The ability of membranes to retain fluids without leaking in devices such as high throughput multiwell plates was examined as a function of membrane polymer, surface modification, and liquid surface tension. Microporous membranes (200–800nm) act as arrays of millions of imperfect microcapillaries. Extrusion and leaking requires pressure plus coalescence of microdroplets. For unmodified membranes, the liquid hold up height (pressure) is critically dependent on liquid surface tension, rising rapidly when the contact angle prevents droplet spreading and coalescence. Topography, as measured by AFM, also plays a role in ease of coalescence. Surface modification has a large impact on hold up pressure and its dependence on liquid surface tension.

Modulated optical nanoprobes (MOONs) are microscopic (spherical and aspherical) particles designed to emit different fluxes of light in a manner that depends on particle orientation. When particle orientation is controlled remotely using magnetic fields (MagMOONs) it allows modulation of fluorescence intensity in any selected pattern including square and sinusoidal waves. The broad range of sizes over which MOONs can be prepared allows them to be tailored to applications from intracellular sensors using submicron MOONs to immunoassays using larger MOONs (1–10μm). In the absence of external fields, or material that responds to external fields, the particles tumble erratically due to Brownian thermal forces. These erratic changes in orientation cause the MOONs to blink. The temporal pattern of blinking contains information about the local rheological environment and any forces and torques acting on the MOONs.

Biological cells, in some sense, are all about topology: Biomembranes separating different volumes from one another, ions or even macromolecules having to cross these membranes in controlled fashion through membrane pores; or certain proteins moving along the DNA to find their target sequence instead of searching for this site in the full 3-dimensional cell volume. Even on the single biopolymer level, topology is an essential ingredient: Intriguingly, in bacteria DNA occurs knotted, i.e., in a state topologically different from a simply connected ring. It is a key question to understand the statistical behaviour of such knotted DNA to understand a number of physiological processes having to overcome this knottedness, or to quantify results from DNA separation techniques such as electrophoresis, in which the knottedness influences the mobility. At the same time, double-stranded DNA continuously opens up floppy single-stranded bubbles, which fluctuate in size, exposing the single Watson-Crick bases to binding proteins. Again, statistical mechanical tools can be employed to examine the bubble dynamics. Here, we introduce some recent results on DNA knots and bubble fluctuations.

The immersion precipitation process makes most commercial microporous membranes, which enjoy widespread use in filtration and purification. This process begins with liquid-liquid demixing of a nonsolvent/solvent/polymer ternary system into polymer-rich and polymer-lean phases. This demixing step determines much of the final morphology. In this work, a ternary Cahn-Hilliard formulation incorporating a Flory-Huggins homogeneous free energy function is used to simulate phase separation during demixing. Then the formulation is coupled with constant-viscosity interface-driven fluid flow. Simulations begin with uniform initial conditions with small random fluctuations, and then with two-layer polymer-solvent/nonsolvent initial conditions to simulate actual membrane fabrication conditions. The results are presented in 2-D and 3-D, which demonstrate the effects of mp (degree of polymerization), Kij (gradient penalty coefficients) and χij (Flory-Huggins interaction parameters) on phase separation behavior.

The slow dynamics associated with the structural relaxation of glass forming materials near the glass transition is very sensitive to the effects of small confining geometries. Based upon the experimental results of triplet state solvation dynamics, we explore the extent to which confinement effects can be rationalized solely in terms of interfacial dynamics which are modified relative to the bulk situation. The importance of the interfacial conditions is emphasized by observing the changes due to the surface chemistry, by comparing relaxation times at and further away from the surface, and by studying the effects of ‘soft’ versus ‘hard’ confining materials. While ‘hard’ confinement by porous solids is observed to result in slower dynamics and an increased glass transition temperature Tg for propylene glycol, our 4.6 nm nanodroplets suspended in a more fluid environment display faster structural relaxation, equivalent to a reduction of Tg as observed in free standing polymer films.

We present a comprehensive study of the hooking mechanism of a single DNA molecule in electrophoretic motion colliding with a single microfabricated obstacle. During a collision, DNA impacts an obstacle and deforms. The impact conditions dictate whether this collision results in a “roll-off” event or “hooking” event. Our objective is to better understand the physics of a collision. Specifically, we note that a finite-sized insulating obstacle induces local electric field gradients that can enhance the size-dependent hooking probability. We validate that the hooking mechanism is analogous to a polymer in a transient, non-homogeneous elongational field with a strength characterized by the Deborah number, De. We then show that hook formation increases with De for finite-sized obstacles in the regime De<40.

