We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear-thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspension's volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.Flow non-linearities attract fundamental interest and have major consequences in a host of practical applications [1,2]. In particular, shear-thickening (ST), a viscosity increase from a constant value (Newtonian flow-Nw) upon increasing shear stress (or rate) at high volume fraction φ, can lead to large-scale processing problems of dense pastes, including cement slurries [3]. Several approaches have been proposed to describe the microscopic origin of shear-thickening [4][5][6][7]. The most common explanation invokes the formation of "hydroclusters", which are responsible for the observed continuous viscosity increase [6,8,9] and which have been observed for Brownian suspensions of moderate volume fractions [10,11]. However, this description no longer holds for bigger particles and denser pastes, where contact networks can develop and transmit positive normal stresses [12]. Moreover, the link between hydroclusters and CST for non-Brownian suspensions is still a matter of debate [13]. Additionally, dense, non-Brownian suspensions can also show sudden viscosity divergence under flow [14][15][16][17] with catastrophic effects, such as pumping failures. In contrast to a continuous viscosity increase at any applied rate, defined as continuous shear-thickening (CST), the appearance of an upper limit of the shear rate defines discontinuous shear-thickening (DST). This CST to DST transition is observed when the volume fraction of the flowing suspension is increased above a critical value, which depends on the system properties, e.g. polydispersity or shape, and on the flow geometry [3,18]. An explanation for its microscopic origin is still lacking [19]. Moreover, experiments have demonstrated that the features of the viscosity increase (slope, critical stress) can be controlled by tuning particle surface properties such as roughness [20] and/or by adsorbing polymers [21,22]. These findings suggest that inter-particle contacts play a crucial role in the macroscopic flow at high volume fractions. A more precise description of these contacts is therefore essential to interpret the rheological behavior.In this paper, we present a unified theoretical framework, supported by both numerical simulations and experimental data, which describes the three flow regimes of rough, frictional, non-Brownian particle suspensions (Nw,CST,DST) and links the Nw-ST (in terms of shear) and the CST-DST transitions (in terms of volume fraction) to the local friction. Our micro...

Reversible addition−fragmentation chain transfer (RAFT) polymerization of the reactive monomer γ-methacryloxypropyltrimethoxysilane (γMPS) mediated by 2-cyanoprop-2-yl dithiobenzoate (CPDB) has been studied in dioxane using 2,2‘-azobis(isobutyronitrile) (AIBN) as initiator. Conditions were optimized for a polymerization temperature of 80 °C, and controlled PγMPS chains (up to M n = 40 000 g mol-1) exhibiting low polydispersity indexes (PDI < 1.3) were synthesized. Their characterization by matrix-assisted laser desorption ionization time-of-flight (MALDI-ToF) mass spectrometry showed that the expected structure was obtained although degradation of the dithiobenzoate chain end occurred. Syntheses of block copolymers based on γMPS and methyl methacrylate (MMA) were performed starting from either the PγMPS (M n = 32 100 g mol-1, PDI = 1.16, PMMA calibration) or the PMMA block (M n = 21 000 g mol-1, PDI = 1.14). The success of the block copolymerization was showed by the shift toward higher molar mass of the size exclusion chromatography (SEC) chromatograms recorded before and after block copolymerization. Proton NMR analyses of P(MMA-b-γMPS) allowed to calculate the molar mass of the PγMPS blocks (17 400 g mol-1) which agreed with the targeted one (18 150 g mol-1).

RAFT polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA, 1100 g mol À1 ) was first performed using cyanoisopropyl dithiobenzoate (CPDB). The control of PEGMA homopolymerization, carried out in THF ([PEGMA] ¼ 40 wt %) at 65 C (reflux) using 2,2 0 -Azobis(isobutyronitrile) (AIBN) as initiator, was shown by the linear increase of molar masses and the low polydispersity indices (PDI) observed with conversion and also by the determination of a high chain transfer constant (C tr ¼ 9.1) for CPDB in this system. Copolymerization of PEGMA with methacrylic acid (MAA) ([PEGMA]/[MAA] ¼ 40/60) was then undertaken first in dioxane at 85 C. High conversion (89%) was obtained in 3 h without any composition drift and with a good level of control (PDI \ 1.40). When the polymerization was performed in water, a strong increase in polymerization rate was observed with almost quantitative conversion (98%) in 2 h without affecting the level of control of the final copolymers (PDI $ 1.30). These last results were tentatively explained by the formation of hydrophobic domains in which the polymerization occurred as in bulk.