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Dynamic fluorescence-based single-molecule imaging of λ-DNA molecules driven through agarose hydrogels by DC electric fields reveals that passage through the hydrogel (98.5% water content) induces mobility orthogonal to the external field. Tortuous paths followed by the DNA molecules, which are heavily entangled in the hydrogel mesh as their contour length is nearly 100 times the hydrogel mesh size of 200 nm, cause them to appear to diffuse orthogonal to the driving force. The higher the driving field, from 2 to 16 V/cm, the higher the off-axis dispersion is, over the same time interval. We measure the off-axis displacement distribution over 3 orders of magnitude of probability density and find a master curve after normalizing for time (t) elapsed, but the power of time for normalizing increases with the external field, from t0.25 to t0.6 with increasing field. Comparing trajectories over the same distance traveled in the electric field direction, we observe whereas for the highest field strengths DNA molecules come closest to taking the shortest trajectory between two points in space, deviations from the shortest trajectory grow larger and larger (up to 40% larger) as one approaches the case of small yet finite external field strength. [ABSTRACT FROM AUTHOR]

We compare, using single-particle optical imaging, trajectories of rotation and translation for micron-sized spheres in index-matched colloidal suspensions near their glass transition. Rotational trajectories, while they show intermittent caged behavior associated with supercooled and glassy behavior, explore a sufficiently wider phase space such that in the averaged mean-square angular displacement there appears no plateau regime, but instead sub-Fickian angular diffusion that follows an apparent power law in time. We infer translation and rotation time constants, the former being the time to diffuse a particle diameter and the latter being the time to rotate a full revolution. Correlation between time constants increases with increasing volume fraction, but unlike the case for molecular glasses, the rotation time constant slows more weakly than the translation time. [ABSTRACT FROM AUTHOR]

Noncontact thermometry with micron-scale lateral spatial resolution and fast time resolution is shown to be enabled by measuring the temperature dependence of two-photon absorption (TPA) on crystalline semiconductors. In the proof-of-concept experiments reported here, for studies of Si, an Er:fiber laser at λ=1.56 μm is split into pump and probe beams; where they overlap, the large TPA signal changes strongly with temperature because the two-photon energy lies between the indirect and direct bandgaps of Si. We show that the TPA coefficient increases by a factor of 2 when the temperature increases from 30 to 300 °C. For studies of GaP, we use instead a Ti:sapphire laser at 790 nm to achieve two-photon excitation above the direct bandgap. In GaP, contributions to the TPA from the dominant direct transition show less temperature dependence than for Si but the additional contribution of the indirect transition gives a similar magnitude as the temperature dependence of TPA on Si. In the current implementation using Si, the spatial resolution of the thermometry is 6×6×50 μm3 and the sensitivity is 0.6 K in a 1 kHz bandwidth. [ABSTRACT FROM AUTHOR]

Fluorescence measurements with single-molecule sensitivity are used to measure the hydrodynamic size and local pH of a weak polyelectrolyte, poly-2-vinyl pyridine end labeled with pH-sensitive dye, the polyelectrolyte having concentration so low (nanomolars) that molecular properties are resolvable only from fluorescence experiments and cannot be accessed by light scattering. We find that the local pH near the dye, inferred from its brightness, is consistently three orders of magnitude higher than the bulk pH. Upon varying the bulk pH, we measure the collapse point at which hydrophobic attraction overwhelms electrostatic repulsion between charged elements along the chain, and conclude that adding monovalent salt shifts this coil-to-globule collapse to higher pH than in the absence of salt. The influence of salt appears to shift the ionization equilibrium of this weak polyelectrolyte in the direction of the chain possessing enhanced electric charge at a given pH. Phenomenologically, this is opposite to the case for strong polyelectrolytes, although the mechanism differs. [ABSTRACT FROM AUTHOR]

Single-particle tracking was used to measure the diffusion in aqueous suspension of dilute colloidal clusters, 2–9 μm in size, fabricated by joining 1.57 μm spheres into planar arrangements of various particle numbers and symmetries (doublet, trimer, square, pentamer, hexamer, and so forth). They were allowed to sediment close to a glass surface and their Brownian motion parallel to the surface, effectively in a two dimensional (2D) geometry, was imaged in a microscope in the presence of 3 mM monovalent salt to essentially screen electrostatic interactions. Geometric asymmetry produced systematically increasing discrepancy between the equivalent hydrodynamic radius of translation and rotation—tabulated in this paper. Our observations include cases where the effective hydrodynamic radius changes more rapidly for translation than rotation, the converse, and also cases where the effective hydrodynamic radius for translation changes significantly, while that of rotation is effectively constant. The significance is to document the connection between translational and rotational 2D mobilities for geometrical shapes not described by the Stokes–Einstein–Debye equations for spherical particles. [ABSTRACT FROM AUTHOR]

