules in one case and millimeter-sized sand grains in another -- there is increasing evidence that they are examples of a more general phenomenon known as "jamming". The goal of this project is to use advanced techniques of optical microscopy to measure structure, topology, forces, and elastic constants throughout these materials. One set of measurements focuses on the forces among droplets inside a concentrated emulsion, which serves as a useful model of a sandpile without static friction. A parallel set of measurements focuses on the formation and melting of random particle aggregates (gels). The results will contribute to a deep and broadly applicable understanding of elasticity and aging in disordered solids. A key component of this project is the participation of high-school, undergraduate, and graduate students, who will receive training in soft matter physics suitable for careers in industry, national labs, or academia.

ABSTRACT

Proposal No.: 0609107
Title: NIRT: Controlling Interfacial Activity of Nanoparticles: Robust Routesto Nanoparticle-based Capsules
PI: Todd Emrick
Institution: University of Massachusetts Amherst

Intellectual Merit:

This proposal was received in response to Nanoscale Science and Engineering initiative, NSF 05-610, category NIRT. The objective of this research is to prepare robust materials from nanometer-scale particles, termed nanoparticles. Gaining an understanding of the structure and physical behavior of nanoparticles is crucial for fully understanding their properties, but a significant challenge given the very small size of the particles. The approach is to prepare nanoparticles in unique ways, and with unique surface properties, that allow them to self-organize in the absence of external forces. This self-organization gives assembled nanoscale materials, such as ultra-thin sheets and capsules, where the thickness of the sheet or capsule wall is five nanometers or less. Performing chemical reactions on these assemblies converts them from nano-assemblies, with no mechanical integrity, into nano-materials that are surprisingly robust given their very small dimensions.

Broader Impact:

The research carries critically important features that pertain both to technological advances and educational activities. Nanoscale devices, such as encapsulant and release systems used in therapeutic drug treatments, can be improved and refined through the use of nanoparticles as components of the therapy. In addition, nanoparticles, when used in conjunction with conventional membranes for water purification, can help remove water-borne contaminants, leaving clean drinking water following filtration. Finally, the self-organization of nanoparticles in solution provides stunningly beautiful microscopic images. These images convey the physical importance and visual appeal of the science done in the laboratories to the broader public, providing a source of science education in concert with artistic appeal.

****NON-TECHNICAL ABSTRACT****
The freezing and melting of crystals are fascinating and technologically important phenomena, yet are very difficult to study in the laboratory. Experiments that follow the motions of individual atoms as they form crystals can provide powerful insight, but major technical challenges prevent many of the needed measurements. This project will use microscopic spherical particles suspended in solution (colloidal particles) as ?model atoms,? with which to study crystallization and melting. These particles follow the same physical principles that govern atoms or molecules, but are much larger and slower than atoms and hence directly visible in optical microscopes. The experiments will offer new insight into the role of transient structures, which are not stable but which can nonetheless control the rate at which crystals grow or melt, or even arrest crystal growth. The research may lead to more efficient crystallization of proteins (a major step in determining their structure and function), and controlled crystallization of atoms in nanoparticles (potentially leading to novel materials). The project will provide training for graduate and undergraduate students and includes outreach to high-school students to expose them to the excitement and career opportunities in cutting-edge science. The project will also support a new series of visits and seminars by physicists working in industrial research, with the goals of creating opportunities for collaborative research and of exposing students to a range of career options.

****TECHNICAL ABSTRACT****
The phenomena of freezing and melting are familiar in everyday life and have been studied intensively for many years, but many important questions remain unanswered. Recent experiments have shown that micron-sized particles or droplets can be used as powerful experimental models, providing direct insight into these phenomena with single-particle resolution. This individual-investigator award supports experiments with colloidal spheres and liquid droplets whose interaction potentials can be tuned to induce crystallization, gelation, or melting. The particle motions will be quantitatively tracked in two or three dimensions using optical microscopy. Of particular interest is the role of thermodynamically metastable states, which have a major effect on the free-energy barriers, the rates, and on the final state in cases where the system is trapped out of equilibrium. The results should be helpful in applications such as crystallization of globular proteins or synthesis of inorganic nanocrystals. Graduate and undergraduate students will play key roles in the project and receive training in laboratory research. An outreach program will expose high-school students to the excitement and career potential of cutting-edge science. Reflecting the investigator?s combined interests in industrial applications of soft matter and in training graduate students, the project will also support a series of visits and seminars by physicists currently working in industry.

This Particles on Curved Interfaces: Geometry, Mechanics, and Self Assembly

The adsorption of solid like particles on soft interfaces is ubiquitous in man-made systems as well as in the biological world. The versatility of these phenomena range from Pickering emulsions drops that are covered by a dense colloidal suspension, to proteins that reside on cellular membranes. Understanding the interactions between particles and interfaces and the mutual forces between particles themselves is crucial for developing effective self assembly methods and for controlling the stability of particle laden emulsions. A variety of physical mechanisms are believed to affect these interactions, from surface tension and contact line pinning to electrostatics and van-der-Waals forces. If particles are adsorbed on a curved interface, their mechanics may also be significantly influenced by the surface geometry. Such geometry induced effects have not been addressed in detail, and the purpose of this proposal is to commence their systematic study. Focusing on simple yet nontrivial geometries, we propose experimental and theoretical studies aimed at illuminating universal aspects of the mechanics of particles on curved surfaces.

