Molecular kinetics and cooperative effects in friction and adhesion of fast reversible bonds

Switchable Adhesion: On-Demand Bonding and Debonding

Adhesives have a long and illustrious history throughout human history. The development of synthetic polymers has highly improved adhesions in terms of their strength and environmental tolerance. As soft robotics, flexible electronics, and intelligent gadgets become more prevalent, adhesives with changeable adhesion capabilities will become more necessary. These adhesives should be programmable and switchable, with the ability to respond to light, electromagnetic fields, thermal, and other stimuli. These requirements necessitate novel concepts in adhesion engineering and material science. Considerable studies have been carried out to develop a wide range of adhesives. This review focuses on stimuli-responsive material-based adhesives, outlining current research on switchable and controlled adhesives, including design and manufacturing techniques. Finally, the potential for smart adhesives in applications, and the development of future adhesive forms are critically suggested.

Size compatibility and concentration dependent supramolecular host-guest interactions at interfaces

The quantification of supramolecular host-guest interactions is important for finely modulating supramolecular systems. Previously, most host-guest interactions quantified using force spectroscopic techniques have been reported in force units. However, accurately evaluating the adhesion energies of host-guest pairs remains challenging. Herein, using a surface forces apparatus, we directly quantify the interaction energies between cyclodextrin (CD)-modified surfaces and ditopic adamantane (DAd) molecules in water as a function of the DAd concentration and the CD cavity size. The adhesion energy of the β-CD-DAd complex drastically increased with increasing DAd concentration and reached saturation. Moreover, the molecular adhesion energy of a single host-guest inclusion complex was evaluated to be ~9.51 kBT. This approach has potential for quantifying fundamental information toward furthering the understanding of supramolecular chemistry and its applications, such as molecular actuators, underwater adhesives, and biosensors, which require precise tuning of specific host-guest interactions.

Ageing of Polymer Frictional Interfaces: The Role of Quantity and Quality of Contact

When two objects are in contact, the force necessary for one to start sliding over the other is larger than the force necessary to keep the sliding motion going. This difference between static and dynamic friction is thought to result from a reduction in the area of real contact upon the onset of slip. Here, we resolve the structure in the area of contact on the molecular scale by means of environment-sensitive molecular rotors using (super-resolution) fluorescence microscopy and fluorescence lifetime imaging. We demonstrate that the macroscopic friction force is not only controlled by the area of real contact but also controlled by the "quality" of that area of real contact, which determines the friction per unit contact area. We show that the latter is affected by the local density of the contacting surfaces, a parameter that can be expected to change in time at any interface that involves glassy, amorphous materials.