Strength Of Hydrodynamic Interaction Research Articles

Scaling law of correlated diffusion of colloidal particles confined to a rugged surface

Using optical microscopy and multiparticle tracking techniques, we investigate the correlated diffusion of colloidal particles over a rugged surface. Our findings demonstrate that the correlated diffusion caused by the hydrodynamic interactions of particles confined to energy landscapes displays a distinctive power-law behavior. The local energy landscape on the rugged surface reduces the long-range hydrodynamic interactions between colloidal particles. The energy landscape influences the strength of hydrodynamic interactions, but not their power-law form. The responding factor of the colloidal particles over the energy landscape to hydrodynamics decays exponentially with the potential energy minimum. We propose a scaling method, with which the correlated diffusion of colloidal particles over various energy landscapes can be scaled onto a master curve. The master curve characterizes the response of the particles over the energy landscape to the hydrodynamics. The scale factors used for the master curve allow for the calculation of the energy landscape. The findings provide physical insights into the confinement hydrodynamics and would be helpful for designing material surfaces and controlling the motion of particles on rough surfaces.

River Jets Versus Wave-Driven Longshore Currents at River Mouths

At river mouths, fluvial jets and longshore currents (LSCs) generated by waves interact hydrodynamically. This idealized numerical modeling study simulates a large number of hydro-morphodynamic conditions (650) to explore the emergent hydrodynamics determined by different mouth bar volumes and geometries, river discharge, wave heights, and directions and their potential stress on river-mouth development. We find that in the absence of a river-mouth bar (RMB), interactions are driven by momentum balances, expressed either as the balance of wave momentum flux (Mw) and jet momentum flux (Mj), or the balance of river jet discharge (QJet) and longshore current discharge (QLSC). When a RMB is present, the topography modifies the structure of the jet by spreading it, and we quantify this mechanism through the lateral jet transfer rate (LJT). Secondly, topography generates complex longshore wave-driven circulation as a result of the protruding shoreface which serves as a platform on which counter LSCs develop. The balance in QJet/QLSC may be used as an indication of the type of circulation. High and oblique waves favor longshore circulation and RMB bypass, whereas low waves and normal-to-coast angles generate diverging LSCs on the mouth bar crest which interrupts the longshore circulation. A quantification of the dynamic diversion is proposed in the form of the non-dimensional Dynamic diversion index (DyD), which scales with the product of Mj and Mw, and can account for the absolute strength of hydrodynamic interactions occurring at river mouths. RMB morphology can affect DyD in multiple ways by strengthening or by weakening the interactions. The DyD effect seems to increase with increasing RMB size, indicating that the RMB scale regulates the interplay of the wave-driven circulation and the river jet which further controls the adjacent topography changes.

2D stokesian simulation of particle aggregation at quiescent air/oil-water interfaces

HypothesisAggregation of particles on a liquid interface is controlled by inter-particle forces and hydrodynamic interactions. Previous experimental work has shown atypical structures despite diffusion limited cluster aggregation like behavior. It is likely that this is primarily due to the role of capillary quadrupoles in allowing particle repositioning after aggregation, which is tested here. ExperimentsUsing Stokesian dynamics and inter-particle forces unique to particles at liquid interfaces, aggregation of particles adsorbed to a liquid interface is studied. Simulations’ parameters are adjusted to control hydrodynamic interaction strength, initial particle position, and inter-particle forces magnitudes to compare to existing experimental results and hypothesis. FindingsIt is found that initial particle position plays a small role on equilibrium interfacial microstructure but has a significant impact on aggregation kinetics. Interfacial hydrodynamic interactions and inter-particle forces have a strong impact on equilibrium microstructure by altering the amount particles can reposition, which is consistent with published results. Capillary forces that allow significant repositioning after contact appear to play a key role in previously observed fractal dimensions of particle laden interfaces.

Pattern formation of skin cancers: Effects of cancer proliferation and hydrodynamic interactions.

