Physical chemistry: stressed molecules break down.
Explore the source record for details and available documents.
Comment↗
Biomedical subjects
Steve Granick
Publications and source records attributed to Steve Granick.
Molecular-dynamics simulations of the transport properties of a single polymer chain in two dimensions.
Molecular-dynamics simulations are conducted to elucidate the critical factors affecting the transport properties of isolated polymer chains in strictly two dimensions. The relevance of surface inhomogeneity is critically examined. We unequivocally find that surface inhomogeneity is critical in obtaining transport behavior consistent with the recent measurements of surface diffusion for polymers adsorbed at the solid-liquid interface. For a systematic investigation of this point, heterogeneity was introduced by decorating the surface with impenetrable elements and we find that chain diffusivity crossed over from Rouse-type behavior to reptationlike with increasing surface coverage of obstacles. This transition in behavior occurred when the mean distance between obstacles is approximately equal to the end-to-end distance, Re, of the two-dimensional chain. Our results underscore the importance of surface disorder (not only literal obstacles but by reasonable extension also to other types of disorder) in determining the transport behavior of chains adsorbed to solids.
Journal Article↗
Electrostatic stitching in gel-phase supported phospholipid bilayers.
We show that mixing zwitterionic lipids with up to 20% mole % cationic lipids produces gel-phase supported lipid bilayers that are morphologically free of defects detectable using noncontact mode atomic force microscopy (AFM). This contrasts with the observation of massive defects when anionic lipid was added, and also when no charged lipid was added. Infrared measurements of headgroup orientation in the presence of cationic lipid show that the mean headgroup orientation changes only minimally when temperature is lowered from the fluid phase to the gel phase. This is consistent with a tentative explanation, based on simple electrostatic arguments, in which cationic lipids "stitch" the bilayers together. On the functional side, this study demonstrates a simple method by which to minimize defects in gel-supported phospholipid bilayers.
Gels↗
How to stabilize phospholipid liposomes (using nanoparticles).
The simple strategy of mixing phospholipid liposomes with charged nanoparticles and using sonication to mix them at low volume fraction produces particle-stabilized liposomes that repel one another and do not fuse. Subsequently, the volume fraction can be raised as high as approximately 50%, reversibly, still without fusion. In studies of liposome longevity, we verified the stability of particle-stabilized liposome suspensions with volume fraction up to 16% for up to 50 days, the longest period investigated. Fluorescent dyes were encapsulated within the particle-stabilized liposomes, without leakage. Although these particle-stabilized liposomes were stable against fusion, approximately 75% of the outer liposome surface remained unoccupied. This opens the door to using particle-stabilized liposomes in various applications.
Lipid Bilayers↗
Lipid diffusion compared in outer and inner leaflets of planar supported bilayers.
The translational diffusion coefficient (D) of lipids located in the outer and inner leaflets of planar supported DLPC (1,2-dilauroyl-sn-glycero-3-phosphocholine) bilayers in the fluid phase was measured using fluorescence correlation spectroscopy of dye-labeled lipids at the low concentration of 0.001% and using iodide quenching of dyes in the outer leaflet to distinguish diffusion in the inner leaflet from that in the outer leaflet. To confirm the generality of these findings, the bilayers were prepared not only by vesicle fusion but also by Langmuir-Blodgett deposition. We conclude that regardless of whether the bilayers were supported on quartz or on a polymer cushion, D in the inner and outer leaflets was the same within an experimental uncertainty of +/-10% but with a small systematic tendency to be slower (by <5%) within the inner leaflet.
Biological Transport↗
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↗
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↗
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↗
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↗
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↗
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↗
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↗
Polymers: a multitude of macromolecules.
Explore the source record for details and available documents.
Chemistry, Organic↗
Materials science: molecules squeezed and stroked.
Explore the source record for details and available documents.
News↗
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↗
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↗
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↗
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↗