We introduce the notion of “Janus balance” (J), defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a “Janus particle”) from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The J value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without the need to know the interfacial energy. It is demonstrated that Janus particles of the same chemical composition but different geometries will have the highest adsorption energy when J=1. Even for particles of homogeneous chemical makeup, the Janus balance concept can be applied when considering the contact angle hysteresis in desorbing the particle from equilibrium into the water or oil phase. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analog of the classical hydrophile-lipophile balance of small surfactant molecules. [ABSTRACT FROM AUTHOR]

We demonstrate the concept to combine topographical smoothness and plasmonic properties to produce flat substrates with surface enhanced Raman spectroscopy activity—properties that may find use in nanotribology and other thin film applications. Preliminary findings to this end are described. A dual-beam focused ion beam (FIB) system is used to drill large arrays of small pores in single crystals of mica, 2–6 μm thick, yielding controlled cross sections (squares, triangles, and circles), sizes (100 nm to many microns), and arrangements (square, hexagonal, and random). When filled with metals, arrays result to embedded nanorods with their long axis oriented normal to the surface. As an extension of this method, arrays of nanorods standing perpendicular to a supporting surface can also be produced. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Methods are described to implement dielectric spectroscopy (frequency range 10[SUP-1] -10[SUP6]Hz) within a surface forces apparatus by using as electrodes silver sheets on the backside of mica. These methods are applied to study the competitive effects of surface alignment, confinement, and shear field on 5CB (5-cyanobiphenyl), a nematic liquid crystal at the experimental temperature of 25°C. In the planar orientation, films could be squeezed to a minimum thickness of &ap;5 Å, the molecule's thickness. In the perpendicular (homeotropic) orientation, films could be squeezed to &ap;25 Å, the expected thickness of the head-to-tail 5CB dimer. It was difficult to discuss responses at f > 10[SUP5] Hz quantitatively because the peak was not visible in the experimental frequency window. Nonetheless, the onset of the relaxation mode for the planar oriented molecules appears at higher frequency than for the homeotropic orientation. A slower relaxation mode, peaked at f &ap; 10 Hz, was assigned to electrode polarization due to the mobility of trace ions within the 5CB samples although these samples were >99.7% pure. The peak frequency was a factor of 3 slower with homeotropic than planar alignment and, in both cases, independent of film thickness except when the film thickness exceeded 10 μm. This was explained using a simple model based on the assumption that trace ions move to oppositely charged electrodes to form electric double layers. A small influence of shear on the dielectric response was observed but only when the dielectric response was measured at the same frequency as the large-amplitude shear deformation. Also described is the use of capacitance to measure force-distance profiles. [ABSTRACT FROM AUTHOR]

Contrasts the adsorption of human serum albumin onto two solid substrates previously primed with the same polyelectrolyte of net opposite charge to form one of two alternative structures. Formation of the structures either by the adsorption of quaternized poly-4-vinylpyridine (QPVP) or by end-grafting QPVP chains of the same chemical makeup.

Studies the thin-film dielectric response of organic films confined within a surface forces apparatus and between parallel sheets of atomically smooth mica. Dielectric properties of the organic film from measured capacitance of the total system; Peak frequency of normal mode dielectric loss in thin-film geometries; Dielectric features attributed to spatial constraints.

Examines the transition from static to kinetic friction in a model lubricated system. Focus on the mechanisms of relaxation, dissipation and transition in the linear response regime of model lubricated systems; Usage of a linear relationship to describe fluid flow resistance to deformation rate; Impact of elastic structure formation on transition.

Examines the kinetic regimes of polyelectrolyte exchange between the adsorbed state and free solution. Application of Fourier transform infrared spectroscopy in the feasibility study; Importance of infrared technique in total mass adsorption and surface concentration information; Observation of a kinetic regime at the highest salt concentration.

