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.
Highlights
Microgravity provides a unique environment in which to investigate the physics of transport processes such as diffusion, convection, and conduction
Dynamic light scattering As a control experiment, we measured the diffusivities of particles of radius 50 nm and 1 μm, respectively, using dynamic light scattering (DLS)
To confirm that Differential dynamic microscopy (DDM) yields quantitative information on the dynamics of bidisperse mixtures, we examined the q-dependence of τL, which was not resolvable with DLS
Summary
Microgravity provides a unique environment in which to investigate the physics of transport processes such as diffusion, convection, and conduction These processes affect structure in systems featuring sub-microscale constituents, including bacterial biofilms,[1, 2] protein crystals,[3, 4] and complex fluids.[5] Monitoring dynamics on these length scales in microgravity is expected to generate fundamental insight into the physics controlling structural evolution. An intriguing alternative is provided by recent enhancements to the Light Microscopy Module (LMM) on the ISS, which increased the time resolution of image acquisition and imparted confocal imaging capabilities. These advances make it possible to access faster dynamics across a broad range of samples but require methods to obtain dynamics from microscopy time series images
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