Dinsmore Research Group
UMass-Amherst Physics

Statistical Mechanics

Measurement of Forces in a 3D Pile of Frictionless Droplets

We made systematic and detailed measurements of inter-particle contact forces inside three dimensional piles of frictionless liquid droplets. We measure long-range, chain-like correlations of the directions and magnitudes of large forces, thereby establishing the presence of force chains in three dimensions. Our correlation definition provides a chain persistence length of 10 mean droplet diameters, decreasing as load is applied to the pile. We also measure the angles between contacts and show that the chain-like arrangement arises from the balance of forces. Moreover, we find that piles whose height is comparable to the chain persistence length exhibit substantially greater strain hardening than tall piles, which we attribute to the force chains. Together, the results establish a connection between the microscopic force network and the elastic response of meso- or macroscopic granular piles. (reference). With these results, we developed a simple model that allows us to predict chain-like correlations and the dependence on size polydispersity, coordination number, and load.
(w/ students Jing Zhou & Hao Wang. Supported by the NSF.)

Confocal microscopy of colloidal gels and model sandpiles

We seek to understand how microstructure determines bulk elasticity. To do this, we watch colloidal particles aggregate using a confocal microscope. From our images, we identify chains of particles and characterize their topology and characteristic chemical dimension. By monitoring the thermal undulations of the particles, we characterize the (overdamped) vibrational modes of the gel and measure the stiffness of individual particle chains. With our model system, we can control the attraction between the particles and determine the effect on structure and viscoelasticity. Click here for movies and more details and publications: 3D structure and topology and microstructure and elasticity.
(w/ Ian Wong, Vikram Prasad and Dave Weitz, Harvard University.)

Melting & Freezing of Colloidal Crystals
We study the kinetics of sublimating crystals with single-particle resolution by experiments with colloidal spheres and by computer simulations. A short-ranged attraction between spheres led to crystallites one to three layers thick. The spheres were tracked with optical microscopy while the attraction was reduced and the crystals sublimated. Large crystallites sublimated by escape of particles from the perimeter. The rate of crystallite shrinkage was greatly enhanced, however, when the size fell below a cross-over value that ranged between 20 and 50 in different regions of the phase diagram. Simultaneous with the enhanced sublimation rate, the crystallites transformed to a dense amorphous structure, which then rapidly vaporized. The dramatic enhancement of kinetics by thermodynamically meta- or unstable phases may play a major role in melting, freezing, and annealing of crystals with short- or long-range interactions. The results should be relevant in diverse systems including colloids, proteins, and atoms such as Argon. (ref.)

More recent work focuses on formation of crystallites following a temperature quench. These experiments show the classical nucleation of the solid phase occurs under some conditions (esp. at low concentration) but at higher concentrations liquid-like clusters appear first, then crystallize.

(w/ John Savage, Liquan Pei, Don Blair, Alex Levine, Bob Guyer and Jon Machta. Supported by the NSF and the Research Corporation.)

Colloids and Electronics
We are fabricating metallic colloidal particles to measure electronic properties of particle-particle junctions formed in solution.

We have demonstrated a straightforward assembly approach in which metallic colloidal spheres serve as the electrodes. The devices are formed by assembly in suspension followed by deposition onto a patterned substrate. The key to this approach is that the inter-electrode (inter-sphere) spacing is spontaneously set to allow tunneling contact with a single layer of nanoparticles. The measured current exhibits the Coulomb blockade owing to the small size and large electrostatic charging energy of the nanoparticles. We show that the device resistance can be tuned by means of a gate electrode. Our results demonstrate an altogether new approach to inexpensive and large-scale fabrication of electronic devices such as transistors with nanometer-scale features. [ref; see also a News and Views summary of this work.]

In separate experiments, we study how internal degrees of freedom affect the charge transport (for example, 'charge shuttling')[ref.].

(w/ Kan Du, Chris Knutson, Kevin McCarthy, and Mark Tuominen, UMass Physics, and Vince Rotello of Chemistry, and Liz Glogowski, Todd Emrick and Tom Russell of Polymer Sci. Eng. Supported by the NSR-NIRT program and by the Center for UMass/Industry Research on Polymers (CUMIRP))

Light Propagation in Strongly-Scattering Particle Arrays
Disordered arrays of microscopic particles are the basis of paints and many other coatings. They also pose a number of challenging problems of condensed matter physics, because of the fact that light propagating through these materials scatters many times before emerging. Films composed of these small particles often scatter light strongly enough that photons effectively diffuse through the sample with a typical random-walk step length, l*, which depends on the structure and refractive index.

Currently, we seek to understand how the arrangement of the particles affects the wave propagation in the strong-scattering limit. As part of this problem, we seek the optimal arrangement of particles to maximize the scattering. We have found that the magnitude of l* can be reduced by a factor of more than two (hence increasing the scattering) by reducing the coordination number of the particles from ~10 (in a close-packed film) to ~ 4. Others have shown that when l* falls below a critical value (~wavelength/2*pi), then the photons become localized in the material. This provides an example of Anderson localization and might offer technological applications.

