Passive and Active Microrheology of Hard-sphere Colloids

Wilson, Laurence G.; Harrison, Andrew; Schofield, Andrew B.; Arlt, Jochen; Poon, Wilson

doi:10.1021/jp8079028

Cited by 90 publications

(89 citation statements)

References 33 publications

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“…Using a power-law fit we find that F $ ðv À v c Þ with close to 1=2. This implies ''velocity or shear thinning'' similar to that observed in microrheology studies of dense hard sphere systems [9,22,23,25] and bulk rheology of colloidal crystals [19,20]. Moreover, the v 1=2 behavior has been reported to be directly related to a change in the structure factor, suggesting a change in the local structure [7,8].…”

supporting

confidence: 56%

Shear Thinning and Local Melting of Colloidal Crystals

Dullens

Bechinger

2011

Phys. Rev. Lett.

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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.

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supporting

confidence: 56%

Shear Thinning and Local Melting of Colloidal Crystals

Dullens

Bechinger

2011

Phys. Rev. Lett.

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“…With passive motion, this transformation is done using the Generalized Stokes Einstein equation that relates the mean-squared displacement of the probe and the fluid parameters [4-7, 9, 10]. With active motion, it is obtained by comparing the force applied to the probe and the distances by which it moved [4,11,12]. The conversion of the time or frequency dependences of some local µ-rheology variables into a macroscopic rheological response remains however a challenging issue.…”

Section: -Introductionmentioning

confidence: 99%

Magnetic wire-based sensors for the microrheology of complex fluids

et al. 2013

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Abstract:We propose a simple µ-rheology technique to evaluate the viscoelastic properties of complex fluids. The method is based on the use of magnetic wires of a few microns in length submitted to a rotational magnetic field. In this work, the method is implemented on a surfactant wormlike micellar solution that behaves as an ideal Maxwell fluid. With increasing frequency, the wires undergo a transition between a steady and a hindered rotation regime. The study shows that the average rotational velocity and the amplitudes of the oscillations obey scaling laws with well-defined exponents. From a comparison between model predictions and experiments, the rheological parameters of the fluid are determined. -IntroductionRheology is the study of flow and deformation of fluids when they are submitted to mechanical stresses. Conventional rheometers determine the relationship between strain and stress in steady or oscillating flow on samples of a few milliliters. µ-rheology in contrast studies the motion of micron-size probe particles that are thermally fluctuating via the interactions with a surrounding medium, or particles that are forced by an external field. In the first case, the µ-rheology is said to be passive, in the second active. In both cases, the motion of the probes is related by the mechanical properties of the medium. Fluids produced in small quantities, e.g. costly protein dispersion or fluids confined in small volumes down to 1 picoliter, such as living cells can only be examined by this technique. With the development of microfluidics systems in the last decade [1], rapid advances were made in the field of µ-rheology. Standard experimental protocols and data treatment softwares are now available and implemented on a regular and controlled basis [2][3][4][5][6][7]. The correspondence between µ-and macro-rheology is nowadays well established. In µ-rheology, the objective is to translate the motion of a probe particle into the relevant rheological

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“…Microrheology has august origins- Einstein (1906) and Perrin (1909) used the diffusion of colloidal particles in materials with known viscoelastic properties to measure Avagadro's number. With advances in microscopy and other techniques [see, e.g., Crocker and Grier (1996)], microrheology has become highly valued as a tool for interrogation of complex fluids [Mason and Weitz (1995); MacKintosh and Schmidt (1999); Habdas et al (2004); Meyer et al (2006); Squires and Brady (2005); Khair and Brady (2006); Wilson et al (2009)]. In passive microrheology, thermal fluctuations drive the Brownian movement of colloidal particles.…”

Section: Introductionmentioning

confidence: 99%

Large amplitude oscillatory microrheology

Swan¹,

Zia²,

Brady³

2014

Journal of Rheology

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SynopsisWe study the motion of a colloidal particle as it is driven by an oscillating external force of arbitrary amplitude and frequency through a colloidal dispersion. Large amplitude oscillatory flows (LAOFs) are examined predominantly from a phenomenological perspective in which experimental measurements inform constitutive models. Here, we investigate a LAOF from a microstructural perspective by connecting motion of the probe particle to the material response while making no assumptions a priori about how stress relaxes in the material. The suspension exerts nonconservative, hydrodynamic forces on the probe, while distortions in the particle configuration exert conservative forces: Brownian and interparticle forces, for example. The relative importance of each of these contributions to particle motion evolves with the degree of displacement from equilibrium. When the force on the probe is weak, the linear microviscoelasticity of the suspension is probed [see, e.g., Khair and Brady, J. Rheol. 49, 1449-1481]. When oscillation rate is slow, the steady microrheology is probed [see, e.g., Squires and Brady, Phys. Fluids 17, 073101 (2005); Khair and Brady, J. Fluid Mech. 557, 73-117 (2006)]. This article develops a micromechanical model that recovers these limiting cases and then uses the same model to reveal the microrheology of colloidal dispersions deformed by a probe driven with arbitrary force amplitude and frequency. A chief result of this work is the discovery of a regime in which the resistance to motion of the probe particle is on average weaker than the resistance the probe experiences when deformed by high frequency oscillation. This hypoviscous effect arises when the reciprocating motion of the probe particle opens a a)

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Passive and Active Microrheology of Hard-sphere Colloids

Cited by 90 publications

References 33 publications

Shear Thinning and Local Melting of Colloidal Crystals

Shear Thinning and Local Melting of Colloidal Crystals

Magnetic wire-based sensors for the microrheology of complex fluids

Large amplitude oscillatory microrheology

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