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.