News – Physics of Surfaces – TU Darmstadt

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Physics of Surfaces

Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy

The evaporation of droplets from liquid mixtures plays a major role in many technical applications. For example, when we paint a wall or print on paper, the liquid components of the ink must evaporate, while the solid ink particles stick to the wall or paper. To better control such technical processes, liquid mixtures are often used, with the individual liquids often evaporating at different rates. The mixing ratio after application of the drops therefore changes over time due to evaporation. These transport processes give rise to concentration gradients in the droplets, which were previously difficult to detect, especially in organic solvents. Now, researchers of the Collaborative Research Center 1194 “ Interaction between Transport and Wetting Processes” have succeeded in detecting such concentration gradients without contact using spatially resolved nuclear magnetic resonance (NMR) and Raman spectroscopy. Since the measurements are contact-free and no chemical marker substances had to be added, the evaporation behavior could be studied largely without interference from the measurement process. The mixture concentration in the droplet was investigated independently of one another using two measurement methods based on different physical principles: NMR spectroscopy investigates the nuclear magnetic resonance in a strong magnetic field, while confocal Raman spectroscopy uses a laser to measure the molecular vibrations. The model mixture studied is composed of two alcohols of different chain positions. The two alcohols can be easily distinguished by NMR spectroscopy. With Raman spectroscopy, it is somewhat more difficult to distinguish the two liquids. Therefore, one of the substances was labeled with deuterium. Since deuterium is heavier than normal hydrogen, the vibrational frequency of the molecules changes, making the two alcohols very distinguishable even with Raman spectroscopy. In the publication by Bell, Kind et al., the participating scientists led by Alena Bell and Jonas Kind were able to show how a concentration gradient forms in the droplet, which changes over time due to evaporation. The two spatially resolved spectroscopy methods provide two independent tools for measuring concentration changes in organic solvents, such as those caused by evaporation and transport processes. The publication is the result of the cooperation of five subprojects of the Collaborative Research Center SFB 1194 “Mutual Influence of Transport and Wetting Processes”. Four of these subprojects are located at TU Darmstadt, one subproject was localized at the Max Planck Institute for Polymer Research in Mainz and is now at the Leibniz Institute for Polymer Research in Dresden. The Collaborative Research Center has been funded by the German Research Foundation since 2016.
Simultaneous Deconvolution of In-Plane and Out-of-Plane Forces of HOPG at the Atomic Scale under Ambient Conditions by Multifrequency Atomic Force Microscopy

Multifrequency atomic force microscopy (AFM) is shown to be an excellent tool for imaging crystal structures at atomic resolution in different spatial directions. However, determining the forces between single atoms remains challenging, particularly in air under ambient conditions. Developed here is a trimodal AFM approach that simultaneously acquires torsional and flexural frequency-shift images and spectroscopic data to transfer these observables into in-plane and out-of-plane forces between single bonds of highly oriented pyrolytic graphite (HOPG) at atomic resolution in air under ambient conditions based on the Fourier method. It is found that the cantilever mean deflection is an excellent indicator to understand that strong attractive interactions between the tip and the surface of HOPG in dynamic AFM imply a local lift of the topmost carbon layer when using higher eigenmodes for the topographical feedback. Cross-talk between torsional and flexural-oscillation modes is shown to be negligible. Interestingly, significant differences are observed in the in-plane forces depending on the orientation of the carbon bonds relative to the direction of torsional oscillation.
Mapping humidity-dependent mechanical properties of a single cellulose fibre

Modelling of single cellulose fibres is usually performed by assuming homogenous properties, such as strength and Young’s modulus, for the whole fibre. Additionally, the inhomogeneity in size and swelling behaviour along the fibre is often disregarded. For better numerical models, a more detailed characterisation of the fibre is required. Herein, we report a method based on atomic force microscopy to map these properties along the fibre. A fibre was mechanically characterised by static colloidal probe AFM measurements along the longitudinal direction of the fibre. Thus, the contact stress and strain at each loading point could be extracted. Stress–strain curves were be obtained along the fibre. Additionally, mechanical properties such as adhesion or dissipation were mapped. Local variations of the effective fibre radius were recorded via confocal laser scanning microscopy. Scanning electron microscopy measurements revealed the local macroscopic fibril orientation and provided an overview of the fibre topography. By combining these data, regions along the fibre with higher adhesion, dissipation, bending ability and strain or differences in the contact stress when increasing the relative humidity could be identified. This combined approach allows for one to obtain a detailed picture of the mechanical properties of single fibres.
Endothelialization of PDMS-based microfluidic devices under high shear stress conditions

Microfluidic systems made out of polydimethylsiloxane (PDMS) offer a platform to mimic vascular flow conditions in model systems at well-defined shear stresses. However, extracellular matrix (ECM) proteins that are physisorbed on the PDMS are not reliably attached under high shear stress conditions, which makes long-term experiments difficult. To overcome this limitation, we functionalized PDMS surfaces with 3-aminopropyltriethoxysilane (APTES) by using different surface activation methods to develop a stable linkage between the PDMS surface and collagen, which served as a model ECM protein. The stability of the protein coating inside the microfluidic devices was evaluated in perfusion experiments with phosphate-buffered saline (PBS) at 10−40 dynes/cm2 wall shear stress. To assess the stability of cell adhesion, endothelial cells were grown in a multi-shear device over a shear stress range of 20−150 dynes/cm2. Cells on the APTES-mediated collagen coating were stable over the entire shear stress range in PBS (pH 9) for 48 h. The results suggest that at high pH values, the electrostatic interaction between APTES-coated surfaces and collagen molecules offer a very promising tool to modify PDMS-based microfluidic devices for long-term endothelialization under high shear stress conditions.

Institute of Materials Science