Examines the adsorption of polyelectrolyte chains of same polymerization degree over a range of ionic strength. Basis of measurements on Fourier transform infrared spectroscopy; Impact of polymer adsorption on silanol dissociation; Effect of infrared dichroism measurement on ionic strength confirmation; Limitations in layer equilibrium adsorption.

Focuses on rheo-optical experiment for parallel plates. Description of a device to perform rheo-optical experiments; Achievement and characterization of parallel alignment at controlled separations; Comparison of capacitance and interferometric measurements of surface-surface spacing.

The friction of dry self-assembled monolayers, chemically attached to a solid surface and comprising a well-defined interface for sliding, is compared to the case of two solids separated by an ultrathin confined liquid. The monolayers were condensed octadecyltriethoxysilane (OTE). The liquid was squalane (C30H62), a film 2.0 nm thick confined between parallel plates of mica. The method of measurement was a surface forces apparatus, modified for oscillatory shear. The principal observations were the same in both cases: (1) Predominantly elastic behavior in the linear response state was followed by a discontinuous transition to a mostly dissipative state at larger deformations. The elastic energy stored at the transition was low, of the order of 0.1 kT per molecule. This transition was exactly repeatable in repetitive cycles of oscillation and reversible with pronounced hysteresis. (2) The dissipative stress in the sliding state was almost independent of peak sliding velocity when this was changed over several decades. Significant (although smaller) elastic stress also persisted, which decreased with increasing deflection amplitude but was almost independent of oscillation frequency. (3) The adhesive energy in the sliding state was significantly reduced from that measured at rest. This similarity of friction in the two systems, dry and wet sliding, leads us to speculate that, similar to plastic deformation of solids, sliding in the confined liquid films is the result of slippage along an interface. [ABSTRACT FROM AUTHOR]

This special topic highlights recent advances in experiments, theory, and coarse-grained modeling on charged macromolecular systems involving synthetic and biological polymers and membranes. The behaviors of the charged systems are very complex and are qualitatively different from those of uncharged polymers. The results presented in this special topic provide deep insights into the fundamentals of how the coupling between the long-ranged electrostatic and topological correlations of charged macromolecules controls their conformations, assembly, dynamics, and transport. There are many insights into the creation and design of new materials based on ionic interactions. [ABSTRACT FROM AUTHOR]

We describe a sensitive method for measuring time-dependent changes in refractive index within ∼5 μm of an interface using off-null time-resolved ellipsometry and a dual-cavity femtosecond laser. The sensitivity to changes in refractive index is two orders of magnitude higher than conventional picosecond interferometry. A thin metal film on a sapphire substrate is heated by ∼10 K using an ultrafast optical pump pulse; the subsequent changes of the phase difference δΔ between p⁁ and s⁁ polarized reflectivity are tracked using off-null ellipsometry using a time-delayed probe pulse. We demonstrate a sensitivity of δΔ≈3×10-7 deg/[Square_Root]Hz using interfaces between Au and water, and Au and various gases including R134a, a common refrigerant. Our data for the damping rate of ≈200 MHz frequency acoustic waves in O2, N2, and Ar at atmospheric pressure agree well with prior results obtained at much lower pressures and frequencies. [ABSTRACT FROM AUTHOR]

