Effect of Directional Movement on Dynamic Light Scattering

Abstract

Dynamic light scattering is a standard technique of nano-particle sizing in colloid systems. However, it is difficult to measure the particle size accurately in the flowing dispersion, which is inevitable in many applications, such as on-line measurement. In this paper, we present that the wave vector of the scattered light, which is averaged to a constant in traditional technique, varies with time when the particles undergoing not only the random motion but also the directional movement. This variation results in an additional term of sinc function to the traditional intensity autocorrelation function of the scattered light, thus affect the final particle size determination, especially for large velocity of the directional movement. The experimental results agree well with our derivation. We believe this will facilitate the extension of the technique of the dynamic light scattering to a wider range of application, especially of the on-line measurement in the flowing dispersion.

Microfluidic in-line dynamic light scattering with a commercial fibre optic system

“…8 Subsequent theoretical developments have accounted for polydispersity, 6 and shear, 16 measurement of the shear rate itself, [12][13][14][15][16][17] and analysis of flow data along the incident beam direction in a flow cuvette. 9 Eqn (8) can be rearranged 4,5,7,8 to define a 'modified correlation function':…”

“…where ω is the beam radius at the focus, and v is flow velocity. Given the restrictions highlighted in eqn ( 5) and (7), several previous flow-DLS setups were designed with large (≳1 cm) flow cells 4,5,[7][8][9][10] such that, at a given flow rate, both the velocity and shear rate become relatively small and their respective decorrelation contributions can be neglected (or the transit term accounted for).…”

“…First, the raw correlogram is normalised ① by the baseline, B, and coherence factor, β. These correlation data are subsequently rearranged into the MCF representation ② using eqn (9). Here, the fitting region is chosen as the data points that lay between 0.2 < (g 2 − B)/ Bβ < 0.8, before being transformed into the MCF.…”

“…8 Subsequent theoretical developments have accounted for polydispersity, 6 and shear, 16 measurement of the shear rate itself, 12–17 and analysis of flow data along the incident beam direction in a flow cuvette. 9 Eqn (8) can be rearranged 4,5,7,8 to define a ‘modified correlation function’:which is a linear relation in τ , with intercept 2 Γ and slope v 2 / ω 2 , providing a convenient and robust method of analysis for (monodisperse) particles under flow.…”

“…Dynamic light scattering (DLS) is a ubiquitous experimental technique used to characterise the hydrodynamic size of particles and relaxation processes in simple and complex systems. [1][2][3] Given its extensive use in biology, physicalchemistry and soft matter science, a number of authors have considered both the theoretical and experimental feasibility and implementation of DLS under flow for particle size measurements, [4][5][6][7][8][9][10][11] flow velocity and shear determination, [12][13][14][15][16][17] and recently, also coupling with microfluidics. 16,[18][19][20] Various technical challenges in flow-DLS have been identified, pertaining to optical alignment of the scattering and flow planes, 11,16 and limitations on the flow velocity relative to particle diffusion, 4,5,12 respectively giving rise to shear flow and transit contributions to the scattering signal, which require complex data analysis, by contrast to that of conventional DLS.…”