Wall-drag effect on diffusion of colloidal particles near surfaces: A photon correlation study

Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films

HYPOTHESIS

Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the 2-dimensional Smoluchowski equation and systematic coarse-graining.

METHODS AND SIMULATIONS

For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects.

FINDINGS

After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid-liquid and liquid-vapour interfaces.

The Stokes-Einstein-Sutherland Equation at the Nanoscale Revisited

The Stokes-Einstein-Sutherland (SES) equation is at the foundation of statistical physics, relating a particle's diffusion coefficient and size with the fluid viscosity, temperature, and the boundary condition for the particle-solvent interface. It is assumed that it relies on the separation of scales between the particle and the solvent, hence it is expected to break down for diffusive transport on the molecular scale. This assumption is however challenged by a number of experimental studies showing a remarkably small, if any, violation, while simulations systematically report the opposite. To understand these discrepancies, analytical ultracentrifugation experiments are combined with molecular simulations, both performed at unprecedented accuracies, to study the transport of buckminsterfullerene C60 in toluene at infinite dilution. This system is demonstrated to clearly violate the conditions of slow momentum relaxation. Yet, through a linear response to a constant force, the SES equation can be recovered in the long time limit with no more than 4% uncertainty both in experiments and in simulations. This nonetheless requires partial slip on the particle interface, extracted consistently from all the data. These results, thus, resolve a long-standing discussion on the validity and limits of the SES equation at the molecular scale.

Anisotropic molecular diffusion in confinement I: Transport of small particles in potential and density gradients

HYPOTHESIS

Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue. In the vicinity of interfaces, density fluctuations as a consequence of layering locally impose statistical drift, which impedes the analysis of spatially dependent diffusion coefficients even further. We hypothesise, that we can derive a model to spatially resolve interface-perpendicular diffusion coefficients based on local lifetime statistics with an extension to explicitly account for the effect of local drift using the Smoluchowski equation, that allows us to resolve anisotropic and spatially dependent diffusivity landscapes at interfaces.

METHODS AND SIMULATIONS

An analytic relation between local crossing times in system slices and diffusivity as well as an explicit term for calculating drift-induced systematic errors is presented. The method is validated on Molecular Dynamics simulations of bulk water and applied to simulations of water in slit pores.

FINDINGS

After validation on bulk liquids, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Significant spatial variations in the diffusivities correlate with interface-induced structuring but cannot be solely attributed to the drift induced by local density fluctuations.

Numerical simulations of confined Brownian-yet-non-Gaussian motion

Brownian motion is a central scientific paradigm. Recently, due to increasing efforts and interests towards miniaturization and small-scale physics or biology, the effects of confinement on such a motion have become a key topic of investigation. Essentially, when confined near a wall, a particle moves much slower than in the bulk due to friction at the boundaries. The mobility is therefore locally hindered and space-dependent, which in turn leads to the apparition of so-called multiplicative noises, and associated non-Gaussianities which remain difficult to resolve at all times. Here, we exploit simple, optimized and efficient numerical simulations to address Brownian motion in confinement in a broadrange and quantitative way. To do so, we integrate the overdamped Langevin equation governing the thermal dynamics of a negatively-buoyant single spherical colloid within a viscous fluid confined by two rigid walls, including surface charges. From the produced large set of long random trajectories, we perform a complete statistical analysis and extract all the key quantities, such as the probability distributions in displacements and their main moments. In particular, we propose a novel method to compute high-order cumulants by reducing convergence problems, and employ it to efficiently characterize the inherent non-Gaussianity of the confined process.

