Instrumental development

Fourier microscopy

FM is performed by inserting a Bertrand lens in the light path of a conventional microscope with Köhler illumination which images the Fourier image formed at the back focal plane of the objective. It allows to determine the structure of fluid and crystalline ordered suspensions (Phys. Rev. E. 77, 061401 (2008)).

Fourier Image Köhler illumination

Fourier-selective microscopy

If an additional aperture is placed at the back focal plane, the q-range from which the image is constructed may be restricted. This can be used to omit any positional information of particles within a crystal and construct the image from zeroth order light and incoherently superimposed 1st order diffractions of uncorrelated objects like grain boundaries, holes, dislocations etc. The Bragg diffracted light is missing from the zeroth order diffraction leading to a colour coding of different crystal structures. missing Such a technique is useful for fast wide area scans of crystal quality and structure.

Multiple crystal structure Diffraction images

Bragg microscopy

Bragg microscopy

BM in principle is an inverted light scattering experiment. Instead of illuminating the sample in a direction normal to its surface and recording the Bragg diffracted light, it is illuminated under an angle fulfilling the Laue condition and only the light of properly oriented crystals is reflected into the objective of the microscope. This e.g. allows to monitor the competing growth of wall and bulk crystals or of oriented twin domains after succession of shear (J. Chem. Phys. 137, 094906 (2012)).

Wall and bulk crystal growth Shear growth

Further, Differential Interference Contrast Microscopy, Phase Contrast Microscopy, Polarization Microscopy, particle location and tracking analysis, and standard AFM are frequently used. Examples show particle trajectories towards a growing crystallite with three domains grown at an HCl releasing seed. Particles are identified to be in a crystalline environment by bond-order analysis.

Particle trajectories Three domain crystal

Continuous deionization of Colloids

Pump circuit

Working with charged sphere suspensions requires salt concentration levels of 10-6M and below. We have developed and perfected a continuous conditioning system, which guarantees such values, but also allows to adjust higher levels under conductometric control (J. Phys. Chem. 96, 8180-8183 (1992)). Further it shear melts all colloidal crystals, homogenizes the suspension prior to investigation, and is flexibly connecting to all our experiments (J. Chem. Phys. 114, 7556 - 7562 (2001)).

Combined Bragg and Small Angle light scattering

Bragg and small angle apparatus SALS data

Both the bragg scattering and the small angle scattering regime are accessible with this instrument with high angular and temporal resolution, thus probing the sample both at the length scale of typical particle distances and lattice constants and the length scale of its micro-structure. The two rotating detector arms collect light from the full azimuthal range and leave the fragile sample undisturbed (PRX submitted 1.2014). This set-up and its prototype (Nuovo Cimento D 16, 1149-1157 (1994)) herited from late K. Schätzel were used to study the solidification kinetics of sterically stabilized PMMA hard spheres (J. Coll. Interface Sci. 207, 119 - 127 (1998), collaboration with W. v. Megen) and PS-microgel hard spheres (J. Chem Phys. 130, 084502 (2009), Phys. Rev. E 79, 010601(R) (2009), Phys. Rev. Lett. 102, 038302 (2009), collaboration with E. Bartsch and H. J. Schöpe). In the shown example the SALS signal exhibits dynamic scaling and originates from a foam-like micro structure formed within the polycrystalline sample during coarsening (Phys. Rev. E 81, 051401 (1-20) (2010)). More recently, we studied the different solidification scenarios of hard spheres with and without additional depletion attraction to learn about the pathways from nuclei to micro-structure in colloidal suspensions (J. Chem. Phys. 143, 064901 (2015))

Combination Light Scattering Experiment

This is one of our main work horses, as it allows the simultaneous measurement of static, dynamic and elastic properties of our samples on a two arm goniometer with three independently optimized illumination and detection optics (J. Colloid Interface Sci. 234, 149-161 (2001). Combination with the continuous deionization further facilitates additional simultaneous measurements of e.g. conductivity) It was e.g. used to determine structure and concentration dependent elasticity (J. Chem. Phys. 109, 10068 - 10074 (1998)), compare different experimental effective charges to theoretical calculations (J. Chem. Phys. 116, 10981 – 10988 (2002), Colloid. Surf. A 270, 220 - 225 (2005), J. Chem. Phys. 125, 194714 (2006)) and test theoretical expectations for the phase behaviour and properties of binary charged sphere mixtures (Phys. Rev. E 80, 021407 (2009), J. Chem. Phys. 133, 104501 (2010).