Intellectual Merit:

(i) We will study the conditions under which the binding energy of solid particles to curved surfaces depends only on the local interfacial curvatures near the adsorbed particle.

(ii) We will study the geometry induced interactions among a few particles on a curved interface. We will develop a set of experiments to test the curvature dependent interactions. In particular, we will explore whether such forces can underlie the puzzling long range attraction that often exists between adsorbed particles.

(iii) We will characterize the behavior of adsorbed particles under nonequilibrium surface flows. We will address flow geometries that allow comparison between geometryinduced forces and the viscous stresses associated with nonequilibrium flow.

(iv) We will address basic problems related to the behavior of a dense suspension of adsorbed particles. In particular, we will explore whether the presence of many adsorbed particles can affect the stability of curved interfaces, and what is the nature of interactions between adsorbed particles in dense suspension.

Broad Impact:

(i) The proposed research will provide training for a graduate student and one or more undergraduate students in a variety of experimental and analytic methods, and the opportunity to combine both perspectives within the same project.

(ii) Both PI's are leading a number of educational projects, intended for graduate and undergraduate students in UMass Amherst and elsewhere. A new component of the proposal is the development of outreach programs for regional high school students and teachers, which will impart awareness, excitement, and active exposure to ongoing research in soft matter physics. An emphasis will be given to participation of students and teachers from low income and underrepresented communities.

(iii) Understanding the basic principles underlying geometry induced forces will yield transformative results, that could be used to exploring new approaches to directed assembly of particles, and to controlled, particle stabilized (Pickering) emulsification.

(iv) Finally, the proposal seeks to explore the most basic geometry induced effects by focusing on simple (axially symmetric) interfaces and solid (spherical) particles. The results of the proposed studies will spur a fruitful line of research that will address the manifestations of geometry induced interactions in more complicated systems, characterized by flexible particles (e.g., proteins), other types of surfaces (e.g., with bending modulus), and complicated surface geometries and particle shapes.

CBET 1438425

This project examines the mechanical properties of liquid interfaces that are coated by small particles. When the number of particles on a liquid interface is large, the particles can form arrangements called mesostructures, which can change the mechanical stiffness, permeability, appearance, or other properties of the interface. The project will use a combination of experiments, theory, and numerical calculations to show how mesostructure formation affects mechanical properties of the interface. Interfaces covered with particles appear in a wide variety of technological and biological applications including microcapsule formulations for food products and drug delivery, separations in mining operations, and improved, low-toxicity dispersants for treating oil contamination in bodies of water. The information obtained in this project will provide new methods for scientists and engineers to predict, analyze, and control the mechanical properties of interfaces, which can lead to innovative and mechanically robust products. This project will also support the training of undergraduate and graduate students for careers in science or engineering, and it will support outreach to K-12 students and teachers.

Particle-coated interfaces, called particle sheets, will be fabricated and their stiffness (moduli) for stretching and bending will be measured. Particle sheets will be subjected to stress or geometric distortion to identify three distinct responses, wrinkling or buckling, tearing or cracking, and the formation of particle-packing defects such as disclinations or dislocations. The response of a particle sheet to uniaxial compression will provide the bending modulus. The response to point-like forces imposed on the sheet by poking will provide the stretching modulus. Changing the curvature of a particle sheet by confining it on a curved interface will elucidate the competition among wrinkles, cracks and particle-packing defects. The experimental and theoretical activities in this project will lead to the development of a structural relaxation phase diagram for thin sheets composed of particles.

Contact between liquid droplets and solids arise in many everyday settings such as rain droplets on a car, condensation of dew on plant leaves, or insects walking on water ponds. To understand or control the behavior of these and related systems, we must first understand the shape of the fluid surfaces and the forces that they can apply. In all of these systems, there is an interface between two fluids (air and water, for example), which meets the surface of a solid object. The angle between the fluid interface and the solid surface (the "contact angle") is the key parameter that determines the interface shape and the forces applied by the fluids. Experiments show that the contact angle has a consistent value when a fluid interface advances across a dry surface, and a smaller value when the interface recedes (moves in the other direction). This variability of the contact angle, known as hysteresis, determines key properties such as whether droplets slide off a surface or remain trapped. There is no general theory for contact-angle hysteresis, but it is typically thought to be an inherent property of the materials being used. This project investigates a new finding that contact angle hysteresis also varies with the shape of the fluid interface. The project is important because understanding the role of geometry in contact-angle hysteresis will give broader insights into the phenomenon that we currently lack. Ultimately, the results of this project will contribute to improved technological processes such as coating of fibers with fluids, stabilizing oil droplets with particles in food or oil-recovery applications, and even assembling electronic components by floating them on a liquid surface.

This project will provide measurements of the of the contact-line shape and the contact angle by direct observation under conditions of advancing and receding fluids. Chemical treatment of the solid surface will be varied and a range of fluids will be used. Experiments focus on millimeter-scale objects so that the interface can be directly visualized. From the known interface shapes, the surface energies will be calculated numerically and, where possible, analytically. These results will allow testing of proposed mechanisms of contact-angle hysteresis. In addition to generating new basic science, the proposed research will support educational projects for local middle-school students and career training for undergraduate and graduate students at the University of Massachusetts Amherst.

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.