Diffusive dynamics of charged nanoparticles in convex lens-induced confinement

Transport through heterogeneous confined geometries is encountered in many processes and applications such as filtration, drug delivery, and enhanced oil recovery. We have used differential dynamic microscopy (DDM) and particle tracking to investigate dynamics of 36 nm negatively-charged polystyrene particles in convex lens-induced confinement (CLiC). The confinement gap height was controlled from 0.085 μm to 3.6 mm by sandwiching the aqueous particle solution between a glass coverslip and a convex lens using a homemade sample holder. With an inverted fluorescence microscope, sequences of micrographs were taken at various radial positions and gap heights for five particle concentrations (i.e. φ = 0.5 × 10^-5, 1 × 10^-5, 5 × 10^-5, 10 × 10^-5, 50 × 10^-5) and ionic strengths ranging from 10^-3 to 150 mM. The resulting image structure functions were fitted with a simple exponential model to extract the ensemble-averaged diffusive dynamics. It was found that particle diffusion was more hindered as a function of increased confinement. In addition, the ensemble-averaged diffusion coefficient was found to depend on the bulk concentration, and the concentration dependence increased as a function of confinement. Increasing particle and salt concentration led to confinement-dependent adsorption onto the geometry surface. Overall, we show that CLiC devices are simple and effective and can be used to study dynamics in continuous confinement from sub 100 nm to 100's of μm. These findings could lead to better understanding of separations and interactions in confining devices.

Particle/wall electroviscous effects at the micron scale: comparison between experiments, analytical and numerical models

We report a experimental study of the motion of 1 μm single particles interacting with functionalized walls at low and moderate ionic strengths conditions. The 3D particle's trajectories were obtained by analyzing the diffracted particle images (point spread function). The studied particle/wall systems include negatively charged particles interacting with bare glass, glass covered with polyelectrolytes and glass covered with a lipid monolayer. In the low salt regime (pure water) we observed a retardation effect of the short-time diffusion coefficients when the particle interacts with a negatively charged wall; this effect is more severe in the perpendicular than in the lateral component. The decrease of the diffusion as a function of the particle-wall distancehwas similar regardless the origin of the negative charge at the wall. When surface charge was screened or salt was added to the medium (10 mM), the diffusivity curves recover the classical hydrodynamic behavior. Electroviscous theory based on the thin electrical double layer (EDL) approximation reproduces the experimental data except for smallh. On the other hand, 2D numerical solutions of the electrokinetic equations showed good qualitative agreement with experiments. The numerical model also showed that the hydrodynamic and Maxwellian part of the electroviscous total drag tend to zero ash→ 0 and how this is linked with the merging of both EDL's at close proximity.

The effect of morphology and particle-wall interaction on colloidal near-wall dynamics

We investigated the near-wall Brownian dynamics of different types of colloidal particles with a typical size in the 100 nm range using evanescent wave dynamic light scattering (EWDLS). In detail we studied dilute suspensions of silica spheres and shells with a smooth surface and silica particles with controlled surface roughness. While the near wall dynamics of the particle with a smooth surface differ only slightly from the theoretical prediction for hard sphere colloids, the rough particles diffuse significantly slower. We analysed the experimental data by comparison with model calculations and suggest that the deviating dynamics of the rough particles are not due to increased hydrodynamic interaction with the wall. Rather, the particle roughness significantly changes their DLVO interaction with the wall, which in turn affects their diffusion.

On-chip transporting arresting and characterizing individual nano-objects in biological ionic liquids

Understanding and controlling the individual behavior of nanoscopic matter in liquids, the environment in which many such entities are functioning, is both inherently challenging and important to many natural and man-made applications. Here, we transport individual nano-objects, from an assembly in a biological ionic solution, through a nanochannel network and confine them in electrokinetic nanovalves, created by the collaborative effect of an applied ac electric field and a rationally engineered nanotopography, locally amplifying this field. The motion of so-confined fluorescent nano-objects is tracked, and its kinetics provides important information, enabling the determination of their particle diffusion coefficient, hydrodynamic radius, and electrical conductivity, which are elucidated for artificial polystyrene nanospheres and subsequently for sub-100-nm conjugated polymer nanoparticles and adenoviruses. The on-chip, individual nano-object resolution method presented here is a powerful approach to aid research and development in broad application areas such as medicine, chemistry, and biology.

Brownian motion of a charged colloid in restricted confinement

We study the Brownian motion of a charged colloid, confined between two charged walls, for small separation between the colloid and the walls. The system is embedded in an ionic solution. The combined effect of electrostatic repulsion and reduced diffusion due to hydrodynamic forces results in a specific motion in the direction perpendicular to the confining walls. The apparent diffusion coefficient at short times as well as the diffusion characteristic time are shown to follow a sigmoid curve as a function of a dimensionless parameter. This parameter depends on the electrostatic properties and can be controlled by tuning the solution ionic strength. At low ionic strength, the colloid moves faster and is localized, while at high ionic strength it moves slower and explores a wider region between the walls, resulting in a larger diffusion characteristic time.

