2.3. DLS

The multichannel DLS instrumentation shown in Figures 1 and ​and2A-C2A-C consists of optical elements defining the light path and scattering geometry around the sample holder as well as a diode laser (Schäfter + Kirchhoff GmbH, Germany) providing 120 mW output power with a wavelength of 660 nm. Furthermore, eight detectors and correlator units are combined in one cabinet together with a CPU for displaying and processing data. Focus optics, objective lens and Faraday isolators are combined to guide the laser light into a single mode fiber cable. A mechanical adjustment mechanism with μm precision allows precise coupling of this laser fiber cable (Schäfter + Kirchhoff GmbH). A collimator of 50 mm focal length, resulting in a minimal beam diameter of 25 μm, was used. The laser light at the fiber output is focused into the sample container mounted onto a black base plate (45 cm × 20 cm). Sample drops were placed inside a glass container and covered with Al's oil. Light scattered by particles in the sample drops is focused onto eight receiver fibers via achromatic lenses. A customized ferrule takes up the eight fibers in a row. With this setup the scattered light emerging from equidistant positions along the laser beam passage through a sample suspension is thus collected by single-mode 4.6 μm core diameter fibers and transmitted to the detector units. An x/y/z-translation step motor control is installed to adjust laser output fiber and receiver optics and to select measurement positions in the sample volume. An additional x/y-translation and rotation hinge allows precise movement of all eight receiver focal points onto the laser beam focal point. Design and construction of all optical elements was performed by XtalConcepts GmbH (Germany). The fiber cables were connected to implemented photomultipliers (Hamamatsu {"type":"entrez-nucleotide","attrs":{"text":"H10682","term_id":"875504","term_text":"H10682"}}H10682, Japan). The corresponding output signals were transferred to one correlator unit each (XtalConcepts, Germany), which allow processing of decay times from 400 ns up to seconds. The generated data were integrated and processed by customized python-based software for further analysis and display. The ACFs were evaluated using the CONTIN algorithm (Constrained regularization method for inverting data) [32] with no cumulant analysis involved, allowing to directly calculate the diffusion constants and obtain R_h values. If not stated otherwise, experiments were performed at 20 °C.

Eight-channel in situ DLS setup: Side view (A) and top view (B). Laser and receiver optical elements are shown in a default position adjusted to the sample holder below located on a motorized stage.

Eight-channel in situ DLS setup (A) Portable hardware cabinet, including the diode laser, eight photomultiplier tubes (PMTs) for detection, autocorrelation units (ACUs) and a CPU (B) Setup of laser fiber and receiver optical elements aligned with the sample holder. A thermostat and heating foil below the holder is used for temperature regulation. The laser beam is schematically shown as dashed red line. (Approx. maximum height: 28 cm) (C) Schematic representation of the scattering geometry, measurement positions 1–8 (1 and 8 are labelled) and the eight-channel detection principle (D) Exemplary eight-channel DLS experiment using gold nanoparticles for setup verification. (E) Averaged radii (grey columns) as processed by the individual detectors. The values of the count rates, shown in parallel, approx. fit the expected decrease of scattered light with increasing distance from the focal point of the laser.