Complexity, rate, and scale in sliding friction dynamics between a finger and textured surface.

Sliding friction between the skin and a touched surface is highly complex, but lies at the heart of our ability to discriminate surface texture through touch. Prior research has elucidated neural mechanisms of tactile texture perception, but our understanding of the nonlinear dynamics of frictional sliding between the finger and textured surfaces, with which the neural signals that encode texture originate, is incomplete. To address this, we compared measurements from human fingertips sliding against textured counter surfaces with predictions of numerical simulations of a model finger that resembled a real finger, with similar geometry, tissue heterogeneity, hyperelasticity, and interfacial adhesion. Modeled and measured forces exhibited similar complex, nonlinear sliding friction dynamics, force fluctuations, and prominent regularities related to the surface geometry. We comparatively analysed measured and simulated forces patterns in matched conditions using linear and nonlinear methods, including recurrence analysis. The model had greatest predictive power for faster sliding and for surface textures with length scales greater than about one millimeter. This could be attributed to the the tendency of sliding at slower speeds, or on finer surfaces, to complexly engage fine features of skin or surface, such as fingerprints or surface asperities. The results elucidate the dynamical forces felt during tactile exploration and highlight the challenges involved in the biological perception of surface texture via touch

How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation

Contact geometry and mechanics predict friction forces during tactile surface exploration

International audienceWhen we touch an object, complex frictional forces are produced, aiding us in perceiving surface features that help to identify the object at hand, and also facilitating grasping and manipulation. However, even during controlled tactile exploration, sliding friction forces fluctuate greatly, and it is unclear how they relate to the surface topography or mechanics of contact with the finger. We investigated the sliding contact between the finger and different relief surfaces, using high-speed video and force measurements. Informed by these experiments, we developed a friction force model that accounts for surface shape and contact mechanical effects, and is able to predict sliding friction forces for different surfaces and exploration speeds. We also observed that local regions of disconnection between the finger and surface develop near high relief features, due to the stiffness of the finger tissues. Every tested surface had regions that were never contacted by the finger; we refer to these as " tactile blind spots ". The results elucidate friction force production during tactile exploration, may aid efforts to connect sensory and motor function of the hand to properties of touched objects, and provide crucial knowledge to inform the rendering of realistic experiences of touch contact in virtual reality

Plasmonic-based Label-free Detection and Imaging of Molecules

abstract: Obtaining local electrochemical (EC) information is extremely important for understanding basic surface reactions, and for many applications. Scanning electrochemical microscopy (SECM) can obtain local EC information by scanning a microelectrode across the surface. Although powerful, SECM is slow, the scanning microelectrode may perturb reaction and the measured signal decreases with the size of microelectrode. This thesis demonstrates a new imaging technique based on a principle that is completely different from the conventional EC detection technologies. The technique, referred to as plasmonic-based electrochemical imaging (PECI), images local EC current (both faradaic and non-faradaic) without using a scanning microelectrode. Because PECI response is an optical signal originated from surface plasmon resonance (SPR), PECI is fast and non-invasive and its signal is proportional to incident light intensity, thus does not decrease with the area of interest. A complete theory is developed in this thesis work to describe the relationship between EC current and PECI signal. EC current imaging at various fixed potentials and local cyclic voltammetry methods are developed and demonstrated with real samples. Fast imaging rate (up to 100,000 frames per second) with 0.2×3µm spatial resolution and 0.3 pA detection limit have been achieved. Several PECI applications have been developed to demonstrate the unique strengths of the new imaging technology. For example, trace particles in fingerprint is detected by PECI, a capability that cannot be achieved with the conventional EC technologies. Another example is PECI imaging of EC reaction and interfacial impedance of graphene of different thicknesses. In addition, local square wave voltammetry capability is demonstrated and applied to study local catalytic current of platinum nanoparticle microarray. This thesis also describes a related but different research project that develops a new method to measure surface charge densities of SPR sensor chips, and micro- and nano-particles. A third project of this thesis is to develop a method to expand the conventional SPR detection and imaging technology by including a waveguide mode. This innovation creates a sensitive detection of bulk index of refraction, which overcomes the limitation that the conventional SPR can probe only changes near the sensor surface within ~200 nm.Dissertation/ThesisVideo for Figure 3.2 C to HVideo for Figure 3.5Video for Figure 5.5Video for Figure 6.7Video for Figure 6.11Ph.D. Electrical Engineering 201

Engineering solid-state nanopores for detection of single transcription factors bound to DNA