Height distribution and orientation of colloidal dumbbells near a wall

Geometric confinement strongly influences the behavior of microparticles in liquid environments. However, to date, nonspherical particle behaviors close to confining boundaries, even as simple as planar walls, remain largely unexplored. Here, we measure the height distribution and orientation of colloidal dumbbells above walls by means of digital in-line holographic microscopy. We find that while larger dumbbells are oriented almost parallel to the wall, smaller dumbbells of the same material are surprisingly oriented at preferred angles. We determine the total height-dependent force acting on the dumbbells by considering gravitational effects and electrostatic particle-wall interactions. Our modeling reveals that at specific heights both net forces and torques on the dumbbells are simultaneously below the thermal force and energy, respectively, which makes the observed orientations possible. Our results highlight the rich near-wall dynamics of nonspherical particles and can further contribute to the development of quantitative frameworks for arbitrarily shaped microparticle dynamics in confinement.

Hindered Diffusion near Fluid-Solid Interfaces: Comparison of Molecular Dynamics to Continuum Hydrodynamics

Studying the near-wall hindered diffusion of a particle suspended in a fluid is critical for understanding other more complex, confined systems. We provide a review of the previous experimental and simulation efforts trying to verify the classic calculations in hydrodynamics by Brenner and Faxén. We discuss some of the challenges of extracting the hindered diffusion constants from the mean squared displacements as often done in the literature. We demonstrate that the use of total force autocorrelation functions is a reliable alternative for calculating the diffusion constants without similar challenges for our molecular dynamics (MD) simulations. We find that the change in the diffusion constant in the perpendicular direction calculated in MD is roughly consistent with the hydrodynamic result by Brenner provided that they are normalized by the diffusion constant at the center between the two walls. However, the discrepancy grows large when the colloidal particle is very close to the wall where molecular details matter. Even though the agreement can be considerably improved when the attractions between the particles are made stronger to reduce slip to better fulfill the no-slip condition in MD, we report that there is an underlying difference between the range of the wall interactions with the colloidal particle predicted by MD and hydrodynamics.

Hard-wall entropic effect accelerates detachment of adsorbed polymer chains

Many previous studies of unbinding kinetics have focused on a two-state model, with fully bonded and free states, which may not extend to more complicated biopolymer dynamics involving other reactions. Here we address the kinetic rate of this process at the segment level, as it is influenced by a growing dangling end of the chain. We use the mean first-passage time approach and treat the polymer as a chain attached to a wall through a succession of spring potentials, with two distinct regions of bonded and free segments. The interaction between the wall and free-moving chain end adds an entropic repulsion to this process. We estimate the average monomer detachment rate K as a function of the free dangling length L. For a flexible polymer, we find an acceleration factor in the average detachment rate depending on L and the details of the spring bond; when L is long, this factor is a simple ratio of its breaking distance to the natural bond length. For a semiflexible filament, we examine the regime where L is shorter than persistence length L_{p}, as the limit opposite to that of the flexible chain. An enhancing factor also appears, speeding up the filament unbinding when the free length grows; for a long rigid rod, this factor becomes two, independently of the bond details. We also examine the total unbinding time of an irreversible detaching process by integrating (1/K) over polymer length and discover that its power-law scaling with chain length is smaller than one, over the commonly seen range of polymer size.

Finding trap stiffness of optical tweezers using digital filters

Obtaining trap stiffness and calibration of the position detection system is the basis of a force measurement using optical tweezers. Both calibration quantities can be calculated using several experimental methods available in the literature. In most cases, stiffness determination and detection system calibration are performed separately, often requiring procedures in very different conditions, and thus confidence of calibration methods is not assured due to possible changes in the environment. In this work, a new method to simultaneously obtain both the detection system calibration and trap stiffness is presented. The method is based on the calculation of the power spectral density of positions through digital filters to obtain the harmonic contributions of the position signal. This method has the advantage of calculating both trap stiffness and photodetector calibration factor from the same dataset in situ. It also provides a direct method to avoid unwanted frequencies that could greatly affect calibration procedure, such as electric noise, for example.