We study pattern formation of skin cancers by means of numerical simulation of a binary system consisting of cancer and healthy cells. We extend the conventional model H for macrophase separations by considering a logistic growth of cancer cells and also a mechanical friction between dermis and epidermis. Importantly, our model exhibits a microphase separation due to the proliferation of cancer cells. By numerically solving the time evolution equations of the cancer composition and its velocity, we show that the phase separation kinetics strongly depends on the cell proliferation rate as well as on the strength of hydrodynamic interactions. A steady-state diagram of cancer patterns is established in terms of these two dynamical parameters and some of the patterns correspond to clinically observed cancer patterns. Furthermore, we examine in detail the time evolution of the average composition of cancer cells and the characteristic length of the microstructures. Our results demonstrate that different sequence of cancer patterns can be obtained by changing the proliferation rate and/or hydrodynamic interactions.

Rheology of polymers in many-body dissipative particle dynamics simulations: Schmidt number effect

The Schmidt number effect on the rheology of finitely extensible nonlinear elastic chains (FENE) in many-body dissipative particle dynamics (MDPD) is investigated in this work. We find that the Schmidt number, ranging from (101) to (103), has limited influence on the polymer properties, such as its radius of gyration (Rg), diffusion coefficient (D) and relaxation time (τ). The simulation results follow Zimm model's predictions well. The hydrodynamic interaction strength parameter h* demonstrates that the full hydrodynamic interaction can be simulated for Schmidt number from (100-106) in MDPD. Next, the rheology of FENE polymers is studied using Lees-Edward boundary condition in shear flow. The shear-thinning and normal stress difference are measured and analysed with MDPD; meanwhile, the volume fraction, solvent quality and chain length are varied to explore their effects on the extent of the Newtonian region. Finally, the non-Newtonian droplet is firstly simulated in MDPD. Its maximum spreading diameter is measured for both Newtonian and non-Newtonian droplet with Weber number (We) ranging from 4.37 to 109.2 on hydrophilic, moderate and hydrophobic surfaces, respectively. The fluid shear-thinning property increases at high shear rate and is further enhanced on more hydrophobic surface, from the maximum spreading diameter results. The non-Newtonian (FENE) droplet can be now well simulated in MDPD and this provides additional insight to further research concerning polymer-solvent-surface interactions, which is crucial in various applications.

Performance analysis of solo Duck wave energy converter arrays under motion constraints

This paper studies the power capture performance of solo Duck wave energy converter (WEC) arrays. The barrier function method combined with a quasi-Newton BFGS optimization algorithm is applied to find the maximum captured power of the array when the Ducks are under motion constraints. Based on this optimized maximum captured power, the effects of separation distance, wave period, incident wave direction and Duck width on the array performance are investigated. For the two Ducks array, results show that the alternative constructive and destructive interaction stripes in the contour plot of the q-factor variation with non-dimensional separation distance are resulted from the diffracted wave pattern from each Duck, and the hydrodynamic interaction strength is reduced when constraints affect the performance. For the three Ducks array, the middle Duck shows larger variability of captured power than the side Ducks due to experiencing double in phase diffracted wave from the side ones. The captured power of the solo Duck WEC array is sensitive to incident wave direction, and arrays with Ducks of smaller width are found to have better performance in power capture efficiency.

The non-Newtonian rheology of hydrodynamically interacting colloids via active, nonlinear microrheology

The non-Newtonian rheology of a dilute suspension of hydrodynamically interacting colloids is studied theoretically via active, nonlinear microrheology. In this Stokes-flow regime, a Brownian probe is driven through the suspension by a fixed external force; its motion distorts the configuration of background bath particles, which in turn alters probe motion. This interplay was utilized in our recent article to obtain the normal elements of the suspension stress via the nonequilibrium statistical mechanics theory [Chu and Zia, J. Rheol. 60(4), 755–781 (2016)]. In the present article, we focus on the normal stress differences N1 and N2, osmotic pressure Π, and their evolution with the strength of the probe force and strength of hydrodynamic interactions. As hydrodynamic interactions grow from weak to strong, the influence of couplings between the stress and the entrained motion on N1 changes with the strength of flow. When flow is strong, hydrodynamic interactions suppress the magnitude of N1, owing to coll...

