Abstract
The effect of time-dependent external fields on the structures formed by particles with induced dipoles dispersed in a viscous fluid is investigated by means of Brownian Dynamics simulations. The physical effects accounted for are thermal fluctuations, dipole-dipole and excluded volume interactions. The emerging structures are characterised in terms of particle clusters (orientation, size, anisotropy and percolation) and network structure. The strength of the external field is increased in one direction and then kept constant for a certain amount of time, with the structure formation being influenced by the slope of the field-strength increase. This effect can be partially rationalized by inhomogeneous time re-scaling with respect to the field strength, however, the presence of thermal fluctuations makes the scaling at low field strength inappropriate. After the re-scaling, one can observe that the lower the slope of the field increase, the more network-like and the thicker the structure is. In the second part of the study the field is also rotated instantaneously by a certain angle, and the effect of this transition on the structure is studied. For small rotation angles (theta le 20^{{circ }}) the clusters rotate but stay largely intact, while for large rotation angles (theta ge 80^{{circ }}) the structure disintegrates and then reforms, due to the nature of the interactions (parallel dipoles with perpendicular inter-particle vector repel each other). For intermediate angles (20<theta <80^{{circ }}), it seems that, during rotation, the structure is altered towards a more network-like state, as a result of cluster fusion (larger clusters). The details provided in this paper concern an electric field, however, all results can be projected into the case of a magnetic field and paramagnetic particles.
Highlights
An externally imposed electric or magnetic field affects a suspension of dielectric or paramagnetic particles by inducing dipoles to the particles, due to the difference in the dielectric B Markus HütterMultiscale and Multidisciplinary Modeling, Experiments and Design tic time-scale of their rheological response is O(1s) and is exploited in robotics and in force-feedback sensors (Karasawa and Goddard 1989)
The ratio between the thermal fluctuations and the strength of the electric field, B∗, and the characteristic time-scale tc are kept constant; only the time-dependence of the field-strength of the electric field is varied (i.e., we vary the increase of the field strength until the final value Ec is reached) through the simulations
This means that the magnitude of the thermal fluctuations relative to the final field strength remains constant throughout the simulations and only the deterministic part in Eq (3), is varied
Summary
An externally imposed electric or magnetic field affects a suspension of dielectric or paramagnetic particles by inducing dipoles to the particles, due to the difference in the dielectric B Markus HütterMultiscale and Multidisciplinary Modeling, Experiments and Design tic time-scale of their rheological response is O(1s) and is exploited in robotics and in force-feedback sensors (Karasawa and Goddard 1989). Specific structures of particles could possibly be achieved inside a complex geometry by controlling the microstructure of the composite while printing, through the use of an external electric or magnetic field This would be possible if photo-reactive resins (Anastasio et al 2018) filled with electrically of magnetically active particles are used: Particles are dispersed in the resin (Kim et al 2003; Kim and Shkel 2004), so that by solidification of the resin one could fixate the desired structure of particles; before curing, resins exhibit low viscosity allowing the structure formation within the time-scales (O(1s)) of stereolithography (SLA) (Bártolo 2011). The formation of particle structures in a time-dependent imposed field is examined numerically
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