Monitoring Spatially Heterogeneous Dynamics in a Drying Colloidal Thin Film

Abstract

We report on a new type of experiment that enables us to monitor spatially and temporally heterogeneous dynamic properties in complex fluids. Our approach is based on the analysis of near-field speckles produced by light diffusely reflected from the superficial volume of a strongly scattering medium. By periodic modulation of an incident speckle beam we obtain pixel-wise ensemble averages of the structure function coefficient, a measure of the dynamic activity. To illustrate the application of our approach we follow the different stages in the drying process of a colloidal thin film. We show that we can access ensemble averaged dynamic properties on length scales as small as ten micrometers over the full field of view.

Elasticity and glassy dynamics of dense emulsions

“…DWS is a photon correlation technique that analyzes directly the fluctuations of multiply scattered light. We adopt a detection layout wherein the sample is illuminated with a expanded laser light source and the backscattered intensity pattern I(r) is recorded at a close distance from the sample's surface [17]. The experimental setup is schematically illustrated in the Fig.…”

“…An analyzer selects a single polarization component of I(r,t) and the size of the detected speckles is adjusted to twice the pixel size using the objective's aperture. The sample illumination consists of speckles created by a focused laser (Coherent Verdi V5, λ = 532 nm) impinging on a ground glass and collected by a lens in order to form a speckle-beam as described in [17,19]. Periodic modulation of the ground glass using a linear [19] or rotary [17] electric motor can be applied to improve the signal to noise ratio for spatially resolved DWS.…”

“…The sample illumination consists of speckles created by a focused laser (Coherent Verdi V5, λ = 532 nm) impinging on a ground glass and collected by a lens in order to form a speckle-beam as described in [17,19]. Periodic modulation of the ground glass using a linear [19] or rotary [17] electric motor can be applied to improve the signal to noise ratio for spatially resolved DWS. Finally, we characterize the fluctuations of the backscattered intensity by computing the instantaneous structure coefficient between intensity pat- [17].…”

“…Periodic modulation of the ground glass using a linear [19] or rotary [17] electric motor can be applied to improve the signal to noise ratio for spatially resolved DWS. Finally, we characterize the fluctuations of the backscattered intensity by computing the instantaneous structure coefficient between intensity pat- [17]. Scattered light is recorded in the image plane with a digital camera.…”

“…We use an experimental approach that allows temporally and spatially revolved dynamic light scattering in highly turbid media [17,18,19].…”