Active microrheology of hydrodynamically interacting colloids: Normal stresses and entropic energy density

A single Brownian “probe” particle is driven by an external force through a colloidal suspension and its motion studied to elucidate the relative impacts of external, Brownian, and interparticle forces on the suspension stress. As the probe moves through the suspension, distortions to and relaxation of the particle arrangement give rise to nonequilibrium stress. The shape of the distorted microstructure is set by the strength of the external force, F0, relative to the entropic restoring force, kT/ath, of the suspension, and by the balance of microscopic forces between the constituent particles. The former is given by the Péclet number, Pe≡F0/(2kT/ath), where kT is the thermal energy and ath is the thermodynamic size of the particles. The latter comprise external, Brownian, and interparticle forces, and the sensitivity of each to flow strength Pe is set by the dimensionless repulsion range, κ≡(ath−a)/a, where a is the hydrodynamic size of the particles. The total stress comprises hydrodynamic and entropic contributions which manifest as Brownian, interparticle, and external force-induced stress. To analyze the influence of these forces on structure and suspension stress as they evolve with flow strength, we formulate and solve a Smoluchowski equation analytically in the dual limits of weak and strong external force and hydrodynamic interactions, and numerically for arbitrary values of Pe and κ. Nonequilibrium statistical mechanics are then utilized to compute elements of the stress tensor. Owing to the axisymmetric geometry of the microstructure about the line of the external force, only the diagonal elements are nonzero. When hydrodynamic interactions are negligibly weak, only the hard-sphere interparticle force matters regardless of the flow strength, and the results of Zia and Brady [J. Rheol. 56(5), 1175–1208 (2012)] are recovered whereby normal stresses scale as Pe2 and Pe in the limits of weak and strong forcing, respectively. That is, entropic forces dominate suspension stress regardless of the value of Pe when hydrodynamic interactions are weak. As the repulsion range κ shrinks, hydrodynamic interactions begin to play a role: When forcing is weak, Brownian disturbance flows provide the dominant contribution to suspension stress, but as Pe increases, the external force-induced stress takes over to dominate the total stress. Interestingly, the total suspension stress decreases as the strength of hydrodynamic interactions increases, regardless of the value of Pe. That is, hydrodynamic interactions suppress suspension stress. Owing to the dependence of hydrodynamic interactions on particle configuration, this stress suppression varies with flow strength: At low Pe, the stress scales as Pe2 and the suppression is quantitative, whereas at high Pe, the stress scales as Peδ, where 1 ≥ δ ≥ 0.799 for hydrodynamic interactions spanning from weak to strong. We identify the origin of such suppression via an analysis of pair trajectories: While entropic forces—interparticle repulsion and Brownian motion—destroy reversible trajectories, hydrodynamic interactions suppress structural asymmetry and this underlies the suppression of the nonequilibrium stress. We relate the stress to the energy density: Hydrodynamic interactions shield particles from direct collisions and promote fore-aft and structural symmetry, resulting in reduced entropic energy storage.

