On the wall slip phenomenon of elastomers in oscillatory shear measurements using parallel-plate rotational rheometry: II. Influence of experimental conditions

The use of parallel-plate rotational rheometry to characterize ex situ pre-prepared samples of rubber-like polymers is motivated by, for example, the investigation of magneto-rheological elastomers. When exceeding a critical excitation amplitude in oscillatory shear experiments, these elastomeric samples are prone to slip at the sample-plate contact interface. This phenomenon, known as wall slip, starts to occur at the sample's outer rim and leads to an imperfect force transfer onto the sample. This results in a systematic error of measured rheological material quantities.A thorough investigation is presented to reveal how this phenomenon is affected by selected experimental conditions, namely the static axial preload and measuring frequency. For this purpose disc-shaped samples composed of an unfilled silicone rubber are prepared by casting and examined by means of a controlled stress rotational rheometer equipped with a serrated rotor configuration.The oscillatory strain sweep experiments suggest that wall slip, exclusively present at the serrated rotor surface, is significantly influenced by the static preload. In contrast, only a slight frequency dependence is observed within the examined experimental conditions.Further insights into the wall slip mechanism were attained by two novel methodologies. It is shown that it is possible to produce a master curve for the various applied preloads. This demonstrates that the physical mechanism behind wall slip is independent of the axial force. Furthermore, we derive an empirical model for the criterion governing the onset of wall slip. This links the critical stress at which wall slip is initiated to the static friction condition and geometrical aspects of the rotor configuration. From this it is anticipated that the conditions for reliable experiments involving ex situ pre-prepared samples composed of low damping elastomers can, in the future, be estimated a priori.     

On the wall slip of polymer blends

The slip flow of the polypropylene (PP)/poly[ethylene‐co‐(vinyl acetate)] (EVA) system was studied in a capillary rheometer for shear rates of 40–1000 s^?1 at four temperatures. Three dies made of aluminum with a length/diameter (L/D) ratio of 15 and diameters of 1.59, 1.19, and 0.79 mm provided the flow data. Calculations of the slip velocity by the Mooney method showed power‐law behavior with the stress. Blends were prepared at various proportions of PP and EVA for observation of the variation of the slip velocity for different compositions and temperatures. Direct microscopic observations of the slip layer on extruded samples showed domains of the dispersed phase unevenly distributed between the slip layer and the core and provided estimates of the thickness of the layer adjacent to the capillary wall. Results showed that the viscosity in the slip layer was 10–100 times lower than that in the bulk for the same value of the shear stress. In terms of the extrapolation length, the development of the slip layer was the result of different disentanglement dynamics of the molecules in the slip layer in comparison with those in the bulk. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 303–316, 2002     

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as “particle ordering,” is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.     

Effect of wall slip on the shear flow of a polymer in a channel with a wavy bottom

The effect of stick and wall slip boundary conditions on the specific features of the shear flow of viscous polymers in a confined two-dimensional channel with a wavy bottom is studied. The distribution of flow-rate disturbances across the transverse cross section of the channel is calculated by the numerical simulation of the Navier-Stokes equation for an incompressible fluid at arbitrary amplitudes and an arbitrary wave number of the wall. The wall slip is modeled by the introduction of a thin layer of a low-viscosity fluid at the bottom face. Slippage leads to a marked enhancement of flow rate disturbances including inertial advection. The results agree with the known analytical solutions for the low-amplitude wall wave.     