Guiding microscale swimmers using teardrop-shaped posts

The swimming direction of biological or artificial microscale swimmers tends to be randomised over long time-scales by thermal fluctuations. Bacteria use various strategies to bias swimming behaviour and achieve directed motion against a flow, maintain alignment with gravity or travel up a chemical gradient. Herein, we explore a purely geometric means of biasing the motion of artificial nanorod swimmers. These artificial swimmers are bimetallic rods, powered by a chemical fuel, which swim on a substrate printed with teardrop-shaped posts. The artificial swimmers are hydrodynamically attracted to the posts, swimming alongside the post perimeter for long times before leaving. The rods experience a higher rate of departure from the higher curvature end of the teardrop shape, thereby introducing a bias into their motion. This bias increases with swimming speed and can be translated into a macroscopic directional motion over long times by using arrays of teardrop-shaped posts aligned along a single direction. This method provides a protocol for concentrating swimmers, sorting swimmers according to different speeds, and could enable artificial swimmers to transport cargo to desired locations.

A combined 3D and 2D light scattering study on aqueous colloidal model systems with tunable interactions

In this article we report on the synthesis and characterization of a system of colloidal spheres suspended in an aqueous solvent which can be refractive index-matched, thus allowing for investigations of the particle near-wall dynamics by evanescent wave dynamic light scattering at concentrations up to the isotropic to ordered transition and beyond. The particles are synthesized by copolymerization of a fluorinated acrylic ester monomer with a polyethylene-glycol (PEG) oligomer by surfactant free emulsion polymerization. Static and dynamic light scattering experiments in combination with cryo transmission electron microscopy reveal that the particles have a core shell structure with a significant enrichment of the PEG chains on the particles surface. In index-matching DMSO/water suspensions the particles arrange in an ordered phase at volume fraction above 7%, if no additional electrolyte is present. The near-wall dynamics at low volume fraction are quantitatively described by the combination of electrostatic repulsion and hydrodynamic interaction between the particles and the wall. At volume fractions close to the isotropic to ordered transition, the near-wall dynamics are more complex and qualitatively reminiscent of the behaviour which was observed in hard sphere suspensions at high concentrations.

Near-wall dynamics of concentrated hard-sphere suspensions: comparison of evanescent wave DLS experiments, virial approximation and simulations

In this article we report on a study of the near-wall dynamics of suspended colloidal hard spheres over a broad range of volume fractions. We present a thorough comparison of experimental data with predictions based on a virial approximation and simulation results. We find that the virial approach describes the experimental data reasonably well up to a volume fraction of ϕ≈ 0.25 which provides us with a fast and non-costly tool for the analysis and prediction of evanescent wave DLS data. Based on this we propose a new method to assess the near-wall self-diffusion at elevated density. Here, we qualitatively confirm earlier results [Michailidou, et al., Phys. Rev. Lett., 2009, 102, 068302], which indicate that many-particle hydrodynamic interactions are diminished by the presence of the wall at increasing volume fractions as compared to bulk dynamics. Beyond this finding we show that this diminishment is different for the particle motion normal and parallel to the wall.

Effects of shear and walls on the diffusion of colloids in microchannels

Colloidal suspensions flowing through microchannels were studied for the effects of both the shear flow and the proximity of walls on the particles' self-diffusion. Use of hydrostatic pressure to pump micron-sized silica spheres dispersed in water-glycerol mixture through poly(dimethylsiloxane) channels with a cross section of 30×24μm(2), allowed variation in the local Peclet number (Pe) from 0.01 to 50. To obtain the diffusion coefficients, image-time series from a confocal scanning laser microscope were analyzed with a method that, after finding particle trajectories, subtracts the instantaneous advective displacements and subsequently measures the slopes of the mean squared displacement in the flow (x) and shear (y) directions. For dilute suspensions, the thus obtained diffusion coefficients (D(x) and D(y)) are close to the free diffusion coefficient at all shear rates. In concentrated suspensions, a clear increase with the Peclet number (for Pe > 10) is found, that is stronger for D(x) than for D(y). This effect of shear-induced collisions is counteracted by the contribution of walls, which cause a strong local reduction in D(x) and D(y).

