Abstract
We present a novel approach for characterizing the properties and performance of active matter in dilute suspension as well as in crowded environments. We use Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) to study large ensembles of catalytically active Janus particles moving under UV illumination. SH-LDV facilitates a model-free determination of the swimming speed and direction, with excellent ensemble averaging. In addition, we obtain information on the distribution of the catalytic activity. Moreover, SH-LDV operates away from walls and permits a facile correction for multiple scattering contributions. It thus allows for studies of concentrated suspensions of swimmers or of systems where swimmers propel actively in an environment crowded by passive particles. We demonstrate the versatility and the scope of the method with a few selected examples. We anticipate that SH-LDV complements established methods and paves the way for systematic measurements at previously inaccessible boundary conditions.
Highlights
Active matter is a rapidly evolving emergent field of soft matter physics triggering intense experimental and theoretical activity [1,2,3]
We presented Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) as a facile and versatile approach to measure velocities of active systems in the bulk and away from a wall
SH-LDV can characterize the swimming performance on isolated, non-interacting swimmers and in bulk environments crowded by passive particles
Summary
Active matter is a rapidly evolving emergent field of soft matter physics triggering intense experimental and theoretical activity [1,2,3]. The shift between reference and illuminating beams is introduced utilizing a pair of stabilized acousto-optic modulators (AOM) delivering a nominal frequency difference of ωB,nominal ± Δωnominal = 3000.0 ± 0.5 Hz. In practice, when setting the AOM, we found centre frequencies in the range: ωB,exp(t = 0) = ωB,nominal ± 0.2 Hz. We checked for the temporal stability and found that over the duration of a typical measurement (30 min) ωB,exp(t) = ωB,exp(t = 0) ± 0.04 Hz. The present SH-LDV instrument further features small angle scattering, which allows an efficient correction scheme to isolate the single-scattering signal from undesired multiple-scattering contributions [41] (for a recently realized prototype for wideangle scattering, see [44]). In the present integral mode, scattered light is collected from the complete cross section of the cell, which is illuminated by the central part of the widened UV beam that is activating the self-propulsion of the particles. Parasitic scattering at optical surfaces, multiple scattering and noise terms were not considered, as they are superimposing and can conveniently be removed
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