Thesis (Ph.D.)--Boston UniversityDetection and characterization of nucleic acid-protein interactions, particularly those involving DNA and transcription factors, remain significant barriers to our understanding of genetic regulation. Solid-state nanopores are extremely sensitive single molecule sensors with the capability to map local chemical and structural characteristics along the length of a biopolymer, providing label-free detection for a wide range of analyte lengths and sizes. Previous studies have utilized solid-state nanopores to detect complexes of DNA bound to many large proteins, but improvements to the sensing resolution of the nanopore platform are necessary for detection of single small transcription factors bound to DNA. This project encompasses two novel nanopore modifications that enhance output signal quality and time resolution in nanopores, and establishes solid-state nanopores as a platform for direct measurement of transcription factor-DNA complexes. First, a novel fabrication process was developed to create locally thinned SiN membranes on a full-wafer scale. These modified nanopore chips provide several advantages over conventional solid-state nanopores, including improved signal-to-background ratio, higher probability of functionality, and clearly marked pore locations for re-imaging and array fabrication. Next, the volume outside the nanopore was modified by electrospinning a sparse, hydrophobic co-polymer nanofiber mesh (NFM) directly onto the nanopore chip. The NFM interacts with analyte molecules as they translocate through the pore, increasing residence time in the sensing volume and improving resolution by more than two orders ofmagnitude for a chemically optimized blend ofpoly(E-caprolactone) and poly(glycerol-co-E-caprolactone). Finally, modified nanopores were used for direct, label-free detection of single transcription factors bound to DNA. Translocations of these complexes reveal a combination oftwo possible sensing modalities; either the complex passes unhindered through the pore, causing a transient drop in current at the location ofthe bound protein, or the protein is unable to translocate and is removed as the DNA is electrophoretically driven through the nanopore. The DNA-binding domain of the transcription factor Early Growth Response Protein 1 (EGRl), known as zif268, is presented as a model system for this research. EGRl activates genes that control cell differentiation and mitogenesis, and participates in many regulatory processes including wound response, tumor suppression, and neuronal plasticity

Optogenetic dissection of striatopallidal pathway in control of motor activity

The striatopallidal (indirect) pathway is considered as the main modulatory locus for the basal ganglia control of motor functions. According to the classic basal ganglia model, the striatopallidal pathway inhibits motor activity mainly via its projection to globus pallidus (GPe). However, striatopallidal medium spiny neurons (MSNs) form extensive feedback and lateral inhibitory networks via their collaterals. Thus, the striatopallidal pathway may control motor activity either through its projections onto GPe or through the striatal collaterals. To further define the circuit mechanism whereby the striatopallidal pathway controls motor activity, we have developed two new optogenetic transgenic mouse lines expressing channelrhodospin-2 (ChR2) or archaerhodopsin-3 (Arch) selectively in the striatopallidal neurons under the Adora2a gene promoter. Consistent with previous optogenetic studies, we found that ChR2 activation and Arch silencing of the striatopallidal neurons in dorsolateral striatum (DLS) suppressed and increased motor activity, respectively. However, contrary to the prediction from the classical model, we found that selective activation of the striatopallidal axon projections in GPe increased locomotor activity. Thus, light stimulation of MSN cell bodies and collaterals in DLS, versus stimulation in GPe axon projections, produced opposite motor responses. This led us to reassess the function of the striatopallidal collaterals and to test the hypothesis that the profuse projections and collaterization within the striatum may contribute to striatopallidal pathway control of motor activity. We found that ChR2-mediated activation of the striatopallidal neurons in DLS induced c-Fos expression in ChR2/GFP-positive MSNs. Conversely, Arch-mediated silencing of the striatopallidal neurons induced c-Fos expression and MAPK phosphorylation in Arch/GFP-negative MSNs surrounding the Arch/GFP-positive MSNs. This c-Fos/pMAPK expression pattern in MSNs is consistent with the suppression of GABA release in GFP-positive cells, resulting in the induction of c-Fos in GFP-negative cells having collateral connections with the GFP-positive cells. Together, our findings revealed a previously unrecognized complexity and novel motor control mechanism of the striatopallidal pathway: activation of striatopallidal projections to GPe increases motor activity while activation of striatopallidal neurons and collaterals in the DLS may contribute to motor suppression. These findings call for a revisit of GPe as a potential locus for deep brain stimulation in Parkinson’s disease

EVAPORATION-INDUCED FORMATION OF WELL-ORDERED SURFACE PATTERNS ON POLYMER FILMS

Various techniques of fabricating surface patterns of small scales have been widely studied due to the potential applications of surface patterns in a variety of areas. It is a challenge to fabricate well-ordered surface area efficiently and economically. Evaporation-induced surface patterning is a promising approach to fabricate well-ordered surface patterns over a large area at low cost. In this study, the evaporation-induced surface patterns with controllable geometrical characteristics have been constructed. The dewetting kinetics on deformable substrate is also investigated. Using simple templates to control the geometry and the evaporation behavior of a droplet of volatile solvent, various gradient surface patterns, such as concentric rings, multiple straight stripes formed with a straight copper wire, etc. have been constructed on PMMA films. The wavelength and amplitude are found to decreases with the decrease of the distance to the objects used in templates. There is also a nearly linear relation between the amplitude and wavelength. The effects of several experimental parameters on the geometrical characteristics of the surface structures are studied, i.e. dimensions of the template, film thickness (solution concentration), substrate temperature, etc. The wavelength and amplitude increase with the increase of the film thickness (solution concentration), with the increase of the dimension of the template. However with the increase of the substrate temperature, the wavelength increases, while the amplitude decrease. Hexagonal network in pre-cast PMMA film have been fabricated by a “breath figure” approach at low humidity and low substrate temperature. The dimensions of the hexagonal holes are dependent on the template size and film thickness. The kinetics of the evaporative dewetting of a liquid (toluene) film on a deformable substrate (PMMA film) with the confinement of a circular copper ring is also studied. The liquid film first dewets from the outside towards the copper ring. When a critical volume is reached, an internal contact line appears, which dewets from the center to the copper ring smoothly with a constant velocity, then switches to a “stick-slip” motion. The average velocity of the smooth motion increases with the increase of the copper ring size and film thickness