Force-induced diffusion in suspensions of hydrodynamically interacting colloids

We study the influence of hydrodynamic, thermodynamic and interparticle forces on the diffusive motion of a Brownian probe driven by a constant external force through a dilute colloidal dispersion. The influence of these microscopic forces on equilibrium self-diffusivity (passive microrheology) is well known: all three act to hinder the short- and long-time self-diffusion. Here, via pair-Smoluchowski theory, we explore their influence on self-diffusion in a flowing suspension, where particles and fluid have been set into motion by an externally forced probe (active microrheology), giving rise to non-equilibrium flow-induced diffusion. The probe’s motion entrains background particles as it travels through the bath, deforming the equilibrium suspension microstructure. The shape and extent of microstructural distortion is set by the relative strength of the external force $F^{\mathit{ext}}$ to the entropic restoring force $kT/a_{\mathit{th}}$ of the bath particles, defining a Péclet number $\mathit{Pe}\equiv F^{\mathit{ext}}/(2kT/a_{\mathit{th}})$; and also by the strength of hydrodynamic interactions, set by the range of interparticle repulsion ${\it\kappa}=(a_{\mathit{th}}-a)/a$, where $kT$ is the thermal energy and $a_{\mathit{th}}$ and $a$ are the thermodynamic and hydrodynamic sizes of the particles, respectively. We find that in the presence of flow, the same forces that hinder equilibrium diffusion now enhance it, with diffusive anisotropy set by the range of interparticle repulsion ${\it\kappa}$. A transition from hindered to enhanced diffusion occurs when diffusive and advective forces balance, $\mathit{Pe}\sim 1$, where the exact value is a sensitive function of the strength of hydrodynamics, ${\it\kappa}$. We find that the hindered to enhanced transition straddles two transport regimes: in hindered diffusion, stochastic forces in the presence of other bath particles produce deterministic displacements (Brownian drift) at the expense of a maximal random walk. In enhanced diffusion, driving the probe with a deterministic force through an initially random suspension leads to fluctuations in the duration of probe–bath particle entrainment, giving rise to enhanced, flow-induced diffusion. The force-induced diffusion is anisotropic for all $\mathit{Pe}$, scaling as $D\sim \mathit{Pe}^{2}$ in all directions for weak forcing, regardless of the strength of hydrodynamic interactions. When probe forcing is strong, $D\sim \mathit{Pe}$ in all directions in the absence of hydrodynamic interactions, but the picture changes qualitatively as hydrodynamic interactions grow strong. In this nonlinear regime, microstructural asymmetry weakens while the anisotropy of the force-induced diffusion tensor increases dramatically. This behaviour owes its origins to the approach toward Stokes flow reversibility, where diffusion along the direction of probe force scales advectively while transverse diffusion must vanish.

The medium amplitude oscillatory shear of semidilute colloidal dispersions. Part II: Third harmonic stress contribution

In Paper I [J. W. Swan et al., J. Rheol. 58, 307–338 (2014)], we derived an exact theoretical description of medium amplitude oscillatory shear for a semidilute colloidal dispersion. Through solution of the Smoluchowski equation governing the spatial distribution of suspended particles in the semidilute limit, we calculated the stresses that arise from an oscillatory linear flow as an expansion in powers of the rate of deformation. Here, this is extended to calculation of the first departures from linearity in the first and third harmonics of the suspension stress driven by oscillatory deformation. The role of hydrodynamic interactions is investigated via the excluded-annulus model in which particles are given an impenetrable core with a radius larger than their hydrodynamic radius. The ratio of these length scales controls the strength of hydrodynamic interactions. The third harmonic of the suspension stress is predicted to be dominated by hydrodynamic stresses at high frequency, a result that is shown to be valid experimentally for the oscillatory shear response of concentrated near hard-sphere dispersions. The calculations anticipate recent experimental observations on model near hard-sphere colloidal dispersions, and quantitative agreement is demonstrated when the predictions are scaled appropriately to account for volume fraction effects. The first departures from linearity in harmonics of the suspension stress are separated into several material functions that are independent of the flow geometry. These functions are generated from detailed numerical solutions, while asymptotic analysis is shown to predict the values of these functions at high frequency. These exact calculations provide a basis for understanding the onset of nonlinear rheological behavior of colloidal suspensions under dynamic oscillatory flow.

Linear viscoelasticity of attractive colloidal dispersions

The rheology of a semidilute dispersion of colloids having short-ranged attractions is studied. A depletion potential is chosen as a model for the attractive interaction and arises from nonadsorbing polymer dispersed with the colloids. The complex viscosity of these materials can be calculated by investigating their response to weak oscillatory shear. A first order expansion in small rates of deformation is used to solve for the microstructure and stress in the dispersion. Additionally, the effect of hydrodynamic interactions is studied via the excluded annulus model, which views the radius at which hard-sphere interactions occur as a barrier that resides beyond the hydrodynamic radius of the particles. This treatment allows a continuous variation of the hydrodynamic interaction strength. The viscoelastic response exhibits a sharp transition when going from weak attraction to strong attraction. Below a critical strength, increasing the interparticle attraction reduces the low frequency viscosity. Strong attractions increase the viscous response and delay the onset of an elastic plateau at high frequencies. An asymptotic analysis shows that the stress response is a result of the interplay of two length scales: The range of attraction and the diffusive boundary layer around particles. Independent of the strength of hydrodynamic interactions, the complex viscosity can be described with high accuracy by two well characterized viscoelastic models: At low frequencies, a Maxwell mode; at high frequencies, the purely repulsive hard-sphere response. Our results demonstrate that beyond hydrodynamic interactions the strength and range of the interaction potential between particles plays a central role in setting the viscoelasticity.

