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Biomedical subjects

Steve Granick

Publications and source records attributed to Steve Granick.

16 recordsLinked to original sources
PubMed2005

Chemical imaging in a surface forces apparatus: confocal raman spectroscopy of confined poly(dimethylsiloxane).

Confocal Raman spectroscopy has been implemented within the molecularly thin films of a surface forces apparatus. Applying this technique to an initial system, we investigate the confinement and shear-induced changes in the Raman spectra of poly(dimethylsiloxane) (PDMS) liquids confined between atomically smooth mica surfaces at thicknesses less than the unperturbed radius of gyration of the polymer. We focus on stretch vibrations of the PDMS methyl group, whose net orientation is perpendicular to the chain backbone. When PDMS was confined to a thickness of approximately the unperturbed radius of gyration (3.5 nm) but no shear, the Raman intensity of the methyl group was anisotropic in the x-y plane, signifying that chains oriented preferentially parallel to the confining surfaces. Relative to the bulk fluid, the relative intensity of the asymmetric to symmetric carbon-hydrogen stretch (2965 and 2907 cm-1, respectively) was enhanced, indicating that asymmetric vibration was enhanced by confinement. Measurements using polarized radiation showed coherent planar anisotropy in the x-y plane whose direction varied stochastically from experiment to experiment. It seems that although coherent in-plane alignment was favored, no preferential alignment direction was favored in the absence of shear. Application of shear caused the time-averaged polymer conformations to become more nearly isotropic in the plane of shear. These measurements are considered to represent the first chemical imaging of chemical species within the contact area of a surface forces apparatus.

Journal Article↗
PubMed2005

Slaved diffusion in phospholipid bilayers.

The translational diffusion of phospholipids in supported fluid bilayers splits into two populations when polyelectrolytes adsorb at incomplete surface coverage. Spatially resolved measurements using fluorescence correlation spectroscopy show that a slow mode, whose magnitude scales inversely with the degree of polymerization of the adsorbate, coexists with a fast mode characteristic of naked lipid diffusion. Inner and outer leaflets of the bilayer are affected nearly equally. Mobility may vary from spot to spot on the membrane surface, despite the lipid composition being the same. This work offers a mechanism to explain how nanosized domains with reduced mobility arise in lipid membranes.

Biophysics↗
PubMed2005

Patterns formed by droplet evaporation from a restricted geometry.

We show that hundreds of concentric rings, each approximately nanometers high and micrometers wide, form when a drop is allowed to evaporate from a confined geometry, leaving behind a dry, nonvolatile polymer solute. This new preparative strategy offers a simple, versatile, generalizable method to produce patterns of unprecedented high fidelity and regularity, as well as a direction to look to design yet more complex patterns.

Journal Article↗
PubMed2004

How confined lubricants diffuse during shear.

The translational diffusion of a fluorescent dye embedded at a dilute concentration in a confined fluid was compared at rest and during shear. The fluid, octamethylcyclotetrasiloxane (OMCTS), was confined between step-free muscovite mica to thickness 3-4 layers. Fluorescence correlation spectroscopy showed that the time scales of intensity-intensity autocorrelation functions were essentially the same during shear and at rest, except they were faster during shear by a factor of 2 to 5. This dynamical probe of how liquids order in molecularly thin films fails to support the hypothesis that shear produced a melting transition.

Journal Article↗
PubMed2004

Modification of a supported lipid bilayer by polyelectrolyte adsorption.

Addition of a weak polyelectrolyte, poly(methacrylic acid) (PMA), to a supported phospholipid bilayer made from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) depresses the melting temperature and alters the morphology of the bilayer in the gel phase. Ellipsometry measurements show that PMA adsorption lowers the phase transition temperature by 2.4 degrees C. Atomic force microscopy (AFM) showed no visible contrast in the fluid phase (above the melting temperature) but a rich morphology in the gel phase. In the gel phase, adsorption leads to formation of significantly less mobile phospholipid islands and other defects. One consequence of this lower mobility is a decrease in the implied cooperativity number of the phase transition, N, when polymer is added. Additionally, AFM images of the gel-phase bilayer show a highly defected structure that anneals significantly more slowly than in the absence of adsorbed polymer. Tentatively, we suggest that PMA preferentially decorates island and defect edges of the DMPC bilayer.

Adsorption↗
PubMed2004

Superlubricity: a paradox about confined fluids resolved.

Using the method of Frantz and Salmeron to cleave mica [Tribol. Lett. 5, 151 (1998)]] we investigate alkane fluids in a surface forces apparatus and confirm several predictions of molecular dynamics (MD) simulation. An oscillatory force-distance profile is observed for the methyl-branched alkane, squalane. Boundary slip is inferred from the frictional sliding of molecularly thin fluids and also from the hydrodynamic flow of thicker films. These findings resolve the paradox that prior experiments disagreed with these aspects of MD predictions, and demonstrate that exceptionally low energy dissipation is possible when fluids move past solid surfaces that are sufficiently smooth.

Journal Article↗
PubMed2004

Polymer lateral diffusion at the solid-liquid interface.

