Experimental Study of Fibrin/Fibrin-Specific Molecular Interactions Using a Sphere/Plane Adhesion Model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shear flow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrin-coated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers. A simple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact. Copyright 2001 Academic Press.     

Self‐sustained entropy‐driven flow in nano‐confined polymer melts

We present a Monte Carlo simulation study of the Brownian motion of polymer chains in a melt confined in a periodically asymmetric channel of nanometric dimensions. We assume no friction between the chains and the channel. A detailed analysis of the conformations of the chains reveals the presence of a favorable entropy gradient along the easy flow direction. For high molecular weight chains, this gradient is seen to drive a self‐sustained polymer flow with Peclet numbers as high as 0.9, which makes our observation experimentally accessible. Much weaker efficiences are observed at low molecular weights. We show that the unexpected directed Brownian motion of polymers in confined geometries is of significant importance in the design and stabilization of platelet nanocomposites which typically age over time under quiescent conditions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 869–875     

Particle laden fluid interfaces: Dynamics and interfacial rheology

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.     

A computational study of the hydrodynamically assisted organization of DNA-functionalized colloids in 2D

We study computationally the self-organization of DNA-functionalized colloidal particles confined to two dimensions and subjected to a linear shear force. We show that hydrodynamic forces allow a more thorough sampling of phase space than thermal or Brownian forces alone. Two particle types are present in each of our dynamic simulations each signifying its own specific oligonucleotide sequence grafted to the particle surface: A-type and B-type. Particles are modeled as interacting via a type-specific DNA attraction where unlike-types have affinities for each other while like-types do not. The particles are small enough to feel Brownian motion while the shear adds motion to the particles. We find the formation of lines of A-type and B-type particles in simulations with an imposed shear. Simulations without imposed shear form a frustrated network with little or no linear order. An orientational distribution function, g2(r), quantifies the degree of linear order. A phase diagram is constructed, finding a linear dependence of the minimum DNA force necessary for line formation on the dimensionless shear rate. A force analysis performed on the structures shows that the lines orient perpendicular to the axis of the elongation component of the shear because it is this orientation that allows the DNA attraction to resist the shear.     

Natural convection in nanofluid flow with chemotaxis process over a vertically inclined heated surface

The thermal energy transport analysis with chemotaxis in the free convective flow of viscous nanofluid over stretchable vertically inclined heated sheet is addressed in this article. The fluid forced and free convection motion is investigated and discussed with physical reasoning. The fluid also contains microorganism heavy-bottom species, and their chemotactic motion is studied. In the light of Buongiorno model, the impact of Brownian motion and thermophoresis slip mechanism on thermal conduction in the nanofluid is analyzed. The work is based on the similarity analysis of governing partial differential equations (PDEs) which lead to non-dimensional ordinary differential equations (ODEs). The solution of resulting flow and heat equations is computed via bvp4c technique. The outcomes are represented in graphical abstract. It is noted that free convective flow field increases near to the surface of sheet then it decays to free stream exponentially. Higher magnitude of thermophoretic force boost up the thermal energy transport in nanofluid flow. The Brownian motion enhances temperature profile and lower down the convection velocity. Chemotaxis motion of species in nanofluid is increasing function of bioconvective Peclet number.     

Electro-dry-adhesion

This work presents novel conductive bioinspired dry adhesives with mushroom caps that enable the use of a synergistic combination of electrostatic and van der Waals forces (electro-dry-adhesion). An increase in shear adhesion bond strength of up to 2046% on a wide range of materials is measured when a maximum electrical field of 36.4 V μm(-1) is applied. A suction effect, due to the shape of the dry adhesive fibers, on overall adhesion was not noted for electro-dry-adhesives when testing was performed at both atmospheric and reduced pressure. Utilization of electrostatics to apply a preloading force to dry adhesive fiber arrays allows increased adhesion even after electrostatic force generation has been halted by ensuring the close contact necessary for van der Waals forces to be effective. A comparison is made between self-preloading of the electro-dry-adhesives and the direct application of a normal preloading pressure resulting in nearly the same shear bond strength with an applied voltage of 3.33 kV on the same sample.     