We investigate the feasibility of tapping-mode atomic force microscopy (TM-AFM) as a force measurement tool for compliant surfaces. For quantitative extraction of the tip-sample interactions, numerical modeling of the cantilever dynamics is required using a defined form for the interaction, with the results compared to experiment. Through TM force measurements on silicon, we illustrate that a forced damped harmonic oscillator model sufficiently represents the motion of the cantilever. Particularly for liquid operation, distance-dependent dissipation must be included in the model for accurate quantification of the tip-sample interactions and for successful reproduction of experimental force curves. This dissipation is not due to damping from the bulk viscous medium, but is likely frictional in origin. This investigation shows that TM force measurement in liquid is feasible and could be particularly advantageous for the measurement of intermolecular interactions from soft and easily deformed molecular layers.

We present the frequency dependence of the proton spin-lattice relaxation rate 1/T1 in variously hydrated proteins. We present also the case of proteins confined in heavily hydrated gels where the rotation has been immobilized. The relaxation efficiency increases according to a power law at low frequencies. The temperature dependence of the protein protons T1 demonstrates that relaxation results from a direct spin-phonon process instead of a Raman process at temperatures above 273K. We propose a theory that accounts for experiments and depends on the dynamical distribution of states, the localization of the disturbances along and transverse to the peptide chains, and the spatial distribution of hydrogen in the structure. In hydrated and confined proteins, the motions of the backbone that dominate the relaxation process are transverse rather than along the peptide chain. We show that the protein structure adjusts to hydration from the lyophilized state to the fully hydrated state in small increment steps.

Fundamental statistical-mechanical expressions for transport coefficients describing transport processes in spatially inhomogeneous systems (such as nanofluids, interfacial systems, atomic clusters, etc.) and derived by the use of the functional perturbation theory (FTP) due to Pozhar and Gubbins (PG) are simplified for the use in engineering and technology. Together with explicit expressions for the charge transport properties of quantum inhomogeneous systems (such as semiconductor quantum dots, wells and wires, artificial atoms/molecules etc.) derived recently, these expressions form a basis for development of algorithms and codes to realize a virtual (i.e., fundamental theory-based, computational) synthesis of nanomaterials with predesigned transport properties for novel nanocluster- or nanopore- based catalysts and adsorbents, integrated nanocircuits, nanoheterostructures, etc.

Particle fluctuations in colloidal suspensions are generally considered to be much slower than the embedded fluid dynamics. It was recently proposed that the fluid self-diffusion nearby an interface follows Lévy statistics, extending the time domain of the fluid dynamics towards the low frequency range. It is then possible to probe colloidal particle motion and especially its evolution during a phase transition looking at the slow dynamics of the fluid molecule close to an particle surface. Using field cycling NMR relaxometry, we show how the slow and confined water dynamics at proximity of a colloidal surface provide an original way to follow the glass transition of a colloidal system made of plate-like Laponite particles, a synthetic clay. The interplay between fluid Levy dynamics and particle jamming is discussed. An analytical model involving correlated elementary water time steps on the colloidal interface is proposed and critically compared to our experimental data. The method permits probing of colloidal motions and especially their evolution during a phase transition in a new way. This method can be extended to other systems including mineral lyotropic liquid crystals and biological molecular structures.

Vibrational dynamics of the OH stretching mode of water in the water pool of reverse micelles (H2O/Aerosol OT (AOT)/isooctane) are studied by nonlinear infrared spectroscopy such as transient grating method and three-pulse photon echo peak shift measurements. The W0 value (W0 =[H2O]/[AOT]) is changed from 2 to 40, which corresponds to a water pool diameter of a few nm to about 20 nm. Polarization dependent transient grating experiments show rapid anisotropy decay of the OH stretching mode, which could be due to resonant intra and intermolecular energy transfer. From the three-pulse photon echo peak shift experiments, the spectral diffusion is found to be dependent on W0.

We present a robust feedback control algorithm and apply it to the nonlinear oscillator array (Frenkel-Kontorova) model of nanoscale friction. The new control approach is based on the concepts of non-Lipschitzian dynamics and global targeting. We show that average quantities of the controlled system can be driven - exactly or approximately - towards desired targets which become additional, linearly stable attractors for the system's dynamics. Extensive numerical simulations show that the basins of attraction of these targets are reached in very short times and the control exhibits very strong robustness. We investigate the efficiency of the control in terms of various parameters (e.g., system size, non-Lipschitzian exponent).

The diffusion-limited kinetics of the growth of depletion zone around a static point trap in a thin, long stripe geometry was studied using a laser photobleaching experiment of fluorescein dye inside a rectangular capillary. The dynamics of the depletion zone was monitored by the θ-distance, defined as the distance from the trap to the point where the reactant concentration has been depleted to the specific fraction of its initial bulk value. A dimensional crossover from two dimensions to one dimension, due to the finite width of the reaction zone, was observed. The crossover seems to occur for all θ values concurrently when the depletion zone touches the boundary for the first time, suggesting that the boundary information spreads faster than diffusion. Monte Carlo simulations were performed to support the experimental results. The crossover time (τc) is found to scale with the width (L) of the rectangular reaction zone as τc ∼ L2, as expected from the Einstein's diffusion law.