Dissipative particle dynamics simulation of dilute polymer solutions—Inertial effects and hydrodynamic interactions

We examine the accuracy of dissipative particle dynamics (DPD) simulations of polymers in dilute solutions with hydrodynamic interaction (HI), at the theta point, modeled by setting the DPD conservative interaction between beads to zero. We compare the first normal-mode relaxation time extracted from the DPD simulations with theoretical predictions from a normal-mode analysis for theta chains. We characterize the influence of bead inertia within the coil by a ratio Lm/Rg, where Lm is the ballistic distance over which bead inertia is lost, and Rg is the radius of gyration of the polymer coil, while the HI strength per bead h* is determined by the ratio of bead hydrodynamic radius (rH) to the equilibrium spring length. We show how to adjust h* through the spring length and monomer mass, and how to optimize the accuracy of DPD for fixed h* by increasing the friction coefficient (γ ≥ 9) and by incorporating a nonlinear distance dependence into the frictional interaction. Even with this optimization, DPD simulations exhibit deviations of over 20% from the theoretical normal-mode predictions for high HI strength with h* ≥ 0.20, for chains with as many as 100 beads, which is a larger deviation than is found for Stochastic rotation dynamics simulations for similar chains lengths and values of h*.

The medium amplitude oscillatory shear of semi-dilute colloidal dispersions. Part I: Linear response and normal stress differences

We study the medium amplitude oscillatory shear (MAOS) of a semi-dilute, hard-sphere colloidal dispersion. Through solution of the Smoluchowski equation governing the statistical distribution of suspended particles in the semi-dilute limit, we calculate the linear viscoelasticity and the normal stress differences that arise from oscillatory shear as an expansion in terms of small rates of deformation. The effects of hydrodynamic versus conservative interactions are studied via the excluded-annulus model for the hard-sphere interactions. This treats the hard-sphere interactions as a steric barrier that resides beyond the hydrodynamic radius of the particles and allows for continuous variation of the strength of hydrodynamic interactions. We see remarkable distinctions between suspensions that are freely draining and those for which hydrodynamic lubrication among the particles dominate their motion. In oscillatory shear flow of a freely draining suspension at high-frequency, the normal stress differences are generated by hard-sphere stresses. In contrast, for a hydrodynamically interacting suspension at high-frequency, the first normal stress difference derives from Brownian stresses while the second normal stress difference is dominated by hydrodynamic stresses. When the first normal stress difference is plotted parametrically against the oscillatory rate of strain, the resulting Lissajous-Bowditch curves for freely draining suspensions and those with full hydrodynamic interactions take on distinct and disparate shapes. Additionally, we use an asymptotic analysis to predict that the third harmonic of the suspension stress for hard spheres is dominated by the hydrodynamic lubrication in the limit of high frequency oscillation. Our calculations demonstrate that under MAOS, hydrodynamic interactions play a central and qualitative role in the stress response of a semi-dilute colloidal dispersion.

