Eric Sembrat's Test Bonanza

Textbooks on colloidal phenomena teach us to describe the electrostatic and dispersion interactions at interfaces using single parameters (Hamaker constants, surface potentials or charge densities), an approach which treats surface as uniform. Real surfaces, be they mineral, polymeric, or biological, present heterogeneous surface chemistry that complicates their interactions.  It is recognized that clustered rather than uniform presentation of attractive chemistries, for instance peptide sequences, enhances biological interactions.  Likewise, heterogeneity in charge and acid-base interactive groups has been invoked to explain colloidal instabilities and bacterial adhesion (that DLVO theory fails to predict).  A lack of understanding the energy landscape of real surfaces prevents their description by convolved versions of DLVO and related approaches.  Our lab, has, however, developed a series of model surfaces with well-characterized electrostatic heterogeneity and, through systematic variations in the interface, demonstrated how heterogeneity alters the surface forces. 

This talk addresses the fundamental aspects of heterogeneous surface interactions and then translates the interactions to static and dynamic particle adhesion.  In particular, the influence of the energy and distribution of the heterogeneity will be addressed. The talk will then demonstrate how interactions at heterogeneous interfaces are particularly sensitive to physical interfacial features such as curvature and mechanical softness, and how this sensitivity can be exploited for separation and microfluidic sensing schemes.  Finally, the talk will address the role of heterogeneity in dynamic adhesion, from capture –hold-release protocols to continuous behaviors such as rolling.  These will be demonstrated with particles and spherical bacteria.  Throughout the talk, parallels between synthetic and biological interfaces will be drawn, probing the extent to which charge and other chemical clusters can be treated conceptually as biological receptors.

Lattices that are on the verge of mechanical instability provide useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packing’s and granular matter, networks of semi-flexible polymers, and protein structure. This talk will explore elastic and mechanical properties and mode structures of model periodic lattices of periodic versions of this lattice, including the origin and nature of zero modes under both periodic (PBC) and free boundary conditions (FBC). It will derive general conditions (a) under which the zero modes under the two boundary conditions are essentially identical and (b) under which phonon modes are gapped with no zero modes in the periodic spectrum but include zero-frequency surface Rayleigh waves in the free spectrum. The gapped states have a topological characterization, similar to that of topological insulators that define the nature of zero-modes at the boundary between systems with different topology.

Even though cosmic rays were discovered more than 100 years ago, their origin remains a mystery. Neutrinos, product of cosmic ray interactions at or near the production site, are the best astrophysical messenger to find the cosmic ray sources. The past few months have seen fast progress in the search for very-high-energy (>100 TeV) astrophysical neutrinos.  IceCube has reported a set of events that are inconsistent with terrestrial origin and have characteristics best explained by an astrophysical origin. In this presentation I will discuss the current status of IceCube's observations including Georgia Tech's role in this work. I will also discuss future prospects for the field of neutrino astrophysics.

With the availability of spectrally pure lasers and the ability to precisely measure optical frequencies, it appears the era of optical atomic clocks has begun.  At the expense of signal-to-noise ratio, in one project at NIST we have used single trapped atomic ions because uncertainties in systematic effects are smallest, reaching Df/f₀ = 0.8 x 10^-17.  At this level, many effects, including those due to special and general relativity, must be calibrated and corrected for.

Characterization of the mechanical properties of cells, as well as the tissues and extracellular matrices (ECM) in which they reside, requires microscale manipulation platforms that allow precise measurement of their local rheology. To achieve this, my laboratory has developed a suite of NdFeB-based magnetic tweezers devices optimized for biomaterials characterization. In this talk, I will present the design and construction of three new microscope-mounted magnetic tweezers devices that allow controlled forces to be applied locally to networks, cells, and tissues while their deformation is determined with nanometer accuracy: (1) high-force devices that enable the application of nN forces; (2) ring magnet devices that enable oscillatory microrheology without prestress; and (3) portable magnetic tweezers that enable visualization of the microscale deformation of soft materials under applied force through simultaneous fluorescence imaging. The utility of these devices will be demonstrated by measuring the mechanics of dense networks of microtubules, which are rigid cytoskeletal polymers. We find that crosslinker dynamics profoundly affect network elasticity and dynamics, and that it is possible to predict macroscale stiffness, strength, and stress propagation from the force-sensitive unbinding kinetics and compliance of single crosslinkers.

DNA coils undergo  striking conformational transitions when it is confined to volumes with dimensions smaller than one of the characteristic lengths of the molecule.  We are particularly interested in confinement to channels less than two persistence length wide, and hundreds of microns long.  In these channels, DNA extends to 50 % of its contour length and more, thus establishing a clear connection between location and the linear "genetic address" expressed in base pairs. We can fabricate nanochannel systems with arbitrary configurations in two dimensions using fused silica, and thus are able to directly observe DNA configurations through fluorescence microscopy.

This talk will explore the physics of confined DNA, the application to epigenetic mapping, the interactions of electric fields with DNA, and the dynamic analysis of functional DNA-modifying enzymes.  In particular, we have studies the fluctuation spectrum of nanoconfined DNA, have mapped cytosine methylation levels, histone modification profiles, have discovered that a collapse of DNA in high electric fields that hints at the complete breakdown of linear theory, have studied the migration of DNA through nanochannel systems, have observed single-molecule restriction mapping, have detected DNA at nanoelectrode junctions, and have observed a previously unknown pre-catalytic compaction of DNA by a widely used DNA-binding protein.

The centimeter-long DNAs in our cells are folded up into micron-scale chromosomes through an array of protein-DNA interactions.  Our group uses single-DNA micromanipulation – stretching and twisting of the double helix – as a tool to analyze a variety of enzymes acting on DNA.  I will describe a few different kinds of “magnetic tweezers” experiments we are doing that are aimed at understanding enzymes that help to package DNA and to change its topology. We also use analogous but larger-scale micropipette-based micromanipulation approaches to study the large-scale structure of metaphase chromosomes; I will discuss experiments that tell us the metaphase chromosomes behave as “chromatin gels”, apparently stabilized in part by DNA entanglement. Recent experiments on effects of depletion of condensin SMC complexes - thought to be major "crosslinkers" of chromosomes - will also be discussed.

Flows of particulate material, such as sand discharging in an hourglass, are ubiquitous in nature and industry. The flow and transport of granules, powders, or grains is complex and can differ considerably from that associated with a single-phase material. This presentation will highlight some unique features of granular materials (such as the discharge from an orifice) and describe some recent work at Caltech on wave propagation, booming sand dunes, and granular flow rheology.   

Quantum Simulation with cold atoms is a very ambitious program in AMO research that is being pursued in many laboratories worldwide. The goal is to use cold atoms in optical lattices and in different environment to simulate important yet unsolved theoretical models. In this talk, I shall review the current progress of this effort, its success, and the serious challenges it faces. I shall discuss the possible solutions and the exciting future it holds.

Entanglement has recently emerged as an important conceptual tool in quantum many-body physics.  I will explain why we care about entanglement in quantum matter and why we are interested in the physics of highly ntangled quantum states.  I will also show how entanglement has led us to new phases of matter, new ways to characterize phases and phase transitions, novel numerical techniques, and useful conceptual advances. I will conclude with a discussion of the prospects for measuring entanglement in many-body systems.