Differential dynamic microscopy of bidisperse colloidal suspensions

Abstract

Research tasks in microgravity include monitoring the dynamics of constituents of varying size and mobility in processes such as aggregation, phase separation, or self-assembly. We use differential dynamic microscopy, a method readily implemented with equipment available on the International Space Station, to simultaneously resolve the dynamics of particles of radius 50 nm and 1 μm in bidisperse aqueous suspensions. Whereas traditional dynamic light scattering fails to detect a signal from the larger particles at low concentrations, differential dynamic microscopy exhibits enhanced sensitivity in these conditions by accessing smaller wavevectors where scattering from the large particles is stronger. Interference patterns due to scattering from the large particles induce non-monotonic decay of the amplitude of the dynamic correlation function with the wavevector. We show that the position of the resulting minimum contains information on the vertical position of the particles. Together with the simple instrumental requirements, the enhanced sensitivity of differential dynamic microscopy makes it an appealing alternative to dynamic light scattering to characterize samples with complex dynamics.

Quantitative optical microscopy of colloids: The legacy of Jean Perrin

“…The difference between these potentials depends on resolving particle separations at the nanometer level. Previous centroid-based methods [41] (purple line) produce a P s ðδÞ with nonsensical features, such as significant overlaps, that cannot be fit by a reasonable interaction potential. We discuss the overlaps in PERI's extracted P s ðδÞ in SM [18], Sec.…”

“…A first notable feature of DDM is that it can be implemented with a variety of imaging contrast mechanisms that can operate in various conditions of signal-to-noise ratio, resolution needs and multiple scattering. Since its introduction about ten years ago, DDM was used to study colloids in bright-field [35][36][37][38][39][40][41], phase-contrast [42], dark-field [43,44] and depolarized [45] microscopy. It has also been demonstrated with fluorescent particles in wide-field [36,46,47], confocal [48,49] and lightsheet [50,51] configurations.…”

“…The time evolution of a specific region [ Fig. 5(b) and Supplemental Movie S5 [41]] reveals that, within a time scale of 7 oscillations (t= 70 s), defects move and the sample relaxes towards a crystalline configuration with fewer impurities. Defects self-annihilate in two fashions: either five to seven pairs meet and extinguish reciprocally [e.g., pairs 1 and 2 in Fig 5(b), from t= 0 to 20 s], or they form transient circular grains (from t= 20 to 70 s; see Supplemental Movie S5 [41]).…”

“…5(b) and Supplemental Movie S5 [41]] reveals that, within a time scale of 7 oscillations (t= 70 s), defects move and the sample relaxes towards a crystalline configuration with fewer impurities. Defects self-annihilate in two fashions: either five to seven pairs meet and extinguish reciprocally [e.g., pairs 1 and 2 in Fig 5(b), from t= 0 to 20 s], or they form transient circular grains (from t= 20 to 70 s; see Supplemental Movie S5 [41]). Our experimental results agree with what has been recently reported by van der Meer et al [24], but, further to that, allow for pure shear response to be examined over much larger domains.…”

“…5(d) shows the crystalline domains after oscillatory shear is applied for approximately 20 min at θ = 15 • and ω = 0.1 Hz. During this time the grains merge and larger crystalline domains develop (see also Supplemental Movie S6 [41]). Hence, the low-amplitude applied shear increases the "quality" of the 2D colloidal system through defect annealing.…”