Detection and characterization of liquid|solid and liquid|liquid|solid interfacial gradients of water nanodroplets in wet N-octyl-2-pyrrolidone

We report on the rotational diffusion dynamics and fluorescence lifetime of lissamine rhodamine B sulfonyl chloride (LRSC) in two thin-film experimental configurations. These are liquid|solid interfaces, where N-octyl-2-pyrrolidone (NOP) containing water and ethylene glycol (EG) thin films are each supported on glass, and a liquid|liquid|solid interface where thin films of water and NOP, both supported on glass, are in contact with one another, forming an NOP|water interface. The reorientation dynamics and fluorescence lifetime of LRSC are measured as a function of distance from the NOP|glass and EG|glass interfaces and from the NOP|water and NOP|glass interfaces in the liquid|liquid|solid experimental configuration. Fluorescence anisotropy decay data from the liquid|solid systems reveal a liquid film depth-dependent gradient spanning tens of micrometers from the NOP|glass interface into the wet NOP phase, while this gradient is absent in EG. We interpret these findings in the context of a compositional gradient in the NOP phase. The spatially resolved fluorescence lifetime and anisotropy decay data for an NOP|water|glass interfacial structure exhibits the absence of a gradient in the anisotropy decay profile normal to the NOP|water interface and the presence of a fluorescence lifetime gradient as a function of distance from the NOP|water interface. The compositional heterogeneity for both interfacial systems is in the form of water nanodroplets in the NOP phase. We understand this compositional gradient in the context of the relative surface energies of the water, NOP, and glass components.

Translational and rotational near-wall diffusion of spherical colloids studied by evanescent wave scattering

In this article we extend recent experimental developments [Rogers et al., Phys. Rev. Lett., 2012, 109, 098305] by providing a suitable theoretical framework for the derivation of exact expressions for the first cumulant (initial decay rate) of the correlation function measured in Evanescent Wave Dynamic Light Scattering (EWDLS) experiments. We focus on a dilute suspension of optically anisotropic spherical Brownian particles diffusing near a planar hard wall. In such a system, translational and rotational diffusion are hindered by hydrodynamic interactions with the boundary which reflects the flow incident upon it, affecting the motion of colloids. The validity of the approximation by the first cumulant for moderate times is assessed by juxtaposition to Brownian dynamics simulations, and compared with experimental results. The presented method for the analysis of experimental data allows the determination of penetration-depth-averaged rotational diffusion coefficients of spherical colloids at low density.

Hydrodynamic measurement of Brownian particles at a liquid-solid interface by low-coherence dynamic light scattering

The hydrodynamics of Brownian particles close to a wall is investigated using low-coherence dynamic light scattering. The diffusion coefficient of the particles in a suspension is measured as a function of distance from the wall. A sudden reduction in the diffusion coefficient near the interface is clearly observed using this method. The theoretically predicted wall-drag effect is experimentally confirmed when the influence of the spatial resolution due to the finite coherence length of the light source is accounted for. The space-dependent dynamics of Brownian particles under the wall-drag effect is obtained for the first time using our spatially resolved dynamic light scattering technique.

Coherence resonance in a unijunction transistor relaxation oscillator

The phenomenon of coherence resonance is investigated in an unijunction transistor relaxation oscillator and quantified by estimating the normal variance (NV). Depending on the measuring points, two types of NV curves have been obtained. We have observed that the degradations in coherency at higher noise amplitudes in our system is probably the result of direct interference of coherent oscillations and the stochastic perturbation. Degradation of coherency may be minimal if this direct interference of noise and coherent oscillations is eliminated.