Active microrheology: Fixed-velocity versus fixed-force

In active microrheology, a probe particle is driven by an external force through a complex medium and its motion studied in order to infer properties of the embedding material. It is conducted in two limiting forms: either the probe is propelled by a fixed force, as with magnetic tweezers, or it is driven at a fixed velocity, as with optical tweezers. Recent work has shown that the mean probe motion can be interpreted as an effective material viscosity, but that this viscosity depends on whether the fixed-force or fixed-velocity mode is employed. We compute the effective viscosity probed by fixed-velocity active microrheology of a dilute colloidal dispersion. A comparison is made between this new result and the effective viscosity probed in the fixed-force mode. In the absence of hydrodynamic interactions, the particle-phase contributions to the effective viscosity for the two modes differ by exactly a factor of two. A simple scaling argument has been previously advanced to rationalize this difference: in the fixed-force mode, the probe is free to diffuse, and thus the relaxation time scale is set by the relative diffusivity between probe and bath. However, in the fixed-velocity mode, thermal motion of the probe particle is “frozen out” because the probe cannot diffuse; the relaxation rate is thus halved. The ratio of the two rates is independent of how quickly the probe particle is driven through the suspension—the extent and shape of microstructural deformation is the same for the two cases. In contrast, when the suspended particles interact hydrodynamically, the distortions to the suspension microstructure in the fixed-velocity versus fixed-force modes differ. We show that, depending on the strength of the hydrodynamic interactions, the ratio of the fixed-velocity to the fixed-force microstructural contributions to the effective viscosity may be as small as 1.3, and only approaches 2.0 when hydrodynamic interactions among the particles are negligibly weak. While this ratio varies both as a function of the strength of the deformation imposed and of the strength of hydrodynamic interactions, the fixed-velocity effective viscosity agrees qualitatively with that already measured for the fixed-force mode: the colloidal dispersion thins in the limit of weak hydrodynamic interactions; and it first thins and then thickens in the limit of strong hydrodynamic interactions, as the strength of deformation increases, recovering characteristics of shear-(force-) thinning and thickening well known in colloidal dispersions. The agreement between the two, and with traditional macrorheological approaches, shows that both fixed-force and fixed-velocity provide a useful tool for the interrogation of complex fluids.

Microwave irradiation induced modifications on the interfaces in SAN/EVA/PVC and PVAc/BPA/PVP ternary polymer blends: Positron lifetime study

Ternary polymer blends of poly(styrene-co-acrylonitrile)/poly(ethylene-co-vinylacetate)/poly(vinyl chloride) (SAN/EVA/PVC) and poly(vinyl acetate)/bisphenol A/polyvinylpyrrolidone (PVAc/BPA/PVP) with different compositions have been prepared by solvent casting method and characterized by positron lifetime spectroscopy and differential scanning calorimetry DSC. Phase modifications have been induced by irradiating the blends with microwave radiation. These changes have been monitored by measuring the free-volume content in the blends. The results clearly show improved interactions between the constituent polymers of the blends upon microwave irradiation. However, the free-volume data and DSC measurements are found to be inadequate to reveal the changes at the interfaces and the interfaces determine the final properties of the blend. For this we have used hydrodynamic interaction (αij) approach developed by us to measure strength of hydrodynamic interaction at the interfaces. These results show that microwave irradiation stabilizes the interfaces if the blend contains strong polar groups. SAN/EVA/PVC blend shows an increased effective hydrodynamic interaction from −3.18 to −4.85 at composition 50/35/15 upon microwave irradiation and PVAc/BPA/PVP blend shows an increased effective hydrodynamic interaction from −3.81 to −7.57 at composition 20/50/30 after irradiation.

Intrachain Dynamics of Large dsDNA Confined to Slitlike Channels

Exploration of intrachain hydrodynamics of dsDNA within channels has been limited to indirect analysis of global coil dynamics. In this Letter, we isolate hydrodynamic interactions within single molecules of dsDNA confined to slitlike channels by making use of density covariance measurements. We show that the strength of hydrodynamic interactions in DNA is dependent on the intrachain correlation length and that screening by symmetry in slitlike confinement results in a screening length that is proportional channel height. Moreover, we directly show the partial draining nature of the blobs formed by dsDNA in slits and predict under what conditions a dsDNA blob should obey nondraining Zimm behavior.

