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DLS : Dynamic Light Scattering for Colloidal Size Measurements

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DLS has rarely been combined with microfluidics, compared to other existing techniques such as X-ray scattering, spectroscopy... Actually, online DLS measurements are difficult to perform due to the high shear rates involved in miniaturized pressure-driven flows.

Top : geometry of a DLS experiment in a microfluidic channel of width w and height h. The velocity profile corresponds to the case of a large aspect ratio w/h, q denotes the scattering vector, ki and ks are the wavevectors of the incident and scattered light. Insert : zoom on the scattering volume V defined by the incident beam of size wi and the scattered one of size ws. Bottom : theoretical calculated correlation functions in a Poiseuille flow for different imposed flow rates, and for a typical DLS experiment as sketched in the left. v0 is the maximal velocity in the scattering volume. Insert : characteristic times ?äc of the correlation function vs. v0. Below v0 = 2 mm/s, the correlation functions are dominated by the brownian motion of the scatterers and the DLS measurements yield the size of the flowing colloids. Above v0 = 2 mm/s, the characteristic times scales with v0-1 and DLS measurements are dominated by the effects of the shear flow in the scattering volume.