中空铜纳米线的拉伸断裂分布与初始滑移分布的关系

构建了系列球形中空结构的纳米线(NW),采用分子动力学(MD)对每个模型300个不同初始态的样本开展拉伸形变模拟。并利用基于密度的噪声应用空间聚类(density-based spatial clustering of applications with noise,DBSCAN)机器学习算法,获得了初始滑移面的位置。基于大数据统计,分析了初始滑移位置分布以及断裂位置分布两者之间的相关性。研究结果表明：当内部中空半径较小时,断裂位置分布形成于塑性形变阶段,初始滑移分布与断裂位置分布之间无显著的相关性;但是对于脆性特征明显的大中空半径的NW,高能内表面诱导产生的滑移面迅速积累,产生颈缩并导致最终的断裂。因此当内部中空结构达到一定尺寸时初始滑移位置的分布与最终断裂位置的分布之间有明确的因果关系。     

中空铜纳米线的拉伸断裂分布与初始滑移分布的关系

构建了系列球形中空结构的纳米线(NW)，采用分子动力学(MD)对每个模型300个不同初始态的样本开展拉伸形变模拟。并利用基于密度的噪声应用空间聚类(density-based spatial clustering of applications with noise，DBSCAN)机器学习算法，获得了初始滑移面的位置。基于大数据统计，分析了初始滑移位置分布以及断裂位置分布两者之间的相关性。研究结果表明：当内部中空半径较小时，断裂位置分布形成于塑性形变阶段，初始滑移分布与断裂位置分布之间无显著的相关性；但是对于脆性特征明显的大中空半径的NW，高能内表面诱导产生的滑移面迅速积累，产生颈缩并导致最终的断裂。因此当内部中空结构达到一定尺寸时初始滑移位置的分布与最终断裂位置的分布之间有明确的因果关系。     

Influence of wall slip and jump in wall temperature on transport of heat energy in hybrid nanofluid

Journal of Thermal Analysis and Calorimetry - It is the first time that partial slip and jump in wall temperature during transfer of thermal energy in hybrid nanofluid are considered...     

Distinguishing between dynamic yielding and wall slip in a weakly flocculated colloidal dispersion

Silica spheres coated with grafted poly(butyl methacrylate) chains and dispersed in a poor solvent at low to moderate volume fractions display high elastic and loss moduli, together with viscosities that diverge at a dynamic yield stress and decrease by several orders of magnitude with increasing stress. The elastic modulus and dynamic yield stress increase rapidly with volume fraction, but remain in constant ratio. Longer grafted chains produce stronger interparticle attractions, resulting in a smaller yield strain and larger elastic modulus. An apparent Newtonian plateau at extremely low shear rates in creep experiments is argued to result from slip at the wall. Thus, attractions between these grafted layers in a poor solvent generate a pseudo plastic solid, a metastable state that persists indefinitely and demonstrates little affinity for the walls of the rheometer, hence the slip. In contrast, depletion flocculated dispersions with higher volume fractions but weaker attractions produce very similar rheology but with a true Newtonian viscosity and adhesion to the wall.     

Boundary slip and wetting properties of interfaces: correlation of the contact angle with the slip length

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.     

Adhesion of viscoelastic materials to rigid substrates. II. Tensile strength of adhesive joints

Measurements have been made of the tensile force required to pull a disk of a model viscoelastic adhesive away from an inert rigid substrate. Over a wide range of temperature and rate of deformation of the adhesive the results were found to yield a single master relation in terms of deformation rate by means of the Williams, Landel and Ferry rate—temperature equivalence for viscous materials. Thus, the strength of adhesion is due mainly to dynamic effects in the adhesive of a viscous nature, in a similar way to the cohesive strength of viscoelastic materials. This similarity is attributed to a common failure mechanism: initial failure at a highly stressed point, followed by spreading of the failure zone under local stresses which are governed by the dynamic response of a compliant material. An increase in the strength of adhesion is observed with decreasing thickness of the adhesive layer. This is also explained by the proposed failure mechanism if failure starts at a critical amount of local deformation energy, a form of Griffith's fracture criterion.     

