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?

Polydimethylsiloxane (PDMS) can be grafted to metal-oxide photocatalysts such as titanium oxide by simple UV irradiation in solution or melt. The PDMS graft metal oxides are still photocatalytically active. They are hydrophobic, liquid repellent, self-cleaning, prevent biofouling and are long-term stable even in UV light.

The assembly of colloidal particles from evaporating suspension drops is seen as a versatile route for the fabrication of supraparticles for various applications. However, drop contact line pining leads to uncontrolled shapes of the emerging supraparticles, hindering this technique. Here we report how the pinning problem can be overcome by self-lubrication. The colloidal particles are dispersed in ternary drops (water, ethanol, and anise-oil). As the ethanol evaporates, oil microdroplets form (‘ouzo effect’). The oil microdroplets coalesce and form an oil ring at the contact line, levitating the evaporating colloidal drop (‘self-lubrication’). Then the water evaporates, leaving behind a porous supraparticle, which easily detaches from the surface. The dispersed oil microdroplets act as templates, leading to multi-scale, fractal-like structures inside the supraparticle. Employing this method, we could produce a large number of supraparticles with tunable shapes and high porosity on hydrophobic surfaces.

A method for mesoporous supraparticle synthesis on superamphiphobic surfaces is designed. Therefore, supraparticles assembled with nanoparticles are synthesized by the evaporation of nanoparticle dispersion drops on the superamphiphobic surface. For synthesis, no further purification is required and no organic solvents are wasted. Moreover, by changing the conditions such as drop size and concentration, supraparticles of different sizes, compositions, and architectures are fabricated.