Brownian dynamics simulations of single-wall carbon nanotube separation by type using dielectrophoresis

We theoretically investigate the separation of individualized metallic and semiconducting single-wall carbon nanotubes (SWNTs) in a dielectrophoretic (DEP) flow device. The SWNT motion is simulated by a Brownian dynamics (BD) algorithm, which includes the translational and rotational effects of hydrodynamic, Brownian, dielectrophoretic, and electrophoretic forces. The device geometry is chosen to be a coaxial cylinder because it yields effective flow throughput, the DEP and flow fields are orthogonal to each other, and all the fields can be described analytically everywhere. We construct a flow-DEP phase map showing different regimes, depending on the relative magnitudes of the forces in play. The BD code is combined with an optimization algorithm that searches for the conditions that maximize the separation performance. The optimization results show that a 99% sorting performance can be achieved with typical SWNT parameters by operating in a region of the phase map where the metallic SWNTs completely orient with the field, whereas the semiconducting SWNTs partially flow-align.     

Effect of Rotary Inertia on the Orientational Behavior of Dilute Brownian and Non-Brownian Fiber Suspensions

The orientational behavior of a dilute suspension of slender Brownian and non-Brownian fibers with rotary inertia in simple shear and turbulent channel flows is numerically investigated. The translational inertia of fibers is neglected. The equations describing the evolution of fibers orientation are integrated along the Lagrangian paths of the fluid elements. The fully developed turbulent channel flow at Re _τ = 180 is provided by a direct numerical simulation (DNS). The coupling between the flow field and the fiber dynamics is one way. The Brownian motion is modeled by a stochastic Wiener process. The results are compared with those of inertia-free particles. In simple shear flow, the inertial non-Brownian fibers align slower than the inertia-free fibers to the shear direction while they tend to the same steady state orientation. For Brownian fibers, the steady state orientation of inertial and inertia-free fibers differ. In turbulent channel flow, the second moment of the orientation distribution function shows an oscillatory behavior at high values of inertia for non-Brownian fibers while the oscillations disappear at lower inertia. For Brownian fibers, the oscillations are weaker due to the damping effect of the Brownian diffusivity.     

Simulation of nonlinear shear rheology of dilute salt-free polyelectrolyte solutions

Brownian dynamics simulations are used to conduct a systematic analysis of the nonlinear shear rheology of dilute polyelectrolyte solutions, exploring its relationship to shear rate, Bjerrum length, and concentration. A simple coarse-grained bead-spring chain model that incorporates explicit counterions is used. It is found that the polyelectrolyte chains exhibit a shear thinning behavior at high shear rate (as characterized by bead Peclet number Pe) that is independent of the electrostatic strength due to the stripping of ions from close proximity to the chain caused by the flow. In contrast, at low values of Pe, the viscosity increases monotonically with increasing Bjerrum length over the range studied here, in contrast to the nonmonotonic trend displayed by the chain size. Furthermore, at fixed Bjerrum length, the reduced viscosity increases monotonically with concentration. The mechanism underlying these observations is essentially the primary electroviscous effect; the ion cloud surrounding a polyelectrolyte chain deforms in flow, causing a significant increase in viscosity as concentration increases. Finally, the authors have also considered the role of hydrodynamic interactions in these simulations, finding that for low concentration studies in shear flow, these do not qualitatively affect the results.     

Periodic orientational motions of rigid liquid-crystalline polymers in shear flow

The collective periodic motions of liquid-crystalline polymers in a nematic phase in shear flow have, for the first time, been simulated at the particle level by Brownian dynamics simulations. A wide range of parameter space has been scanned by varying the aspect ratio L/D between 10 and 60 at three different scaled volume fractions Lphi/D and an extensive series of shear rates. The influence of the start configuration of the box on the final motion has also been studied. Depending on these parameters, the motion of the director is either characterized as tumbling, kayaking, log-rolling, wagging, or flow-aligning. The periods of kayaking and wagging motions are given by T=4.2(Lphi/D)gamma(-1) for high aspect ratios. Our simulation results are in agreement with theoretical predictions and recent shear experiments on fd viruses in solution. These calculations of elongated rigid rods have become feasible with a newly developed event-driven Brownian dynamics algorithm.     