Measurements are presented of how polymer surface diffusion at the solid-liquid interface is controlled by surface coverage. The method of measurement was fluorescence correlation spectroscopy (FCS), and the system was poly(ethylene oxide) (PEG) adsorbed onto methyl-terminated self-assembled monolayers in buffered aqueous solution. The translational diffusion coefficient at first increased with increasing surface concentration, presumably because the number of adsorption sites per molecule decreased. Ultimately it slowed by 1 order of magnitude, presumably reflecting jamming by neighboring chains.

Journal Article↗
PubMed2003

Noncontact temperature measurement in microliter-sized volumes using fluorescent-labeled DNA oligomers.

A new method of noncontact temperature measurement in microliter-sized volumes is demonstrated, based on the temperature sensitivity of the fluorescence lifetime of rhodamine-G when it is attached to a DNA oligomer. As temperature changes, the spacing between the fluorescent dye and a designed sequence of DNA bases is modulated by conformation changes of the DNA chain, and as a result the ability of dye molecules to fluoresce is also modulated according to differential quenching by bases on the DNA. In the system that we studied, the temperature sensitivity of the fluorescence lifetime was 36-42 ps/ degrees C depending on specific solution conditions. Although this strategy of temperature measurement is demonstrated using a specific sequence of DNA, it can also be generalized to a dye attached to any other intrinsic quencher of fluorescence whose conformation changes with temperature.

DNA, Single-Stranded↗
PubMed2003

Slippery questions about complex fluids flowing past solids.

Viscous flow is familiar and useful, yet the underlying physics is surprisingly subtle and complex. Recent experiments and simulations show that the textbook assumption of 'no slip at the boundary' can fail greatly when walls are sufficiently smooth. The reasons for this seem to involve materials chemistry interactions that can be controlled--especially wettability and the presence of trace impurities, even of dissolved gases. To discover what boundary condition is appropriate for solving continuum equations requires investigation of microscopic particulars. Here, we draw attention to unresolved topics of investigation and to the potential to capitalize on 'slip at the wall' for purposes of materials engineering.

Friction↗
PubMed2002

Materials science of the gel to fluid phase transition in a supported phospholipid bilayer.

We report the results of in situ AFM measurements examining the phase transition of bilayers formed from the zwitterionic phospholipid, DMPC, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, supported on mica. The images show that the fluid to gel phase transition process features substantial tearing of the bilayer due to the density change between the two phases. The gel to fluid transition is strongly affected by the resultant stress introduced into the gel phase, which changes the degree of cooperativity, the shape of developing fluid phase regions, and the course of the transition.

Aluminum Silicates↗
PubMed2002

Contrasting friction and diffusion in molecularly thin confined films.

We study, using fluorescence correlation spectroscopy, translational diffusion in molecularly thin liquids confined within a surface forces apparatus. The diffusion coefficient decreases exponentially from the edges towards the center of the Hertzian contact and further suggests the presence of a small number of distinct diffusion processes. This holds alike a crystallizable fluid (OMCTS) and a glass-former (1,2-propane diol), both of which displayed static friction. We conclude that friction, the average of an ensemble of molecules, masked massively heterogeneous molecular mobility.

Journal Article↗
PubMed2002

Limits of the hydrodynamic no-slip boundary condition.

A controversial point in fluid dynamics is to distinguish the relative importance of surface roughness and fluid-surface intermolecular interactions in determining the boundary condition. Here hydrodynamic forces were compared for flow of Newtonian fluids past surfaces of variable roughness but similar, poorly wetted, surface chemistry. The critical shear stress and shear rate to observe deviations from predictions using the no-slip boundary condition increased nearly exponentially with increasing roughness and diverged at approximately 6 nm rms roughness. We conclude that local intermolecular interactions dominated when the surface was very smooth, but roughness dominated otherwise. This quantifies the limits of both ideas.

Journal Article↗
PubMed2002

Hydrophobicity at a Janus interface.

Water confined between adjoining hydrophobic and hydrophilic surfaces (a Janus interface) is found to form stable films of nanometer thickness whose responses to shear deformations are extraordinarily noisy. The power spectrum of this noise is quantified. In addition, the frequency dependence of the complex shear modulus is a power law with slope one-half, indicating a distribution of relaxation processes rather than any dominant one. The physical picture emerges that whereas surface energetics encourage water to dewet the hydrophobic side of the interface, the hydrophilic side constrains water to be present, resulting in a flickering, fluctuating complex.

Journal Article↗
PubMed2002

Phospholipid membranes as substrates for polymer adsorption.

A largely unsolved problem in soft materials is how surface reconstruction competes with the rate of adsorption. Here, supported phospholipid bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) were employed as substrates for the adsorption of a weak polyelectrolyte, polymethacrylic acid, whose time-dependent ratio of charged to uncharged functional groups served to probe the local dielectric environment. Chains that encountered sparsely covered surfaces spread to maximize the number of segment-surface contacts at rates independent of the molar mass (which was varied by a factor of 30), but dependent on the phase of the lipid bilayer, gel or liquid crystal. Surface reconstruction rather than molar mass of the adsorbing molecules seemed to determine the rate of spreading. The significance of these findings is the stark contrast with well-known views of polymer adsorption onto surfaces having structures that are 'frozen' and unresponsive, and is relevant not just from biological and biophysical standpoints, but also in the formulation of many cosmetics and pharmaceutical products.

Adsorption↗