Session J2: Force Probes of Materials' Structure and Function

Nanoscale Mechanical Resonators for Probing Physical Phenomena: Fluid Dynamics of High-frequency Flows

With their miniscule sizes, high frequencies, and small force constants, nanoelectromechanical systems (NEMS) resonators are expected to emerge as tools for sensing a variety of analytes, for probing biological entities, and for measuring molecular-scale forces. Because many of these foreseeable applications are in fluids, it is natural to consider the operation of NEMS resonators in fluids. When immersed in a fluid, however, the NEMS resonator loses most of its vibrational energy to the fluid. In other words, the quality factor (Q) of the resonator decreases significantly. Reductions in Q result in a reduction in the resonator's sensitivity to added mass or force. In order to understand the fluid dynamics of NEMS, we have revisited a well-known fluid dynamics problem: Stokes' second problem of the oscillating plate in a fluid. At the typical frequencies of NEMS resonators, Stokes' second problem needs to be reformulated using a relaxation time approach in order to accurately describe the fluidic effects. Our experiments and theory show that the fluid relaxation time in conjunction with the resonator frequency determines the nature of the flow; linear dimension and geometry appear to have weak effects. Our results support a universality in oscillating flows and suggest a deep connection between simple and complex fluids. With this understanding, we are making progress toward reducing NEMS dissipation in water.   

Consistency and discrepancy between single molecule force spectroscopy experiments and theoretical models

Single molecule force spectroscopy is a well-established tool to study molecular interactions in a wide range of binding affinities on the single-molecule level. Information about the strength of the molecular bond can be quantified in terms of the dissociation rate k$_{off}$, and the reaction length x$_{b}$ (i.e., the distance between potential minimum and maximum along the direction of pulling). The analysis and interpretation of the underlying force-distance curves is still challenging and various models describing the experimental data are under discussion. In this talk, I will present experimental data for a protein-RNA interaction related to posttranscriptional regulation on the single molecule level, and the interaction between DNA bases forming two or three hydrogen bonds. I will use these examples to discuss the advantages and limitations of this technique, and the consistency and discrepancy to theoretical models.   

Magnetic force microscopy of superconductors: vortex manipulation and measuring the penetration depth

We use a low temperature magnetic force microscope (MFM) to image superconductors. The interaction between the magnetic tip and individual vortices allows us to both image vortices and to manipulate them. The manipulation results depend on sample thickness and on the superconducting properties. Here I concentrate on YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) samples and on Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$, an underdoped pnictide. In thin films, if the force exerted by the tip is strong enough to overcome the pinning potential a vortex jumps as a whole to a new pinning site. The behavior in thick YBCO single crystals depends on the doping level. In a slightly overdoped sample vortices stretch rather than jump when we perturb them strongly [1]. The dragging distance in this crystal is anisotropic: it is easier to drag vortices along the Cu-O chains than across them, consistent with the tilt modulus and the pinning potential being weaker along the chains. We also find that when we ``wiggle'' the top of a vortex we can drag it significantly farther than when we do not, giving rise to a striking dynamic anisotropy between the fast and the slow directions of the scan pattern. In an underdoped YBCO single crystal, where superconductivity is so anisotropic that a vortex should be viewed as a stack of two dimensional pancakes, we show that vortices kink rather than tilt when we perturb them [2]. Since the discovery of the pnictides, a new family of high temperature superconductors, we have also been developing ways to determine the absolute value of the magnetic penetration depth, which is notoriously difficult to measure, as well as its dependence on temperature. For that we either use the Meissner repulsion of the magnetic MFM tip from the sample or the magnetic interaction between the tip and the magnetic field from a vortex. The temperature dependence that we find allows us to comment on the symmetry of the order parameter [3]. \\[4pt] Work done in collaboration with Lan Luan and Kathryn A. Moler (Stanford)\\[4pt] [1] O. M. Auslaender et al., Nat. Phys. 5, 35 (2009).\\[0pt] [2] Lan Luan et al., Phys. Rev. B 79, 214530 (2009).\\[0pt] [3] Lan Luan et al., Phys. Rev. B 81, 100501 (2010).   

Nanoscale Magnetic Resonance Imaging

Magnetic resonance imaging (MRI), based on the sensitive detection of nuclear spins, enables three dimensional imaging without radiation damage. Conventional MRI techniques achieve spatial resolution that is at best a few micrometers due to sensitivity limitations of conventional inductive detection. The advent of ultrasensitive nanoscale magnetic sensing opens the possibility of extending MRI to the nanometer scale. If this can be pushed far enough, one can envision taking 3D images of individual biomolecules and, perhaps, even solving molecular structures of proteins. In this talk we will discuss issues related to nanoscale magnetic resonance imaging, especially its implementation using magnetic resonance force microscopy (MRFM). We will also consider the future possibility of using NV centers in diamond for detection of nanoMRI.   

Histone Post-Translation Modifications Influence Chromatin Mechanical Stability

Histone proteins organize the human genome into chromatin fibers while their post-translation modification (PTM) regulates genome replication, expression and repair. The mechanistic connections between histone PTMs and biological functions remain enigmatic. We find with a combination of magnetic tweezers mechanical measurements and biochemical studies that a number of histone PTMs influence the DNA mismatch repair process by mechanically destabilizing chromatin. The location of the PTM within the chromatin structure appears to determine the mechanism by which it alters the mechanical stability. These findings have direct implications for understanding the repair of the human genome.