Abstract
Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening and shear jamming. The essential contribution of particle surface roughness and adhesive forces confirms that stress-activated frictional contacts can play a key role in these phenomena. Here, by employing a system of microparticles coated by responsive polymers, we report experimental evidence that the relative contributions of friction, adhesion, and surface roughness can be tuned in situ as a function of temperature. Modifying temperature during shear therefore allows contact conditions to be regulated, and discontinuous shear thickening to be switched on and off on demand. The macroscopic rheological response follows the dictates of independent single-particle characterization of adhesive and tribological properties, obtained by colloidal-probe atomic force microscopy. Our findings identify additional routes for the design of smart non-Newtonian fluids and open a way to more directly connect experiments to computational models of sheared suspensions.
Highlights
Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening and shear jamming
Shear thickening (ST) is a generic phenomenon that occurs when the shear stress σ increases faster than linearly with the shear rate γ_, so that the viscosity η σ=γ_effectively increases with shear rate. It can be observed in a broad range of materials, but nowhere is it more prominent than in dense suspensions of solid particles[1,2]. This phenomenon may take the severe form known as discontinuous shear thickening (DST)[1,3,4,5,6,7], where the suspension’s viscosity increases by orders of magnitude at a critical shear rate, or in the most extreme cases, the suspension may even solidify under shear—an occurrence known as shear jamming (SJ)[8,9,10]
A distinctive feature of PNIPAM brushes is that they undergo a swelling–deswelling transition in water across a lower critical solution temperature (LCST) of 30–33 °C32
Summary
Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening and shear jamming. It can be observed in a broad range of materials, but nowhere is it more prominent than in dense suspensions of solid particles[1,2] This phenomenon may take the severe form known as discontinuous shear thickening (DST)[1,3,4,5,6,7], where the suspension’s viscosity increases by orders of magnitude at a critical shear rate, or in the most extreme cases, the suspension may even solidify under shear—an occurrence known as shear jamming (SJ)[8,9,10]. While fluid films allow suspended particles to slide past each other at low shear, beyond a critical shear stress the hydrodynamic lubrication films between particles break down This occurrence results in particles that are effectively in asperity–asperity contact and can engage in frictional interactions via boundary lubrication. Comparing the nanotribology and rheology of these model colloids allows us to test the relative contributions of friction, adhesion, and surface roughness and to tune them during shear
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