Trapped colloidal particle at constant speed
Puertas, A.M.1, Wilson, L.2 and Poon, W.2
1 Departamento de Fisica Aplicada, Universidad de Almeria, Spain; 2 SUPA and COSMIC, Univ. of Edinburgh, United Kingdom
In a dense colloidal system, a
single particle can be used as a probe to provoke microscopic stresses and
study their relaxation, i.e. for microrheology. Connection with bulk rheology
can be made through the stationary relation 
<v><F>, where the is an effective
friction coefficient and <v> and <F> are the average velocity and
force, respectively. The tracer can be dragged at constant F, constant v or
allowing fluctuations of both F and v. In this work, we study by means of
Brownian dynamics simulations the microrheology and dynamics of a single tracer
trapped in a harmonic potential which travels at constant speed through a
system of hard spheres, and compare with experiments where a tracer is trapped
with optical tweezers. The results are analyzed with a single particle model,
where the bath properties are embedded in an "effective fluid". The
stationary tracer position distribution yields the effective friction
coefficient and temperature. The friction coefficient shows velocity thinning,
with a low speed plateau which gives (in the simulations) the correct bulk
shear viscosity of hard spheres, and a high velocity plateau which agrees (in
the experiments) with the high shear bulk viscosity. The tracer position
correlation function, on the other hand, shows two distinguishable relaxations,
arising from the friction with the solvent and collisions with other particles.
This relaxation cannot be rationalized within the effective fluid model.