Confocal Laser Scanning Microscopy
Confocal laser scanning microscopy enables spatially and temporally resolved visualisation of the shape and reorganisation dynamics of liquid and solid materials down to a depth of several tens of micrometres into the material. We are particularly interested in the wetting behaviour of droplets on hydrophobic, superhydrophobic and microstructured substrates, essential for all coating processes. The wetting behaviour of a surface is reflected in the shape and dynamics of the contact line between droplet, surface and surrounding medium. We visualise, quantify and model how drops roll over surfaces, how drops detach from individual defects and how liquids penetrate into rough or porous structures. We correlate the time- and spatially-resolved microscopic information with the friction experienced by a droplet as it rolls or slides over a surface. Furthermore, we correlate the microscopic information with the simultaneously determined adhesion force that holds a particle on the surface.
On soft substrates, even resting droplets deform the substrate. This causes drops to be surrounded or even encased by a so-called wetting ring. To obtain time- and spatially-resolved information about the reorganisation dynamics of the substrate and the wetting ring, we move droplets over the substrate at a well-defined speed. The speed can be varied over more than 5 orders of magnitude. A special feature is that the self-built confocal setup makes it possible to keep the previously defined area of the drop in focus even during the movement. This enables, for example, the visualisation of a speed-dependent phase separation in the wetting ring (see figure).
References:
Wong WSY, Corrales TP, Naga A, Baumli P, Kaltbeitzel A, Kappl M, et al. Microdroplet Contaminants: When and Why Superamphiphobic Surfaces Are Not Self-Cleaning. Acs Nano. 2020;14(4):3836-46 doi.org/10.1021/acsnano.9b08211
Saal A, Straub BB, Butt HJ, Berger R. Pinning forces of sliding drops at defects. Epl. 2022;139(4). doi.org/10.1209/0295-5075/ac7acf
Geyer F, M. D’Acunzi, A. Sharifi-Aghili, A. Saal, N. Gao, A. Kaltbeitzel, T.F. Sloot, R. Berger, H.J. Butt, D. Vollmer*, “When and how self-cleaning of superhydrophobic surfaces works”, Science Advances, 2020, 6: eaaw9727 (11p). DOI: 10.1126/sciadv.aaw9727
L. Hauer, Z. Cai, A. Skabeev, D. Vollmer, J.T. Pham, “Phase separation in wetting ridges of sliding drops on soft and swollen surfaces”, Phys. Rev. Lett., 2023, doi: 10.1103/PhysRevLett.130.058205.
Schellenberger F, Papadopoulos P, Kappl M, Weber SAL, Vollmer D, Butt HJ. Detaching Microparticles from a Liquid Surface. Physical Review Letters. 2018;121(4). doi.org/10.1103/PhysRevLett.121.048002
Naga A, Kaltbeitzel A, Wong WSY, Hauer L, Butt HJ, Vollmer D. How a water drop removes a particle from a hydrophobic surface. Soft Matter. 2021;17(7):1746-55. doi.org/10.1039/D0SM01925A