A VANADIUM BROMOPEROXIDASE CATALYZES THE FORMATION OF HIGH-MOLECULAR-WEIGHT COMPLEXES BETWEEN BROWN ALGAL PHENOLIC SUBSTANCES AND ALGINATES(1)

The interaction between phenolic substances (PS) and alginates (ALG) has been suggested to play a role in the structure of the cell walls of brown seaweeds. However, no clear evidence for this interaction was reported. Vanadium bromoperoxidase (VBPO) has been proposed as a possible catalyst for the binding of PS to ALG. In this work, we studied the interaction between PS and ALG from brown algae using size exclusion chromatography (SEC) and optical tweezers microscopy. The analysis by SEC revealed that ALG forms a high-molecular-weight complex with PS. To study the formation of this molecular complex, we investigated the in vitro interaction of purified ALG from Fucus vesiculosus L. with purified PS from Padina gymnospora (Kütz.) Sond., in the presence or absence of VBPO. The interaction between PS and ALG only occurred when VBPO was added, indicating that the enzyme is essential for the binding process. The interaction of these molecules led to a reduction in ALG viscosity. We propose that VBPO promotes the binding of PS molecules to the ALG uronic acids residues, and we also suggest that PS are components of the brown algal cell walls.

Agnostic particle tracking for three-dimensional motion of cellular granules and membrane-tethered bead dynamics

The ability to detect biological events at the single-molecule level provides unique biophysical insights. Back-focal-plane laser interferometry is a promising technique for nanoscale three-dimensional position measurements at rates far beyond the capability of standard video. We report an in situ calibration technique for back-focal-plane, low-power (nontrapping) laser interferometry. The technique does not rely on any a priori model or calibration knowledge, hence the name "agnostic". We apply the technique to track long-range (up to 100 microm) motion of a variety of particles, including magnetic beads, in three-dimensions with high spatiotemporal resolution ( approximately 2 nm, 100 micros). Our tracking of individual unlabeled vesicles revealed a previously unreported grouping of mean-squared displacement curves at short timescales (<10 ms). Also, tracking functionalized magnetic beads attached to a live cell membrane revealed an anchorage-dependent nonlinear response of the membrane. The software-based technique involves injecting small perturbations into the probe position by driving a precalibrated specimen-mounting stage while recording the quadrant photodetector signals. The perturbations and corresponding quadrant photodetector signals are analyzed to extract the calibration parameters. The technique is sufficiently fast and noninvasive that the calibration can be performed on-the-fly without interrupting or compromising high-bandwidth, long-range tracking of a particle.

Electrostatically confined nanoparticle interactions and dynamics

We report integrated evanescent wave and video microscopy measurements of three-dimensional trajectories of 50, 100, and 250 nm gold nanoparticles electrostatically confined between parallel planar glass surfaces separated by 350 and 600 nm silica colloid spacers. Equilibrium analyses of single and ensemble particle height distributions normal to the confining walls produce net electrostatic potentials in excellent agreement with theoretical predictions. Dynamic analyses indicate lateral particle diffusion coefficients approximately 30-50% smaller than expected from predictions including the effects of the equilibrium particle distribution within the gap and multibody hydrodynamic interactions with the confining walls. Consistent analyses of equilibrium and dynamic information in each measurement do not indicate any roles for particle heating or hydrodynamic slip at the particle or wall surfaces, which would both increase diffusivities. Instead, lower than expected diffusivities are speculated to arise from electroviscous effects enhanced by the relative extent (kappaa approximately 1-3) and overlap (kappah approximately 2-4) of electrostatic double layers on the particle and wall surfaces. These results demonstrate direct, quantitative measurements and a consistent interpretation of metal nanoparticle electrostatic interactions and dynamics in a confined geometry, which provides a basis for future similar measurements involving other colloidal forces and specific biomolecular interactions.

Unexpected slow near wall dynamics of spherical colloids in a suspension of rods

In this paper, we will show the influence of an additional rodlike component, that is, fd-virus, on the diffusion of spherical polystyrene colloids close to a wall. The sphere diffusivity normal to the wall, D perpendicular, is strongly affected by the presence of the rods, while the effect on the parallel diffusivity, D||, is less pronounced except in the immediate vicinity of the wall. We show that this observation cannot be explained by describing the effect of the rods as a simple mean field depletion potential alone.