Combination Light Scattering Experiment

Super-heterodyne Doppler Velocimetry

For electrokinetic measurements we employ a low angle, incoherent, super-heterodyne, integral Laser Doppler Velocimetry in reference beam mode. This new machine (Condens. Matter 24, 464109 (19pp) (2012)) combines the advantages of integral measurements across the complete cell cross section (allowing simultaneous determination of electro-phoresis and electro-osmosis) with super-heterodyning (allowing separation of the actual heterodyne signal from low frequency noise and homodyne contributions) and low angle measurements (allowing measurements independent of system structure). This machine is used for concentration dependent mobility measurements in electric fields (Phys. Rev. Lett. 98, 176105 (1-4) (2007)) as well as structural changes under hydrostatic and/or electro-phoretic flow (J. Chem. Phys. 119, 3360 – 3370 (2003), Colloid Surfaces B 56, 210 – 219 (2007)). The example shows the distribution of coexisting meta-stable phases under electro-phoretic shear as inferred from simultaneous Velocimetry and structure measurements in the XY cross section.

Superheterodyne Doppler Velocimetry Sheared crystals

Super-heterodyne Dynamic Light Scattering

A recent development was the introduction of a correction scheme for multiple scattering contributions to Super-heterodyne scattering. In our set-up, this contribution in frequency space is Lorentz-shaped and multiple scattering suppression amounts to a simple but efficient background correction. A large illumination beam simultaneously provides excellent ensemble averaging. That way we have recently measured the time dependent solid state diffusion in annealing colloidal crystals (J. Chem. Phys. 146, 204904 (2017)). A large angle instrument for measuring glass transition dynamics is under development.

Examples of super-heterodyne spectra of a suspension in Hagen–Poiseuille flow for increasing particle concentrations as indicated. Note the characteristic broad, Lorentz-shaped multiple scattering hump.
Temporal development of the zero field self-diffusion coefficient for two series of crystallization/coarsening experiments at n = 0.5 1018 m-3 (circles) and n = 2.5 1018 m-3 (diamonds).

Multi-Speckle Correlation

Dynamic Light Scattering (DLS) is a standard technique to investigate the particle dynamics of colloidal suspensions. These particle dynamics are described by the Intermediate Scattering Function (ISF). The ISF is not directly accessible with a standard DLS experiment in non-ergodic samples. Many techniques have been developed to measure the ensemble averaged particle dynamics in non-ergodic samples: Interleaved sampling (Prog. Colloid Polymer Sci. 100, 121-126 (1996), Echo method (Rev. Sci. Instrum. 75, 2419 (2004)). and Multispeckle Correlation Spectroscopy (MSCS) (J. Chem. Phys. 104, 1758-1761 (1996)). In our group, we use MSCS with a special optical detection setup to determine the ensemble averaged as well as the subensemble resolved particle dynamics simultaneously (AIP Conf. Proc. 1518, 304-307 (2013)). This makes the MSCS a powerful tool to investigate e.g. the ageing behavior of colloidal glasses or the dynamical heterogeneities in meta-stable colloidal samples (Nature Physics 12,712-717 (2016)).

Sedementing particles
Measurement of a sample with sedimenting SiO2 particles. Crystallization at the bottom, fluid at the top. Left: Speckle Pattern recorded at the CCD. Right: calculated relaxation time constants at different positions.
Correlation spectrum Multi-Speckle Setup

2-Colour Cross Correlation

Recently, we developed an easy-use and low-cost fiber-based two-color dynamic light-scattering apparatus ( Phys. Rev. E 85, 031401 (2012)), which, hovewer, avaits for the use by a PhD student or a PostDoc (see positions).

Differential Dynamic Microscopy