system
Given the following post, respond as if you're commenting on Reddit:
human
What makes an object or surface sticky/smooth?

I feel silly asking this question but I've always wondered!
gpt
This is actually a very complicated topic, so I'll start with a basic overview of some key points. You can ask about any parts you want more detail on, and we'll take it from there.

Stickiness and smoothness are derived from a number of factors, including surface chemistry and structure at the very core. Chemistry or the makeup of a material is obviously crucial. Stickiness is a macroscopic form of attractive forces, while smoothness is basically about a low friction surface. These are not exactly opposites, but they do run counter to each other in many ways.

At the molecular level, a surface can be sticky towards different things based on any number of forces or interactions. Electrostatics (positively and negatively charged molecules or parts of molecules) can stick things together, as can ionic interactions, hydrogen bonding, Van der Waals forces, and a bunch of others. You can also have chemical reactions that form covalent bonds between two different materials, but that's usually more permanent. Velcro-like entanglement can also happen with longer polymer molecules, for instance.

Smoothness, chemically, is about reducing attractive forces and often increasing (but not necessarily) repulsive forces. This reduces the observed frictional forces. The most well known materials like this would be Teflon and similar perfluorinated compounds, although there are many others as well.

One important note about the chemistry component: nothing repels or attracts everything. It's always targeted based on the most important interactions you want to control.

Structure is however equally important. A piece of steel can feel smooth if polished, but very rough if it's scraped up. There is also the famous example of gecko feet, which have thousands of tiny hair like structures, each of which are a tiny bit sticky (via Van der Waals forces). Add up those thousands of little sticky forces, and geckos can climb walls. Or the famous lotus effect, a structural form of water resistant coating based on the structures observed in lotus leafs. By optimizing the surface area (either to reduce it maximize it) and angles, you can greatly affect the surface interactions of a material, but it's hard to give a universal rule for this. It must be examined in the context of a specific material and it's interesting with specific substances.