Расчет гидродинамического взаимодействия в сверхтекучей жидкости методами вычислительной теории потенциала

The problems of superfluid liquid hydrodynamics are quite relevant today. Previously uninvestigated problem of multiphase flow of superfluid liquid is considered in the paper. The tasks of streamline of groups of objects of different shapes by a flat stationary flow of superfluid liquid is formulated in terms of velocity potential in the form of differential equations in specific derived and boundary integral equations of potential theory. It was shown that for such a flow, the main force, affecting objects, is the strength of hydrodynamic interaction. Calculations were made using a standard algorithm for the boundary element method. The dependences of hydrodynamic interaction forces on the distance between the streamlined objects were obtained for circular and elliptical shaped objects. The cases of a large number of regularly situated circular objects were considered. It was shown that such a system is stable relating to perturbations along the flow velocity vector, and is unstable with respect to perturbations perpendicular to the vector. The considered problems illustrate the features of multiphase flow of superfluid liquid and can serve as model problems for studying such flows.

An effective tool to characterize adhesion at the interface of binary polymer blends: a free volume study

Abstract In this work, an effective method of characterizing the adhesion at the interface of partially miscible and immiscible binary polymer is used. Three polymer blends namely PS/PMMA, PP/NBR and PVC/SAN were irradiated with e-beam and microwaves, in order to bring about interfacial modification, and were characterized in terms of free volume measured by Positron Lifetime technique. The changes in free volume parameters upon e-beam and microwave irradiation on the two systems clearly shows improved interaction between the blend constituents as a result of cross-linking through hydrogen bond formation. However, the extent of interfacial modification brought in at different weight compositions is not forthcoming. Therefore, the hydrodynamic interaction parameter α, which accounts for friction at the interface between the dissimilar chains of the blend constituents, is evaluated from the very same free volume data. Since α represents the strength of hydrodynamic interaction, it could be taken as an indicator of the adhesion strength at the interface. Present results showed that both e-beam and microwave irradiations can be used to stabilize the interfaces in binary polymer blends as evidenced by hydrodynamic interaction.

Influence of variable hydrodynamic interaction strength on the transport properties of coiled polymers

We have computed the hydrodynamic radius Rh and intrinsic viscosity [eta] of a large number of random coil polymer models by path integration. We examine the effects of chain length, solvent quality, monomer size and shape, and chain stiffness on the approach of these transport properties to the limit of large molecular mass. For many of the models, we have also calculated the ensemble-averaged solvent velocity field in the vicinity of the coil as it moves with constant drift velocity under the action of an external force. Naive scaling arguments predict alpha=nu and beta=3(nu-1) , where alpha , beta , and nu are the exponents controlling the chain length behavior of Rh , [eta] , and Rg , respectively. We present evidence for a "draining crossover" that quantifies the slow convergence of the transport properties to their asymptotic scaling behavior. Indeed, the convergence is so slow that effective alpha and beta exponents rarely agree with the naive predictions at typical molecular masses. For the same chain models, Rg converges rapidly to its asymptotic behavior, indicating that the effect is not due to a crossover from theta to swollen behavior, as often stated. Solvent quality, monomer size, and chain stiffness all influence the draining crossover. Our results call into question the common practice of extracting metrical data, e.g., characteristic ratios, directly from polymer solution transport properties.

Interfacial stabilization of binary polymer blends through radiation treatment: A free volume approach

AbstractPartially miscible blends of PS/PMMA and PVC/SAN with different compositions have been prepared by solvent casting technique and characterized by Positron Annihilation Lifetime Technique. Phase modification in these systems has been brought out using e‐ beam and microwave irradiation. These changes have been studied through free volume monitoring. The changes in free volume parameters upon e‐beam and microwave irradiation on the two systems clearly shows improved interaction between the blend constituents as a result of cross‐linking through hydrogen bond formations. The free volume data indicates the overall changes in the blend with respect to free volume, but the extent of interfacial modification occurred for different weight compositions are not forthcoming upon irradiation. Therefore, the hydrodynamic interaction parameter α which accounts for excess friction thus dissipation of energy at the interface between the constituents of polymer blend system is evaluated for this purpose from the very same free volume data. The parameter α represents the strength of hydrodynamic interaction or it is a measure of the adhesion strength at the interface. Present data based on hydrodynamic interaction parameter indicated that both e‐beam and microwave irradiation can be used to stabilize the interfaces in binary polymer blends but the better interface modifier on the two blend systems studied. But which route that is e‐beam or microwave depends on the presence of non‐polar/polar groups respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)