We describe an apparatus to measure the diffusion of dilute fluorophores in molecularly thin liquid films within a surface forces apparatus (SFA). The design is a significant modification of the traditional SFA in that it allows one to combine nanorheology with the single-molecule sensitive technique of fluorescence correlation spectroscopy. The primary enabling idea was to place a miniaturized SFA onto the stage of an optical microscope equipped with a long working distance objective and illuminated by a femtosecond laser. A secondary enabling idea was that the silver coating on the backside of mica, normally used in the traditional SFA design for interferometric measurements of the film thickness, was replaced by multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum. To illustrate the utility of this instrument, we contrast the translational diffusion of rhodamine dye molecules (in the solvent, 1,2-propane diol), in the unconfined bulk state and confined between mica sheets to the thickness 2.5 nm. The diffusion coefficient is found to decrease by 2 orders of magnitude under confinement. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The design, calibration, and performance are described of an apparatus to study the shear response of ultrathin liquid films. The device, a modification of the surface forces apparatus, measures the resistance to shear of liquids confined between two atomically smooth solid surfaces. The surfaces are separated by distances on the order of the size of the liquid molecules (liquid film thickness <10 nm). Shear forces with periodic time dependence are applied to one surface while the second is held fixed, and any motion so induced is analyzed to determine the behavior of the liquid film. The frequency and amplitude of the shear forces applied can be varied over a wide range (0.03–60 Hz frequency and 0.1–1000 nm amplitude) to achieve different values for the magnitude of the shear rate. The dynamic response of the device is linear in the applied force at a given frequency; nonetheless, nonlinear dependence of the liquid’s shear resistance on the shear rate, net normal pressure, and film thickness can be observed with the technique. The mechanical and electrical characteristics of the device are modeled to gain insight into its behavior and facilitate analysis of the measured data. The central results of this approach are expressions for the magnitude of the shear rate and an effective friction coefficient of the liquid film in terms of easily measured electrical quantities. For convenience the friction coefficient is restated as an apparent dynamic viscosity in analogy to continuum hydrodynamics, but the validity of the approach does not depend on a particular understanding of the structure in the liquid layer. The applications and limitations of the device are discussed, as well as other potential uses to which the apparatus may be applied by rational extension to the approach presented. [ABSTRACT FROM AUTHOR]

Reports on the measurement of the competitive adsorption of polystyrene chains, carboxylic acid-terminated and unfunctionalized, onto silicon oxide from dilute cyclohexane solution. Discussion on the termination of anionic polymerization; Results of infrared measurements; Details of surface excess of polystyrene.

We report here the first direct measurement of intermolecular forces between a polymer layer and a solid surface. When solid surfaces bearing adsorbed polymer approach one another, interactions develop that may be categorized as polymer–polymer and polymer–solid (and a negligible contribution of solid–solid interactions at the large solid–solid separations imposed by adsorbed polymer). Direct measurements of the forces between polymer layers adsorbed on mica have been made recently with apparatus of the type developed by Israelachvili. Attractive forces are attributed to a combination of osmotic (polymer–polymer) and bridging (polymer–solid) interactions. Bridging occurs if macromolecules adsorbed one one surface can reach a second surface. In the work presented here, poly(α-methylstyrene) (PαMS) adsorbed on one mica sheet was brought into contact in cyclohexane with a bare mica sheet. Adhesion was measured as a function of separation and contact time and found to be more than 20 times greater than between two polymer–coated surfaces under otherwise identical conditions. This direct measurement of molecular forces between dissimilar surfaces enables estimation of the PαMS segmental sticking energy on mica in cyclohexane as (1)/(3) kT. [ABSTRACT FROM AUTHOR]

A simple globular-shaped liquid (octamethylcyclotetrasiloxane, OMCTS) was placed between two rigid mica plates at variable spacings comparable to the size of this molecule and the linear shear viscoelasticity of the confined interfacial film was measured. Strong monotonic increase of the shear relaxation time, elastic modulus, and effective viscosity were observed as the spacing was decreased below about 10 molecular dimensions. The frequency dependence of the viscoelastic spectra measured at different film thicknesses appeared to scale with reduced variables. The data are inconsistent with the abrupt first-order transition, from bulk fluid to solid with decreasing film thickness, whose possibility has been hypothesized, and suggest a glasslike transition instead. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A technique is described for micropositioning a surface forces apparatus with low mechanical and thermal drift. The technique is less exacting and more stable than competing ones. [ABSTRACT FROM AUTHOR]

A simple interferometric method is described to measure the time-dependent deformation of piezoelectric bimorphs driven by ac voltage. Applying this method to a bimorph assembly used to measure viscoelastic forces in a surface forces apparatus, we find that a sinusoidal-shaped harmonic drive voltage generates sinusoidal deformation but that a triangular-shaped harmonic drive voltage fails to produce triangular-shaped deformation if the drive frequency exceeds 1/100 of the resonance frequency. Sinusoidal displacement amplitudes as small as 0.1 nm were resolved. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We introduce the notion of "Janus balance" (J), defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a "Janus particle") from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The J value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without the need to know the interfacial energy. It is demonstrated that Janus particles of the same chemical composition but different geometries will have the highest adsorption energy when J = 1. Even for particles of homogeneous chemical makeup, the Janus balance concept can be applied when considering the contact angle hysteresis in desorbing the particle from equilibrium into the water or oil phase. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analog of the classical hydrophile-lipophile balance of small surfactant molecules.

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.