Shear Thinning and Local Melting of Colloidal Crystals

Abstract

Phenomena such as shear thinning and thickening, occurring when complex materials are exposed to external forces, are generally believed to be closely connected to changes in the microstructure. Here, we establish a direct and quantitative relation between shear thinning in a colloidal crystal and the surface area of the locally melted region by dragging a probe particle through the crystal using optical tweezing. We show that shear thinning originates from the nonlinear dependence of the locally melted surface area on the drag velocity. Our observations provide unprecedented quantitative evidence for the intimate relation between mechanical properties and underlying changes in microscopic structure. The response of materials to external stresses and strains is of central importance for many applications in science and technology [1,2]. At macroscopic length scales the microstructure of many materials and complex fluids is often neglected and continuum models are applied to describe their flow behavior [1][2][3]. As a consequence of progressive miniaturization, the effect of the microscopic structure on mechanical properties becomes increasingly significant, which has invoked the development of micromechanical models [3][4][5][6]. Also in complex fluids there are many suggestions that the rearrangement of the local structure is the basis for behavior like shear thickening and thinning [1,2,[7][8][9][10]. The key here is the simultaneous characterization of the external forces and changes in the microstructure. We achieve this using active microrheology of colloidal crystals, which enables us to directly connect shear thinning to local melting.A two-dimensional, hexagonal colloidal crystal of melamine spheres of radius R c ¼ 1:5 m in water is deformed using a probe particle trapped in an optical tweezer. The lattice spacing a and number density are 3:5 m and 0:087 m À2 respectively, and the size of singledomain crystallites is typically larger than 250 Â 250 m 2 . Adding a very small amount (less than one probe particle per $3000 small particles) of large polystyrene probe particles (R p ¼ 7:75 m) to the suspension results in a crystal with built-in probe particles as shown in Fig.

Active microrheology in active matter systems: Mobility, intermittency, and avalanches

“…Other studies have examined anomalous diffusion properties of the probe 9,10 , features of the driven particle velocity fluctuations, and threshold-tomotion or depinning type phenomena [11][12][13] . In ordered systems, a driven particle can induce localized melting 14 or shear thinning effects 15 . Driven particles or intruders have also been used to study the onset of jamming as a function of the system density [16][17][18][19][20] .…”

Avalanches, plasticity, and ordering in colloidal crystals under compression

“…Due to their size scale, colloids provide the advantage that microscopic information on the individual particle level can be directly accessed, something which is normally difficult or impossible in smaller scale systems such as nanoparticles, molecules, or atoms 2,3 . Additionally, there are various methods such as optical techniques 4,5 for controlling colloidal ordering and manipulating individual colloids. Examples of phenomena that have been studied with colloids include two-dimensional melting transitions 6,7 , solid-to-solid phase transitions 8 , glassy dynamics 3 , commensurate and incommensurate phases [9][10][11][12] , depinning behaviors [13][14][15][16] , self-assembly 17,18 , and dynamic sorting [19][20][21] .…”

Exact correlations in a single-file system with a driven tracer

“…The SS IC is the one expected to be realized in most experiments [29,[31][32][33], as the tracer particle is usually held and the rest of the fluid left untouched. Taking mesurements for the QU IC would require holding all the particles initially at fixed positions using either optical/magnetic tweezers or a periodic field, and then turning them off before starting the noisy dynamics.…”

“…Due to recent developments of experimental techniques like active microrheology and microfluidics [27,28] there has been a resurgence of interest in studying locally driven interacting particle systems. In this paper we consider the case where the TP is driven externally and study its effect on the surrounding unbiased bath particles.Single driven TPs (DTPs) in quiescent media have been used to probe rheological properties of complex media such as polymers [29,30], granular media [31,32] or colloidal crystals [33], and also to study directed cellular motion in crowded channels [34] or the active transport of vesicles in a crowded axon [35]. Some practical examples of DTPs are a charged impurity being driven by an applied electric field or a colloidal particle being pulled by an optical tweezer in the presence of other colloidal particles performing random motion.…”

“…Single driven TPs (DTPs) in quiescent media have been used to probe rheological properties of complex media such as polymers [29,30], granular media [31,32] or colloidal crystals [33], and also to study directed cellular motion in crowded channels [34] or the active transport of vesicles in a crowded axon [35]. Some practical examples of DTPs are a charged impurity being driven by an applied electric field or a colloidal particle being pulled by an optical tweezer in the presence of other colloidal particles performing random motion.…”