Limitations of differential electrophoresis for measuring colloidal forces: a Brownian dynamics study

Differential electrophoresis experiments are often used to measure subpiconewton forces between two spheres of a heterodoublet. The experiments have been interpreted by solving the electrokinetic equations to obtain a simple Stokes law-type equation. However, for nanocolloids, the effects of Brownian motion alter the interpretation: (1) Brownian translation changes the rate of axial separation. (2) Brownian rotation reduces the alignment of the doublet with the applied electric field. (3) Particles can reaggregate by Brownian motion after they break, forming either heterodoublets or homodoublets, and because homodoublets cannot be broken by differential electrophoresis, this effectively terminates the experiment. We tackle points 1 and 2 using Brownian dynamics simulations (BDS) with electrophoresis as an external force, accounting for convective translation and rotation as well as Brownian translation and rotation. Our simulations identify the lower particle size limit of differential electrophoresis to be about 1 microm for desired statistical accuracy. Furthermore, our simulations predict that particles around 10 nm in size and at ambient conditions will break primarily by Brownian motion, with a negligible effect due to the electric field.     

Molecular bond formation between surfaces: anchoring and shearing effects

Specific molecular bonds between apposing surfaces play a central role in many biological structures and functions. They display a widely varying anchoring to the cell surface, and they are subject to forces that affect their binding characteristics due to their hydrodynamic environments. Here, we examine both anchoring and shearing aspects using simplified model systems aimed at gaining insight into the formation of a 2D bond collection under stress using two different surface anchors. The highly specific streptavidin-biotin molecular bond was chosen as the model receptor-ligand pair, and grafted colloids were used as model surfaces. To explore the role of the surface anchor, we grafted biotin onto the particle surface following two different approaches: first, the grafting was performed directly on the particle amine functions; second, a 35-nm-long PEG spacer was used. Hybrid particle classes were brought into contact in a homogeneous shear (between 200 s(-)(1) and 1200 s(-)(1)) using a cone plate geometry. The bond association and dissociation kinetics were given by the time course assemblage of hybrid particles into doublets. We observed saturating kinetics profiles that we interpreted as a linkage-breakage equilibrium, which yielded the on and off rates. We found that the biotin-PEG spacer was needed in order to observe significant binding at any shear rate. We also showed that only the number of collisions per unit time, generated by the shear, affected the on rate of the binding. Neither the exerted forces nor the collision lifetime had any effect. The off rate decreased with shear, possibly because of the shortening of the force duration, which results from the increasing shear rate.     

Single-chain dynamics in a semidilute polymer solution under steady shear

We use Brownian dynamics computer simulations to investigate single-chain dynamics in a semidilute polymer solution undergoing a steady, uniform shear flow. In the presence of the shear flow, the system used in the present study exhibits anisotropic structure factors, often referred to as butterfly patterns, which rotate with increasing shear rate [P. P. Jose and G. Szamel, J. Chem. Phys. 127, 114905 (2007)]. The rotation of these patterns correlates with shear thinning of the solution. In order to elucidate the microscopic origin of this behavior, we have investigated the change in the single-chain dynamics in the solution: We have focused on the relaxation of the end-to-end vector, the Rouse modes, and the radius of gyration tensor. In equilibrium and for small shear rates, these quantities show double exponential relaxation. With increasing shear rate, they show oscillatory relaxation, which hints at the tumbling motion of the chain. In the high shear rate regime, the frequency of the oscillations of the end-to-end vector autocorrelation function shows a power law dependence on the shear rate. We have compared the single-chain dynamics in the semidilute solution with that in a dilute solution. An analysis of the instantaneous values of the radius of gyration tensor, the end-to-end distance, and the normal stress along the system's trajectory reveals a synchronization of the fluctuations of these quantities.     

Nanoscale high-frequency contact mechanics using an AFM tip and a quartz crystal resonator

The transmission of high-frequency shear stress through a microscopic contact between an AFM tip and an oscillating quartz plate was measured as a function of vertical pressure, amplitude, and surface properties by monitoring the MHz component of the tip's deflection. For dry surfaces, the transmission of shear stress is proportional to the vertical load across the contact. This provides a measure of the forces of adhesion between the substrate and the tip. When stretching soft polymeric fibers created by pulling on the surface of a pressure sensitive adhesive, the transmitted shear stress decreased linearly with extension over the entire range of pulling. This contrasts with the static adhesive force, which remained about constant until it discontinuously dropped at the point of rupture.     

