To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow. DOI: 10.1103/PhysRevLett.119.158001 Shear thickening is an increase in the viscosity η of a concentrated suspension of particles in a fluid as the shear stress σ or shear rate rises beyond a critical value [1]. When suspensions shear thicken at high volume fractions ϕ it is frequently accompanied by complex behavior that includes S-shaped flow curves [2,3] and slow stress decays [4]. The degree of shear thickening can range from a few fold to orders of magnitude increase in η as a function of σ. These distinctions are typically used as working definitions for continuous shear thickening (CST) and discontinuous shear thickening (DST) in the literature [5]. We define weak and strong thickening using the power β as the slope of logðηÞ plotted against logðσÞ [6], where weak thickening occurs at 0.1 ≤ β ≤ 0.7 and strong thickening occurs at 0.7 < β ≤ 1.0. These categories are convenient classifications of the magnitude of the rheological response rather than a fundamental physical transition. Shifting the value of demarcation between weak and strong thickening has no qualitative impact on the state diagrams presented.Dilatancy is sometimes observed with strong shear thickening. Reynolds showed that a dilatant suspension expands in volume because particles cannot otherwise find direct flow paths within the confined environment [7]. This tendency to expand generates a normal thrust, and causes the first normal stress difference N 1 to switch from negative to positive values if boundaries are spherical in shape and surface tension is negligible [5]. The onset stresses for shear thickening and dilatancy do not necessarily coincide [6,8]. Similarly, a sheared suspension that freely expands in volume will not shear thicken because of the lack of a confining stress [9,10].To date, neither hydrodynamics nor friction has successfully explained the full range of flow phenomena in concentrated suspensions. When particles are pushed into cl...

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The shear rate-dependent rheological properties of soft to rigid colloidal suspensions are studied using computational models. We show that a contact force defined based on an elastohydrodynamic deformation theory captures an important rheological behavior of colloidal suspensions: While near hard-sphere particles exhibit a strong and continuous shear-thickening the evolves to a constant viscosity state, soft suspensions undergo a second shear-thinning regime at high Péclet numbers when the hydrodynamic stresses become larger than the modulus of the colloidal particles. We measure N 1 and N 2 to be large and negative in the shear-thickening regime; however, for soft spheres at the onset of second shear-thinning N 2 reduces in magnitude and eventually becomes positive. We show that for near hard-sphere suspensions, colloidal pressure, shear stress, and normal stress difference coefficients tend to diverge near the maximum packing fraction while

We identify the sequence of microstructural changes that characterize the evolution of an attractive particulate gel under flow and discuss their implications on macroscopic rheology. Dissipative particle dynamics is used to monitor shear-driven evolution of a fabric tensor constructed from the ensemble spatial configuration of individual attractive constituents within the gel. By decomposing this tensor into isotropic and nonisotropic components we show that the average coordination number correlates directly with the flow curve of the shear stress versus shear rate, consistent with theoretical predictions for attractive systems. We show that the evolution in nonisotropic local particle rearrangements are primarily responsible for stress overshoots (strain-hardening) at the inception of steady shear flow and also lead, at larger times and longer scales, to microstructural localization phenomena such as shear banding flow-induced structure formation in the vorticity direction. DOI: 10.1103/PhysRevLett.118.048003 Thixotropic elastoviscoplastic (TEVP) materials are a broad class of structured fluids that include (but are not limited to) most colloidal gels [1], nano emulsions [2], crude oils [3,4], and many biological systems such as blood clots and actin networks [5,6]. As a result of their complex underlying microstructure, TEVPs exhibit a wide range of rich and complex thermo-mechanical properties: Below a critical stress, the microstructural network formed by individual particles remains intact and resists large deformations by external forces. At this stage the macroscopic response of the material is similar to that of a viscoelastic solid. By progressively increasing the applied load, the material reaches its "yield stress" and starts to flow [7]. At this point the particle network that is responsible for solidlike response of the macroscopic sample undergoes plastic rearrangements over an increasingly wide range of length scales [8]. Upon complete yielding of this network, plastic flow results ultimately in a viscouslike response; however, as a result of constant formation and breakage events, the particle-level microstructure continues to evolve giving rise to thixotropic behavior. The many-body nature of the problem means that local forces exerted on a single particle change its energy landscape, which consequently defines its subsequent association or dissociation rate to neighboring particles [9,10]. When combined with multibody hydrodynamic effects in these fluids [11], the resulting microstructure-flow relationship becomes complex and may show long time scale transient behavior and multiple steady states [1,12]. This leads to a wide range of timedependent responses that can also be observed, including microphase separation [13], vorticity aligned structure formation [14][15][16], local rigid plug formation and shear banding [17], plus shear-induced rejuvenation of the particle network [18].Although the general form of the flow curve (relating the shear stress to shear rate) and some transient pheno...

a b s t r a c tIn this study two main groups of viscosity measurement techniques are used to measure the viscosity of a simple fluid using Dissipative Particle Dynamics, DPD. In the first method, a microscopic definition of the pressure tensor is used in equilibrium and out of equilibrium to measure the zero-shear viscosity and shear viscosity, respectively. In the second method, a periodic Poiseuille flow and start-up transient shear flow is used and the shear viscosity is obtained from the velocity profiles by a numerical fitting procedure. Using the standard Lees-Edward boundary condition for DPD will result in incorrect velocity profiles at high values of the dissipative parameter. Although this issue was partially addressed in Chatterjee (2007), in this work we present further modifications (Lagrangian approach) to the original LE boundary condition (Eulerian approach) that will fix the deviation from the desired shear rate at high values of the dissipative parameter and decrease the noise to signal ratios in stress measurement while increases the accessible low shear rate window. Also, the thermostat effect of the dissipative and random forces is coupled to the dynamic response of the system and affects the transport properties like the viscosity and diffusion coefficient. We investigated thoroughly the dependency of viscosity measured by both Eulerian and Lagrangian methodologies, as well as numerical fitting procedures and found that all the methods are in quantitative agreement.