Magnetorheology from surface coverage of spin-coated colloidal films

Abstract

In magnetorheological fluids, the viscosity usually increases with the field and the non-Newtonian character of these complex fluids may vary significantly. We provide a new method to measure the relative viscosity of a superparamagnetic colloid, by applying a magnetic field during a spin-coating process, which involves evaporation of the solvent. We define the compact equivalent height to take into account the discrete nature of the suspension, and we compare experimental results under different conditions. We extend the model of Cregan et al. (J. Colloid Interface Sci., 2007, 314, 324) to turn it into an evaporation rate independent one. The generality of the resulting model facilitates measurement of the magnetic field dependent viscosity. We also discuss the morphologies of the final dried colloidal deposits and the possible mechanisms involved in their formation.

Dynamics, crystallization and structures in colloid spin coating

“…21,22 . When working with a dilute aqueous colloidal dispersion of superparamagnetic particles, the dominant effect of the applied magnetic field was to change the rheology.…”

“…Later, Cregan et al 32 generalized this result by considering solvent evaporation in both stages. Rotation-rate dependent diffusion and advection of the solvent, in both vapor and liquid phases, could affect evaporation rate 19,22,29,31,32 , thereby yielding different thicknesses for the deposited layers. Due to drainage and evaporation of the solvent, film thickness (and its rate of thinning) changes continuously as a function of time.…”

“…In this kind of colloidal system, inter-particle magnetic dipole interactions can be affected by applying a magnetic field while spin coating 21,22,[44][45][46] .…”

“…An understanding of the mechanisms of these phase transitions could lead to strategies for making more controllably ordered or disordered films. This review also demonstrates that colloid spin coating offers a remarkably reproducible way to study crystallization in systems far from equilibrium, and that new techniques associated with spin coating [20][21][22] hold significant promise for advancing the field further.…”

Weak ac field-induced patterns in vertical deposition of colloids at various evaporation rates

“…The main purpose is to bring these affected colloidal systems into nonequilibrium transitions in a controlled way [1][2][3][4][5][6][7][8][9]. These transitions lead to macroscopic patterns, and are more complex in nature as their control parameters are influenced by external fields [10,11].…”