Ultrafast Quenching of Binary Colloidal Suspensions in an External Magnetic Field

Abstract

An ultrafast quench is applied to binary mixtures of superparamagnetic colloidal particles confined at a two-dimensional water-air interface by a sudden increase of an external magnetic field. This quench realizes a virtually instantaneous cooling which is impossible in molecular systems. Using real-space experiments, the relaxation behavior after the quench is explored. Local crystallites with triangular and square symmetry are formed on different time scales, and the correlation peak amplitude of the small particles evolves nonmonotonically in time in agreement with Brownian dynamics computer simulations.

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

“…Therefore, our simulation results can be verified in real-space microscopy experiments of two-dimensional superparamagnetic particles [36][37][38][39][40][41] see also 42,43 for alternative setups. An external magnetic field induces repulsive dipole forces.…”

“…The magnetic field is applied perpendicular to the twodimensional interface containing the particles. In the following, the dipole-strength ratio M is fixed to 0.1, corresponding to recent experimental samples 38,39,48 . The relative composition X = N B /(N A + N B ) of B particles is fixed at 50%; hence we are considering an equimolar mixture.…”

“…The Brownian time scale is set by the short-time diffusion constant D A of the A-particles. Knowing that this diffusion constant scales with the inverse of the radius of a particle, D B was chosen such that D B /D A = 1.61 corresponding to the physical diameter ratio of the experimental samples 38,39,48 . We perform standard nonequilibrium Brownian dynamics (BD) and Monte Carlo (MC) computer simulations [50][51][52] in the canonical ensemble where the particle numbers N i (i = A, B), the temperature T and the area A is fixed.…”

Irreversible Rearrangements, Correlated Domains, and Local Structure in Aging Glasses

“…This rapid and deep quench is a unique feature of the NIPA particle experiments which permits measurement of aging over a broad range of time scales [12] that were not accessible to previous aging experiments [20]. A similarly deep quench was reported recently by Assoud et al [21]; their experiment used rapid magnetic field changes to quench longrange repulsive magnetic particles from liquid to glass.…”

The role of quench rate in colloidal gels

“…This is typically the case in both experiments [1,3] and computer simulation [1,10,[18][19][20]. Recent developments in controlling colloid-colloid interactions allow the interactions (and thus the effective temperature) to be changed at will, even on timescales much faster than the colloid dynamics, such that controllable quenches may be carried out [21]. It has also become possible to control attractive interactions between colloids such as temperature-dependent depletion attractions [22,23], multiaxial electric fields [24] and the critical Casimir effect [25][26][27].…”