Constitutive equations for the flow behavior of entangled polymeric systems: application to star polymers

A semimicroscopic derivation is presented of equations of motion for the density and the flow velocity of concentrated systems of entangled polymers. The essential ingredient is the transient force that results from perturbations of overlapping polymers due to flow. A Smoluchowski equation is derived that includes these transient forces. From this, an equation of motion for the polymer number density is obtained, in which body forces couple the evolution of the polymer density to the local velocity field. Using a semimicroscopic Ansatz for the dynamics of the number of entanglements between overlapping polymers, and for the perturbations of the pair-correlation function due to flow, body forces are calculated for nonuniform systems where the density as well as the shear rate varies with position. Explicit expressions are derived for the shear viscosity and normal forces, as well as for nonlocal contributions to the body force, such as the shear-curvature viscosity. A contribution to the equation of motion for the density is found that describes mass transport due to spatial variation of the shear rate. The two coupled equations of motion for the density and flow velocity predict flow instabilities that will be discussed in more detail in a forthcoming publication.     

Brownian dynamics simulations of polyelectrolyte adsorption in shear flow

Brownian dynamics simulations are used to study the adsorption of an isolated polyelectrolyte molecule onto an oppositely charged flat surface in the absence and the presence of an imposed shear flow. The polyelectrolyte is modeled as a freely jointed bead-rod chain where excluded volume interactions are incorporated by using a hard-sphere potential. The total charge along the backbone is distributed uniformly among all the beads, and the beads are allowed to interact with one another and the charged surface through screened Coulombic interactions. The simulations are performed by placing the molecule a fixed distance above the surface, and the adsorption behavior is then studied as a function of screening length. In the absence of an imposed flow, the chain is found to lie flat and extended on the adsorbing surface in the limit of weak screening, whereas in the limit of strong screening it desorbs from the surface and attains free-solution behavior. For intermediate screening, only a small portion of the chain adsorbs and it becomes highly extended in the direction normal to the surface. An imposed shear flow tends to orient the chain in the direction of flow and also leads to increased contact of the chain with the surface.     

Lubrication breakdown in hydrodynamic simulations of concentrated colloids

We report simulation studies of model colloid spheres interacting via squeeze, shear and rotation lubrication interactions. The results are both sensitive to the numerical scheme and its associated timestep. We find evidence that suggests the ideal smooth hard sphere problem is singular in that clusters of particles form within which very narrow gaps exist between neighbours — gaps which are unphysically narrow in colloid terms. The inclusion of a surface interaction or Brownian forces can prevent this catastrophe although for high enough shear rates and/or weak enough coats very narrow gaps again may form. Shear induced structure in these suspensions can be very sensitive to system size, but using large boxes extended along the flow direction gives structures likely realistic of true colloid flow under simple shear.     

Alignment of particles in sheared viscoelastic fluids

We investigate the shear-induced structure formation of colloidal particles dissolved in non-Newtonian fluids by means of computer simulations. The two investigated visco-elastic fluids are a semi-dilute polymer solution and a worm-like micellar solution. Both shear-thinning fluids contain long flexible chains whose entanglements appear and disappear continually as a result of Brownian motion and the applied shear flow. To reach sufficiently large time and length scales in three-dimensional simulations with up to 96 spherical colloids, we employ the responsive particle dynamics simulation method of modeling each chain as a single soft Brownian particle with slowly evolving inter-particle degrees of freedom accounting for the entanglements. Parameters in the model are chosen such that the simulated rheological properties of the fluids, i.e., the storage and loss moduli and the shear viscosities, are in reasonable agreement with experimental values. Spherical colloids dispersed in both quiescent fluids mix homogeneously. Under shear flow, however, the colloids in the micellar solution align to form strings in the flow direction, whereas the colloids in the polymer solution remain randomly distributed. These observations agree with recent experimental studies of colloids in the bulk of these two liquids.     

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.