It has been known for more than 200 years that the maximum static friction force between two solid surfaces is usually greater than the kinetic friction force. In contrast to solid-solid friction, there is a lack of understanding of liquid-solid friction, i.e. the forces that impede the lateral motion of a drop of liquid on a solid surface. Here, we report that the lateral adhesion force between a liquid drop and a solid can be divided into a static and a kinetic regime. This striking analogy with solid-solid friction is a generic phenomenon that holds for liquids of different polarities and surface tensions on smooth, rough and structured surfaces.When two solid objects are brought into contact, a threshold force FTHRD must be overcome in order for one of the objects to slide 1-3 . This phenomenon can be visualised in a typical classroom experiment where a solid block attached to a spring is pulled over a solid surface (Fig. 1a). The static force FS is applied to the stationary block and then increased until it exceeds FTHRD, upon which the block begins to slide. After that, a lower kinetic force FKIN is required to maintain the block's motion 3 . However, it is not clear whether these forces develop in a comparable manner when a drop of liquid resting on a solid surface starts to slide. This gap in our understanding is astonishing, given the fact that liquid drops are omnipresent in our lives and their motion is relevant for numerous applications, including microfluidics 4 , printing 5 , condensation 6,7 , and water collection 8,9 . Hence insight on the behaviour of drops that start sliding over solid surfaces is needed.A sessile drop of liquid is usually in molecular contact with the supporting solid surface. In contrast, two solid bodies are in direct contact only at asperities owing to surface roughness 10,11 . Thus, the real contact area of a solid-solid contact is much smaller than the apparent contact area. Consequently the sliding of drops might be fundamentally different.However, by simply observing a drop of water on a pivot window pane, we know that also sessile drops start sliding when a critical tilt angle is reached, i.e. when the gravitational force acting on the drop overcomes the lateral adhesion force. The question may therefore be raised whether a static and a kinetic regime are also present for sessile drops. The general questions is: How do drops start sliding over solid surfaces and how do the forces develop while the drops slide?

We measured the forces required to slide sessile drops over surfaces. The forces were measured by means of a vertical deflectable capillary stuck in the drop. The drop adhesion force instrument (DAFI) allowed the investigation of the dynamic lateral adhesion force of water drops of 0.1 to 2 μL volume at defined velocities. On flat PDMS surfaces, the dynamic lateral adhesion force increases linearly with the diameter of the contact area of the solid-liquid interface and linearly with the sliding velocity. The movement of the drop relative to the surfaces enabled us to resolve the pinning of the three-phase contact line to individual defects. We further investigated a 3D superhydrophobic pillar array. The depinning of the receding part of the rim of the drop occurred almost simultaneously from four to five pillars, giving rise to peaks in the lateral adhesion force.

Here we investigated the stability of an aptamer, which is formed by two RNA-strands and binds the antibiotic streptomycin. Molecular dynamics simulations in aqueous solution confirmed the geometry and the pattern of hydrogen bond interactions that was derived from the crystalstructure (1NTB). The result of umbrella sampling simulations indicated a favored streptomycinbinding with a free energy of ‫ܩ∆‬ ୠ୧୬ୢ°= -101.7 kJ mol -1 . Experimentally, the increase in oligonucleotide stability upon binding of streptomycin was probed by single-molecule force spectroscopy. Rate dependent force spectroscopy measurements revealed a decrease in the natural off-rate (k off-COMPLEX = 0.22 ± 0.16 s -1 ) for the aptamer-streptomycin complex compared to the aptamer having an empty binding pocket (k off-APTAMER = 0.49 ± 0.11 s -1 ). This decrease in the natural off-rate corresponds to a decrease in the Gibbs free energy of ∆∆G ୱ୦ୣୣ୰ ≈ 3.4 kJ mol -1 .The simulated binding pattern and the experimental results led to the conclusion that hydrogen-bonds between both RNA strands mainly contribute to the decrease in natural-off rate and Gibbs free energy of the aptamer system studied.

It was recently suggested that the electrostatic double-layer force between colloidal particles might weaken at high hydrostatic pressure encountered, for example, in deep seas or during oil recovery. We have addressed this issue by means of a specially designed optical trapping setup that allowed us to explore the interaction of a micrometer-sized glass bead and a solid glass wall in water at hydrostatic pressures of up to 1 kbar. The setup allowed us to measure the distance between bead and wall with a subnanometer resolution. We have determined the Debye lengths in water for salt concentrations of 0.1 and 1 mM. We found that in the pressure range from 1 bar to 1 kbar the maximum variation of the Debye lengths was <1 nm for both salt concentrations. Furthermore, the magnitude of the zeta potentials of the glass surfaces in water showed no dependency on pressure.