Supercritical water: a fascinating medium for soft matter

Properties of water change dramatically at high temperatures and high pressures near and/or above the critical point ( = 374 °C, = 22.1 MPa). The dielectric constant, for example, decreases from 78 at 25 °C and 0.1 MPa to 6 at the critical point, the value of which is comparable to that of 1-dodecanol. As fascinating characteristics of soft matter rely on unique properties of ambient liquid water, the change should have significant impacts on soft matter. However, our knowledge of soft matter under such extreme conditions is virtually nonexistent. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are described. Implications of the findings for geological processes in deep-subsurface are also discussed.

Self-aggregation of Cryptococcus neoformans capsular glucuronoxylomannan is dependent on divalent cations

The capsular components of the human pathogen Cryptococcus neoformans are transported to the extracellular space and then used for capsule enlargement by distal growth. It is not clear, however, how the glucuronoxylomannan (GXM) fibers are incorporated into the capsule. In the present study, we show that concentration of C. neoformans culture supernatants by ultrafiltration results in the formation of highly viscous films containing pure polysaccharide, providing a novel, nondenaturing, and extremely rapid method to isolate extracellular GXM. The weight-averaged molecular mass of GXM in the film, determined using multiangle laser light scattering, was ninefold smaller than that of GXM purified from culture supernatants by differential precipitation with cetyl trimethyl ammonium bromide (CTAB). Polysaccharides obtained either by ultrafiltration or by CTAB-mediated precipitation showed different reactivities with GXM-specific monoclonal antibodies. Viscosity analysis associated with inductively coupled plasma mass spectrometry and measurements of zeta potential in the presence of different ions implied that polysaccharide aggregation was a consequence of the interaction between the carboxyl groups of glucuronic acid and divalent cations. Consistent with this observation, capsule enlargement in living C. neoformans cells was influenced by Ca(2+) in the culture medium. These results suggest that capsular assembly in C. neoformans results from divalent cation-mediated self-aggregation of extracellularly accumulated GXM molecules.

Brownian diffusion close to a polymer brush

In an effort to control particle diffusion near surfaces, we have studied the dynamics of colloidal hard spheres and soft compliant star copolymers on surfaces coated with polymer brushes using evanescent wave dynamic light scattering. The same experiments provide information on the brush structure and confined particle motion. The penetration into dense polydisperse brushes is size- and solvent-dependent.

Towards absolute calibration of optical tweezers

Aiming at absolute force calibration of optical tweezers, following a critical review of proposed theoretical models, we present and test the results of Mie-Debye-spherical aberration (MDSA) theory, an extension of a previous (MD) model, taking account of spherical aberration at the glass-water interface. This first-principles theory is formulated entirely in terms of experimentally accessible parameters (none adjustable). Careful experimental tests of the MDSA theory, undertaken at two laboratories, with very different setups, are described. A detailed description is given of the procedures employed to measure laser beam waist, local beam power at the transparent microspheres trapped by the tweezers, microsphere radius, and the trap transverse stiffness, as a function of radius and height in the (inverted microscope) sample chamber. We find generally very good agreement with MDSA theory predictions, for a wide size range, from the Rayleigh domain to large radii, including the values most often employed in practice, and at different chamber heights, both with objective overfilling and underfilling. The results asymptotically approach geometrical optics in the mean over size intervals, as they should, and this already happens for size parameters not much larger than unity. MDSA predictions for the trapping threshold, position of stiffness peak, stiffness variation with height, multiple equilibrium points, and "hopping" effects among them are verified. Remaining discrepancies are ascribed to focus degradation, possibly arising from objective aberrations in the infrared, not yet included in MDSA theory.

Colloidal dynamics near a wall studied by evanescent wave light scattering: Experimental and theoretical improvements and methodological limitations

The dynamics of colloidal spheres near to a wall is studied with an evanescent wave scattering setup that allows for an independent variation of the components of the scattering wave vector normal and parallel to the wall. The correlation functions obtained with this novel instrumentation are interpreted on the basis of an expression for their short time behavior that includes hydrodynamic interactions between the colloidal spheres and the wall. The combination of the evanescent wave scattering setup and the exact expression for the short time behavior of correlation functions allows for an unambiguous measurement of the particle mobility parallel and normal to the wall by means of light scattering. It is possible to measure the viscous wall drag effect on the dynamics of particles with radii as small as 27 nm, where, however, the method reaches its limits due to the low scattering intensities of such small particles.