Ordered arrays of colloidal particles can have remarkable electro-optical properties if we can control them well enough. We are developing a new design of "photonic crystals" made from colloidal particles: a close-packed lattice of hollow spheres, whose shell has a large refractive index. We make these spheres and assemble them into macroscopic colloidal crystals. These colloidal crystals are beautifully iridescent (here are some pictures). Such materials are expected to exhibit very unusual properties, provided that the refractive index, n is large enough. (So far, we have n=2.0 and are working toward n=3.5.) For example, the emission and propagation of light could be controlled in new and wonderful ways. The most dramatic example would be a complete "photonic band gap," in which light of some frequencies cannot propagate or be emitted in any direction. A list of publications about photonic crystals and links to a few research groups are available on the web.

(w/ Xiaotao Peng, UMass Physics. Supported by the Research Corporation through the Cottrell Scholars Program and by the NSF-sponsored MRSEC on Polymers)

Particles and Droplets

Semi-permeable Capsules by Self-Assembly

By controlling the assembly of particles on droplets, we make hollow shells having well-defined pores with sizes anywhere from the nanometer to micron size scale (ref.). The capsules are made using a simple technique that is compatible with a variety of (non-toxic) materials. (More info here.) In more recent work, we showed that smart capsules can be made using colloidal microgel particles that expand and contract with temperature (ref.). Currently, we are studying the shapes of these shells in three dimensions as they are crumpled by osmotic pressure.

We also investigate assemblies of quantum-dot nanoparticles, which can be crosslinked on the surface to make an ultra-thin membrane (ref.). The ability to synthesize nanoparticles with a broad range of properties leads to many exciting opportunities for making functional membranes and capsules.

An important part of this work is to measure and understand the behavior of microscopic particles adsorbed at the interface between oil and water. We use optical microscopy to track the motions of particles, thereby measuring the interaction potential and probing the two-dimensional rheology of the particle layer.

(w/ Chuan Zeng, Matt Gratale, Ryan McGorty, Yutaka Maki, Yao Lin, Habib Skaff, Liz Glogowski, Tom Russell, and Todd Emrick)

Past projects:

Crystallization of Colloidal Particles (1994-1997)

The spontaneous ordering of microscopic spheres in water is described by the principles of thermodynamics. That is, the particles form "solid" (ordered lattices) and "fluid" (disordered) phases just as atoms do. For studying fundamental issues of thermodynamics, colloids are uniquely valuable because the interactions can be tailored, the particles are visible, and they move slowly enough to follow. We have have been studying suspensions containing spheres of two different sizes (i.e. "binary"), like microscopic billiard balls and marbles.

Frequently, one associates crystallization with an (enthalpic) attraction among atoms. As the temperature is reduced, this attraction overwhelms entropy and the atoms freeze. In a mixture of hard spheres, there are no attractions, yet the larger ones form crystals. An explanation is given by the depletion force theory; one can think of the smaller particles as creating an effective attraction among the larger ones. Publications describe measurements of phase diagrams and of the depletion force between two large spheres. (w/ Dave Pine and Arjun Yodh, UPenn)

Manipulating particles using entropy: entropic force fields at rigid and flexible surfaces (1995-1998)

We have demonstrated a novel and widely-applicable technique for controlling the motions and positions of particles in suspension with much smaller particles. Etching sub-micron structures into the inert walls of a container creates "entropic force fields," which make the particles self-assemble in a pattern which we can control. With this approach, macromolecules can be precisely positioned on a substrate in predetermined patterns or can be made to move in a deterministic way -- by maximizing the entropy. Our motivations include making 2-D and 3-D colloidal crystals with useful photonic properties (such as photonic band gaps; see above). In addition, the work will contribute to a deeper understanding of the effects of complex surfaces on dynamics and phase behavior in colloidal mixtures, in porous media and in biological materials. (w/ Arjun Yodh, UPenn.)

What happens when you put colloidal spheres inside vesicles?

[flexible vesicle]
See "
Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature," by A.D. Dinsmore, D.T. Wong, Philip Nelson, A.G. Yodh; Phys. Rev. Lett. 80, 409 (1998).

Doped Semiconductor Nanoparticles: Synthesis, Structure, Light Emission (1997-1999)

with Y. Tian (Sarnoff, Inc.); J. Yang (PixTech); J. O. Cross (Argonne National Lab); D. Hsu, S. B. Qadri, T. A. Kennedy, and B. R. Ratna (NRL).

We synthesized and characterized doped semiconductor nanoparticles (esp. ZnS:Mn) for potential use as light-emitting materials (phosphors) in displays. We are especially interested in "field emission displays" (FEDs). Like a CRT (the big heavy thing on your desk), an FED works by electron-beam excitation and will potentially be very bright and efficient. Unlike CRTs, however, FEDs will be lightweight and compact.

[surfactant template] We synthesized the nanoparticles by precipitation in water. The simple trick for controlling particle size is to mix surfactant and water to make the bicontinuous cubic phase (shown at right). This structure gives us an array of 6-nm "reaction chambers." We characterized the nanoparticles using electrons (cathodoluminescence and electron microscopy), x-rays (diffraction, EXAFS) and microwaves (EPR).

Here is what we found [ref]:

  • They are bright: Emission from tiny particles (as small as 100 nm) can be comparable to commercially available bulk materials.
  • They are made at low T: Using nanoparticles, we can make bright phosphors after firing at temperatures of just 525 degrees C, hundreds of degrees cooler than is needed for bulk materials. We found that the nanoparticles undergo solid-solid phase transitions at low temperatures.
  • Small size is useful: The small particle size could allow very uniform phosphor films which, in turn, would allow new screen designs.

Dinsmore group page.