Highlights

Close-packed nanocrystal monolayers can be self-assembled by simple drop casting into ultra-thin free-standing membranes. Researchers at the University of Chicago MRSEC have shown that these membranes are remarkably strong, with a Young’s modulus on the order of several GPa, yet highly flexible. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370K. The purely elastic response of these membranes, coupled with exceptional robustness and resilience at elevated temperatures makes them excellent candidates for a wide range of sensor applications.

The Ismagilov and Mrksich groups at the University of Chicago MRSEC have recently established that a microfluidic system utilized in conjunction with surface immobilization chemistries can be used to pattern surfaces with well-defined gradients of adhesion molecules for the attachment of cells. The image shows the patterned surface after placement in a suspension of B16F10 mouse melanoma cells and fixing and immunostaining with antibody against vinculin (found in the focal adhesion structures integrating the cytoskeleton with the extracellular matrix). Each gradient microisland contained a nonuniform distribution of active ligands for cell adhesion.

The general technique of preparing gradients of immobilized species with specific patterns is expected to be an important tool for understanding the influence of nonuniform microenvironments on cell function, including polarization and migration.

Myelin figures are long thin cylindrical structures that grow when water is added to the concentrated lamellar phase of certain surfactants such as soap. The Sidney Nagel and Tom Witten groups at the University of Chicago developed a method to produce isolated myelin figures, based on previous investigations of ring stain formation pioneered at the MRSEC. This allowed them to study their growth and stability in detail.

A University of Chicago MRSEC team led by Philippe Guyot-Sionnest and Woowon Kang have been investigating the transport properties of colloidal quantum dots under magnetic field [1].

They uncovered two effects of the magnetic field on the conductance of the quantum dot solids. One effect is wavefunction squeezing under large magnetic field (10T) and low temperature which reduces overlap and thus conductivity. The other effect is a spin-blockade in which electrons of spin quantized with the external field cannot pass by a dot already occupied by an electron of the same spin orientation, as shown in the schematic above. At low magnetic field (0.02T) weak hyperfine coupling can randomize the spins, favoring transport.

This type of nanomaterial may provide components of future technologies based on spin control with potential applications in information storage and computing.

A collaboration of experimentalists and theorists at the Chicago MRSEC has discovered a new, general route for creating nanoparticle monolayers that retain order across millions of particles, without holes, while staying compact over macroscopic distances[1].

This new method exploits the observation that fast initial drying drives the nanoparticles into an interface between the suspending solvent and the surrounding air, where they form a two dimensional, single-particle-thin skin. This skin drapes itself over the substrate during the final drying stage, accommodating roughness, curvature, and even holes without losing integrity.

When a marble or ball-bearing is dropped onto a bed of fine, loose sand, one first observes a broad splash of sand at impact. Then, a tall jet of granular material shoots up vertically. Experiments at the Chicago MRSEC in collaboration with researchers from the APS at Argonne have tracked the birth and evolution of these granular jets using the fastest x-ray based imaging performed to date (6000 video frames per second)[1]. The images show granular jets emerging from a bed of fine glass spheres after impact by a heavy steel sphere dropped from above. Results for four different ambient air pressures are given. Note the two-stage jet shape visible at intermediate pressures.

These jets are among the most spectacular manifestations of liquid-like behavior in granular materials. Although resembling similar phenomena in ordinary liquids, these jets form in the absence of any cohesive forces or surface tension yet their overall shape depends on the ambient pressure. The measurements at Argonne provide a new understanding of how granular jets are formed and establish how ambient gas pressure affects them.

In March, a group of physicists from the Chicago MRSEC visited Washington DC to talk about science to Congressional Representatives, their staff, and others. The message: basic research is vital to America’s economy and our childrens’ futures. In the photo, current MRSEC Director, Sidney Nagel (left) demonstrates the properties of materials to CUNY Prof. Myriam Sarachik (former President of the APS) and U.S. Rep. Vernon J. Ehlers(R-MI) as Leo Kadanoff (right), former MRSEC Director and currently President-Elect of the APS, looks on. This effort was sponsored by the Condensed-Matter Divison of the APS. Our MRSEC scientists are aware that research is only half of a scientist’s job–the other half is telling people about it.

Working collaboratively, research groups at the Chicago MRSEC have developed new label-free analytical systems that utilize ultra-small sample sizes of cellular lysate, yet allow these single samples to be assayed for multiple kinase activities. The systems involve the integration of solid-phase biochip peptide arrays, mass spectrometric detection, and microfluidic networks. [1] We expect this strategy to aid drug discovery, diagnostics, and other basic research which rely substantially on enzyme assays.