Browsing by Subject "Viscoelasticity"
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Item Effect of Viscoelasticity on the Analysis of Single-Molecule Force Spectroscopy on Live Cells(Elsevier Inc, 2012-07-03) Gupta, V.K.; Neeves, K.B.; Eggleton, C.D.Single-molecule force spectroscopy is used to probe the kinetics of receptor-ligand bonds by applying mechanical forces to an intermediate media on which the molecules reside. When this intermediate media is a live cell, the viscoelastic properties can affect the calculation of rate constants. We theoretically investigate the effect of media viscoelasticity on the common assumption that the bond force is equal to the instantaneous applied force. Dynamic force spectroscopy is simulated between two cells of varying micromechanical properties adhered by a single bond with a constant kinetic off-rate. We show that cell and microvilli deformation, and hydrodynamic drag contribute to bond forces that can be 28–90% lower than the applied force for loading rates of 10³–10⁷ pN/s, resulting in longer bond lifetimes. These longer bond lifetimes are not caused by changes in bond kinetics; rather, they are due to the mechanical response of the intermediate media on which the bonds reside. Under the assumption that the instantaneous bond force is equal to the applied force—thereby ignoring viscoelasticity—leads to 14–39% error in the determination of off-rates. We present an approach that incorporates viscoelastic properties in calculating the instantaneous bond force and kinetic dissociation parameter of the intermolecular bond.Item Unraveling the Complex Fluid Dynamics of Simple Liquids at Nanometer Length Scales and Gigahertz Frequencies(2020-01-01) Uthe, Brian; Pelton, Matthew; Physics; PhysicsThe flow of simple liquids, such as water, is often described using Newtonian fluid mechanics which predicts a purely viscous response by the liquid to the motion of an object moving in that liquid. Treating the liquid response as Newtonian and use of the no-slip boundary condition � the assumption that the tangential velocity is zero between the object and the liquid � together, are commonly used to describe the interaction of an object interacting with a liquid. These two assumptions need to be revisited for flows of simple liquids at the nanometer scale. A reduction in the size of an object causes a corresponding decrease in the mechanical response time of that structure. A nanoscale structure has a characteristic response time on the picosecond scale, which is comparable to the molecular relaxation time of simple liquids. Deviations from a purely viscous (Newtonian) response by simple liquids to the motion of nanoscale objects have been observed, with the liquid exhibiting a viscoelastic response commonly associated with complex liquids. Additionally, molecular dynamics studies predict slip lengths for wetting liquids at the single-nanometer scale and numerous experimental studies have reported sub-micrometer slip lengths. In this work, we present two studies investigating the shear and compressible flow of simple liquids at the nanoscale. We use time-resolved spectroscopy to excite and probe the gigahertz frequency dynamics of metal nanoparticle vibrations suspended in varying concentrations of glycerol in water. In the first study we excite the extensional vibrational mode of highly monodisperse gold bipyramids to generate primarily shear flow in the liquid, which is sensitive to liquid slip. We show that the inherent viscoelasticity of the liquid, at this frequency and length scale, causes a drastic enhancement in the effects of slip at the nanoparticle-liquid interface and use of the no-slip boundary condition fails to quantitatively predict the experimental damping. In the second study we excite the breathing mode vibrations of highly spherical, monodisperse gold nanoparticles which produce compressible flow in the liquid. Use of the highly spherical particles eliminates shear motion at the interface between the nanoparticle surface and the liquid ensuring that the effects of velocity slip are minimized for the nanoparticle-liquid interaction. We obtain excellent agreement between the experimentally measured damping rates and theory, thus validating the underlying compressible viscoelastic constitutive model recently developed for the liquid.