A mechanism responsible for directed transport and molecular separation in a symmetric channel is proposed. We found that under the action of spatial harmonic oscillations of the channel, the system exhibits a directed transport in either direction, presenting multiple current reversals as the amplitude and/or frequency of the oscillations are varied. The particles of different masses may be forced to move with different velocities in the same or in the opposite directions by properly adjusting driving parameters. The directed transport can be produced in both directions even in the absence of thermal noise, the latter can speed up or slow down the transport depending on the system parameters.

A phase–field method is used to model two–phase flow in microfluidic devices, where capillary and viscous stresses dominate over inertial forces. Dissipative and reactive couplings in the hydrodynamic equations are derived from a Cahn–Hilliard–van der Waals free energy, which accounts for the equilibrium thermodynamics of the fluid system, including phase behavior, interfacial tension and wetting properties. The singularities inherent to the free boundary description are smoothed out by the presence of a diffuse interface over which interfacial stresses are distributed, such that complex phenomena like droplet breakup and coalescence or contact line dynamics can be resolved numerically. The reliability of the scheme used to solve the discretized transport equations is tested against different benchmarks for free flow conditions. The model is then applied to the simulation of the flow of droplets in microdevices, resulting in a satisfactory agreement with the behavior observed in experiments.

We have performed x-ray photon correlation spectroscopy (XPCS) measurements on a polymer-bilayer system comprised of 100 nm polystyrene film on top of an 80 nm polybromostyrene film, supported on a Si substrate. In order to distinguish the dynamics at the top interface from that at the polymer-polymer interface we have performed the measurement at grazing incidence. In this geometry, a standing wave is set up in the film. We derive a relation for the intensity of the standing wave and the resulting diffuse scattering. This model is compared with the measured diffuse scattering from which we extract a value of 0.7±0.4 dyne/cm for the surface tension between PS and PBrS at 180C. XPCS was then measured in each of two standing wave conditions, first where diffuse scattering only occurs at the polymer-vacuum interface and then where it only occurs at the interior polymer-polymer interface. The measured time correlation functions for each of the two interfaces show clear differences, with the polymer-polymer interface exhibiting much slower dynamics.

We review recent results on intrinsic frictional properties of adsorbed monolayers, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of a dynamical master equation approach we determine the velocity of a biased impure molecule - the tracer particle (TP), constrained to move inside the adsorbed mono-layer probing its frictional properties, define the frictional forces exerted by the monolayer on the TP, as well as the particles density distribution in the monolayer.

When two solids are in relative sliding motion, the intervening layer separating the two surfaces (for example the boundary lubricant) is typically far from thermal equilibrium. With the help of a generic model reflecting the boundary lubricant, it will be shown that it is often not possible to characterize a sliding contact by means of a single effective temperature. The reason is that the probability distribution (PD) of microscopic variables differs in a characteristic fashion from equilibrium PDs. Non-equilibrium velocity PDs are not Gaussian but tend to be exponential, thus favoring rare events. Leaving dynamic equilibrium by non-uniform sliding conditions leads to yet additional effects, in particular to enhanced dissipation. This is shown in a model describing rubbing polymer brushes in good solvent conditions. Shortly after returning the sliding velocity, the brush interdigitation is distinctly larger than during steady-state sliding. Based on this observation, predictions can be made at what amplitude the loss is maximum for a given driving frequency.

In our study of the aging phenomena in silica suspensions we immersed precipitated silica particles in methyl-terminated PDMS (silicone oil), mineral oil, and glycerol/water and measured their viscoelastic properties as a function of time. We found that the samples based on silicone oil, initially of a pasty consistence, became fluid-like after a period of about three weeks. Aqueous samples aged much faster than the oily samples. Samples containing mineral oil did not exhibit any changes in viscosity and elasticity. In this paper we discuss a possible mechanism for these aging behaviors.

The dissipation mechanisms of nanoscale friction between a scanning force microscopy (SFM) tip and amorphous polystyrene are found to reside solely within the polymer's intrinsic molecular mobility, and are discussed with respect to the glass transition temperature. In both the glassy and the rubbery states, lateral force microscopy friction results revealed the dissipative behavior as activated relaxation processes with potential barrier heights of 7 kcal/mol and 83 kcal/mol, respectively. These values correspond to hindered phenyl (side chain) rotation and to the α-relaxation, respectively. The velocity relationship with friction, F(v), was found to satisfy simple fluctuation surface potential models with F ∝const -ln(v) and F ∝const -ln(v)2/3. Within ∼27 K above the glass transition temperature, friction displayed a shear thinning type behavior, also found in materials that exhibit multiple phases.