Effect of nanometric-scale roughness on slip at the wall of simple fluids

It is commonly acknowledged that roughness decreases the aptitude of simple liquids to exhibit flow with slip at solid interfaces. Most available studies have, however, been conducted on substrates for which both the surface chemistry and the roughness were varied simultaneously, making it difficult to identify their respective role on wall slip. To overcome this difficulty, we have developed a series of surfaces formed by grafting hyperbranched polymeric nanoparticles on a smooth, dense, self-assembled monolayer of SiH-terminated short poly(dimethylsiloxane) oligomers, allowing us to vary independently the surface density, the height, and the width of the grafted nanoparticles, and thereby the roughness parameters, while keeping similar surface chemistry. On such substrates, the boundary condition for the flow velocity of hexadecane has been characterized through near-field laser velocimetry. We demonstrate that decreasing the wavelength of the roughness at a fixed height strongly decreases slip, while increasing the height of the nanoparticles at a fixed aspect ratio of the roughness also dramatically affects slippage.     

Nonequilibrium molecular dynamics of the rheological and structural properties of linear and branched molecules. Simple shear and poiseuille flows; instabilities and slip

Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edward's [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls.     

Adhesion of viscoelastic materials to rigid substrates. III. Energy criterion for failure

Measurements are described of the strength of a model adhesive joint subjected to (1) tensile rupture, with the interface containing a small unbonded region of varying size, and (2) pure shear deformation, in the form of a partly unbonded sheet. These, and previous measurements of resistance to peeling separation, are all shown to be consistent with an energy criterion for adhesive failure. The characteristic failure energy per unit area of interface has been determined for the model adhesive material as a function of the effective rate of detachment, over a wide range covering almost the entire spectrum of viscoelastic response. The values obtained are found to increase from levels only slightly higher than thermodynamic considerations would predict, i.e., 10²?10³ ergs/cm², at low rates of crack propagation, up to a value of about 10⁶ ergs/cm² at high rates when the material responds in a glasslike manner. These results suggest that the failure energy has two components: the (reversible) work of adsorption and the (irreversible) work of deformation of the adhesive in effecting separation.     

Effect of liquid slip in electrokinetic parallel-plate microchannel flow

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in parallel-plate microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. Our parametric results suggest that electrokinetic transport phenomena and liquid slip at channel walls are both important and should be considered simultaneously. Their significance depends on channel wall material, electrolyte concentration, and pH. For pressure-driven-flow, liquid slip counteracts the effect by the electrical double layer and induces a larger flow rate. A higher apparent viscosity would be predicted if slip is neglected. For electroosmotic flow, liquid slip alters the flow rate by about 20% for a thick electrical double layer. Our results provide design guidelines to precisely control time-dependent microflow in hydrophobic microfluidic microelectromechanical system devices.     

Passive solute separation in AC electroosmosis including surface charge-coupled hydrodynamic slip effects

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called “crossover phenomenon” for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.     

Effect of surface roughness on rate-dependent slip in simple fluids

Molecular dynamics simulations are used to investigate the influence of molecular-scale surface roughness on the slip behavior in thin liquid films. The slip length increases almost linearly with the shear rate for atomically smooth rigid walls and incommensurate structures of the liquid/solid interface. The thermal fluctuations of the wall atoms lead to an effective surface roughness, which makes the slip length weakly dependent on the shear rate. With increasing the elastic stiffness of the wall, the surface roughness smoothes out and the strong rate dependence is restored again. Both periodically and randomly corrugated rigid surfaces reduce the slip length and its shear rate dependence.     