DNA-psoralen interaction: a single molecule experiment

By attaching one end of a single lambda-DNA molecule to a microscope coverslip and the other end to a polystyrene microsphere trapped by an optical tweezers, we can study the entropic elasticity of the lambda-DNA by measuring force versus extension as we stretch the molecule. This powerful method permits single molecule studies. We are particularly interested in the effects of the photosensitive drug psoralen on the elasticity of the DNA molecule. We have illuminated the sample with different light sources, studying how the different wavelengths affect the psoralen-DNA linkage. To do this, we measure the persistence length of individual DNA-psoralen complexes.

Diffusion and velocity relaxation of a Brownian particle immersed in a viscous compressible fluid confined between two parallel plane walls

The diffusion tensor and velocity correlation function of a Brownian particle immersed in a viscous compressible fluid confined between two parallel plane walls are calculated in point approximation. The fluid is assumed to satisfy stick boundary conditions at the walls. It is found that the velocity correlation function decays asymptotically with a negative t(-2) long-time tail due to coupling to overdamped sound waves. The coefficient of the long-time tail is calculated and shown to be independent of fluid viscosity.

Anisotropy of Brownian motion caused only by hydrodynamic interaction with a wall

The diffusivity of spherical colloidal particles close to a planar hard wall is studied by dynamic light scattering with evanescent illumination. A novel setup allows us to independently vary the scattering vector components parallel Q parallel and normal Q perpendicular to the wall. An expression for the initial decay rate Gamma of the time autocorrelation functions is derived as a function of both Q parallel and Q perpendicular, as well as the penetration depth of the evanescent wave, where hydrodynamic interactions of particles with the wall are included. This makes it possible to study the viscous wall drag effect quantitatively for particles as small as 85 nm in radius.

Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.05. In this region, the hydrodynamic mobility and diffusion coefficient are substantially reduced below bulk values. The procedure involved measuring the mean and the variance of vertical displacements of a Brownian particle settling under gravity toward the plate. The spatially varying diffusion coefficient was calculated from the displacements at small times (where diffusive motion was dominant). The mobility relationship for motion normal to a flat plate was tested by measuring the average distance of travel versus time as the particle settled under the constant force of gravity. For the simple Newtonian fluid used here (aqueous salt solution), the magnitude of the diffusion coefficient and mobility, plus their dependence on separation distance, showed excellent agreement with predictions. This new technique could be of great value in measuring the mobility and diffusion coefficient for near-contact motion in more complex fluids for which the hydrodynamic correction factors are not known a priori, such as shear-thinning fluids.

Dynamic light scattering from an optically trapped microsphere

Using a single microscope objective lens to optically trap, illuminate, and collect backscattered light of a dielectric microsphere, we measure the temporal-intensity-autocorrelation functions (ACFs), and intensity profiles to obtain the trap stiffness and friction coefficient of the bead. This is an interesting study of an harmonically bound Brownian particle, with nanometer resolution. We extend the work of Bar-Ziv et al. [Phys. Rev. Lett. 78, 154 (1997)] to more general situations allowing for the use of our simpler geometry in other applications. As examples, we present measurements of the parallel Stokes friction coefficient on the trapped bead as a function of its distance from a surface and the entropic force of a single lambda-DNA molecule.

Direct measurements of constrained brownian motion of an isolated sphere between two walls

We report the results of direct measurements, using video microscopy in combination with optical tweezers, of constrained diffusion of an isolated uncharged PMMA sphere in a density-matched fluid confined between two parallel flat walls. Our experimental methodology allows us to study the hindered diffusion of the sphere as an explicit function of its distance from the walls, without interference from sedimentation or from electrostatic interaction between the particle and the walls. The measured diffusion coefficients are used to test the predictions of the wall drag effect predicted by several approximate theoretical analyses. We find a quantitative agreement with the behavior predicted using a hydrodynamic analysis that independently superimposes the wall drag effects arising from each wall. Our results imply, indirectly, that neglect of multiple interactions with the colloid sphere of the perturbations of the pressure and velocity fields induced by each wall leads to an underestimate of the influence of the wall on the drag force experienced by the particle.