1Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
2Department of Chemistry and Catalysis, Technical University of Munich, 85748 Garching, Germany
In recent years, a lot of research went into exploring different strategies to achieve long-lasting and highly liquid-repellent surfaces. This research resulted in a greatly improved understanding on how surface properties (hydrophobicity and geometry) and repellency (i.e. contact angles, impalement pressure) of liquid drops correlate. However, there is poor understanding on how surface properties and contamination (size, concentration, hydrophilicity) correlate. Therefore, we developed a method to slowly move a drop over a contaminated surface while imaging the region close to three-phase contact line by laser scanning confocal micrposcopy simultaneously. This enabled us to monitor in slow motion how a drop takes up particulate contamination while rolling over a contaminated surface.1 Using a lateral adhesion force device, we quantified how the friction force evolve during self-cleaning. Next I will discuss the challenges to characterize the wetting properties of porous surfaces, using metal organic frameworks as an example.2 Finally, I will discuss when and why superhydrophobic surfaces can suppress biofilm formation.3
1) Geyer, F.; D'Acunzi, M.; Sharifi, A.; Saal , A.; Gao N.; Kaltbeitzel, A.; Sloot, T.-F.; Berger, R.; Butt, H.-J.; Vollmer D., When and how self-cleaning of superhydrophobic surfaces works, submitted
2) Jayaramulu, K.; Geyer, F.; Petr, M.; Zboril, R.; Vollmer, D.; Fischer, R. A., Shape Controlled Hierarchical Porous Hydrophobic/Oleophilic Metal-Organic Nanofibrous Gel Composites for Oil Adsorption. Advanced Materials 2017, 29 (12).
3) Geyer, F.; D'Acunzi, M.; Yang, C.-Y.; Müller, M.; Baumli, P.; Kaltbeitzel, A.; Mailänder, V.; Encians, N.; Vollmer, D.; Butt, H.-J., How to Coat the Inside of Narrow and Long Tubes with a Super-Liquid-Repellent Layer. Advanced Materials 2018.
The control of droplet transport on solid surfaces and associated wetting phenomena is important for a wide range of technological applications. Quite often the volume of the droplets is subject to external, time-dependent variations, such as e.g. when there is mass exchange through the droplet interface (evaporation/condensation phenomena), or when droplets sit on porous materials so that liquid is absorbed through the pores. Under these conditions, an important issue is to understand how the properties of the solid substrate may affect the dynamics and shape of the droplet.
We consider droplets with a time-dependent volume on solid surfaces that have simple, macroscopic, and smooth (pinning-free) variations. We show that the droplet exhibits a complex dynamics characterised by a series of sudden and fast lateral movements which occur at some specific values of the droplet size. These fast moves (snaps) are not a consequence of pinning but rather are triggered by a series of underlying bifurcations that emerge as the droplet size increases or decreases. When the droplet snaps, it may randomly shift hence observing completely different trajectories depending on whether the volume is increasing or decreasing.
School of Science and Technology, Clifton Lane, Nottingham Trent University, NG11 8NS, Nottingham, UK
The evaporation of binary liquids droplets proceeds in three distinct stages: chaotic (Stage I), convective (Stage II) and outward radial flow (Stage III). Stage I is characterised by random and strongly circulating vortices, attributed to solutal Marangoni flows driven by the preferential evaporation of the more volatile component (ethanol). Stage III is characterised by an outward capillary flow of the less volatile single component droplet (water). In this work, we use high-speed Fourier Domain (FD) optical coherence tomography (OCT) to measure the flow profiles during the evaporation of binary liquid droplets. We fully explore the influence of gravity on Stage II flow by systematically changing the substrate tilt with respect to gravity through a full 180 degrees i.e. from sessile to pendant configuration. Our results demonstrate conclusively that, in contrast to the accepted view and conventional calculations of the Marangoni and Rayleigh numbers, gravitational effects strongly influence the flow dynamics in microliter size binary droplets in Stage II. We also use gas chromatography to measure the time evolution of the concentration of the more volatile component within the droplet and confirm that a simple analysis of volume data provides the same information. Finally, we establish a flow phase diagram demonstrating the conditions under which of the three stages of evaporation occur.
Base4 Innovation Ltd, Broers Building, 21 JJ Thomson Avenue, Cambridge, CB3 0FA, UK
Optoelectrowetting devices remove the limitations imposed by conventional microfluidic channels and enable complex droplet manipulations on chip. We use the freedom of movement provided by our prototype sequencing platform to easily manipulate single molecules of DNA, delivering reagents with a series of droplet operations including sorts, moves and merges. Our proprietary Cascade Reaction detects the presence of a single nucleotide within a microdroplet and returns an intense fluorescent signal. The technology is still in development but shows tremendous promise for delivering high accuracy single molecule DNA sequences including long reads and differentiation between methylation states directly, without bisulphite conversion.