Investigation of shear failure mechanisms of pressure‐sensitive adhesives

We report new experiments investigating the failure mechanisms in shear, of thin layers of acrylic pressure‐sensitive adhesives (PSA). We have developed a novel experimental device able to shear a soft adhesive layer confined between a rigid hemispherical lens and a rigid glass substrate. Using the resources of in situ contact visualization, the nonhomogeneous deformation of the layer and the shear failure processes were observed optically. Depending on the rheological properties of the adhesive, ratios of the contact radius over the layer thickness of 10–30 were achieved, mimicking well the contact conditions encountered in a thin adhesive layer within a joint. When the adhesive was weakly crosslinked, we observed a fluid‐like behavior and could measure a reasonable value for the viscosity of the PSA, implying that flow can occur in the layer and failure will occur by creep. On the other hand, for a more crosslinked adhesive, closer to what is used in applications, a stick‐slip peeling behavior was observed, which involves a coupling between peeling mechanisms at the leading edge of the contact and interfacial slippage. Such a process suggests a failure by fracture rather than by creep. Interestingly, the peeling mechanisms and the associated stress levels change significantly when the layer becomes as thin as 20 μm, implying a fracture process that is controlled by a critical energy release rate in shear G_IIc rather than by a critical shear stress causing failure of the interfacial bonds. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3316–3330, 2005     

Determination of the Navier slip coefficient of microchannels exploiting the streaming potential

For most microchannels made of hydrophobic materials such as polymers, velocity slip occurs at the wall, affecting volumetric flow rate of electroosmotic flow Q(eof) and streaming potential (??(str)/?z). Since most techniques exploit Q(eof) or (??(str)/?z) to determine the zeta potential, ζ, it is very difficult to measure ζ of hydrophobic walls, if the slip coefficient b is not found a priori. Until now, Q(eof) and (??(str)/?z) are known to depend on ζ and b in a same functional form, which makes it impossible to estimate ζ or b separately using measurements of Q(eof) and (??(str)/?z). However, exploiting the analytic formula for Q(eof) and (??(str)/?z) derived in the present work, it is found that the effect of ζ and that of b on Q(eof) and (??(str)/?z) can be separated from each other by varying the bulk ionic concentration. Thus, the slip coefficient as well as the zeta potential of hydrophobic microchannels can be found with reasonable accuracy by means of a nonlinear curve fitting method using measured data of Q(eof) and (??(str)/?z) at various bulk ionic concentrations. The present method allows an accurate estimation of slip coefficient of hydrophobic microchannels, which is quite simple and cheap compared with methods employing microparticle velocimetry.     

Modeling the failure of an adhesive layer in a peel test

The failure of an adhesive layer in a 180° peel test is analyzed by the modeling of the peel as an elastica. The adhesive is assumed to fail by a process of cavitation and fibrillation. The resistance of the fibrillated material to deformation gives rise to a rate‐dependent peel force F. Governing equations, which take into account the large deformation of the fibrillated materials, are derived. Numerical solutions are obtained for the special case of a linear, viscous adhesive. Assuming a critical strain failure criterion for the fibrils, we show that the peel rate is inversely proportional to the square root of the adhesive thickness and directly proportional to F^3/4. The connection between the peel test and the tack test is also discussed. The maximum traction on the peel arm is found to be directly proportional to the peel force and inversely proportional to the adhesive thickness. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2277–2291, 2002     

Reconciling slip measurements in symmetric and asymmetric systems

In the past decade, the slip of simple liquids on solid surfaces has been demonstrated by many groups. However, the slip of liquids on wettable surfaces is heavily debated. Using colloid probe atomic force microscopy (AFM), we found the slip length of di-n-octylphthalate in a symmetric wettable system (silica) to be around 11 nm, which raises the question of what the measured slip length in an asymmetric hydrophilic-hydrophobic system would be. To answer this question, we investigated liquid slip in one symmetric nonwettable system (hydrophobic DCDMS or OTS) and in one asymmetric hydrophilic (silica)-hydrophobic (DCDMS) system by the same method at driving velocities of between 10 and 80 μm/s. The slip results obtained from the three systems are in agreement with each other, and this comparison provides a means to self-assess the accuracy and reproducibility of the measured force curves and the fitted slip length in our systems. Furthermore, this method provides access to reliable values of the actual slip length on any investigated flat surface in an asymmetric system, avoiding the difficulty of preparing a symmetric probe/flat surface system in a colloid probe AFM force measurement.