Published online by Cambridge University Press: 12 February 2015
We present a study of the effect of finite detector integration/exposure time $E$, in relation to interrogation time interval
${\rm\Delta}t$, on analysis of Brownian motion of small particles using numerical simulation of the Langevin equation for both free diffusion and hindered diffusion near a solid wall. The simulation result for free diffusion recovers the known scaling law for the dependence of estimated diffusion coefficient on
$E/{\rm\Delta}t$, i.e. for
$0\leqslant E/{\rm\Delta}t\leqslant 1$ the estimated diffusion coefficient scales linearly as
$1-(E/{\rm\Delta}t)/3$. Extending the analysis to the parameter range
$E/{\rm\Delta}t\geqslant 1$, we find a new nonlinear scaling behaviour given by
$(E/{\rm\Delta}t)^{-1}[1-((E/{\rm\Delta}t)^{-1})/3]$, for which we also provide an exact analytical solution. The simulation of near-wall diffusion shows that hindered diffusion of particles parallel to a solid wall, when normalized appropriately, follows with a high degree of accuracy the same form of scaling laws given above for free diffusion. Specifically, the scaling laws in this case are well represented by
$1-((1+{\it\epsilon})(E/{\rm\Delta}t))/3$, for
$E/{\rm\Delta}t\leqslant 1$, and
$(E/{\rm\Delta}t)^{-1}[1-((1+{\it\epsilon})(E/{\rm\Delta}t)^{-1})/3]$, for
$E/{\rm\Delta}t\geqslant 1$, where the small parameter
${\it\epsilon}$ depends on the size of the near-wall domain used in the estimation of the diffusion coefficient and value of
$E$. For the range of parameters reported in the literature, we estimate
${\it\epsilon}<0.03$. The near-wall simulations also show a bias in the estimated diffusion coefficient parallel to the wall even in the limit
$E=0$, indicating an overestimation which increases with increasing time delay
${\rm\Delta}t$. This diffusion-induced overestimation is caused by the same underlying mechanism responsible for the previously reported overestimation of mean velocity in near-wall velocimetry.