Polystyrene foam, filters, porous asphalt, aerogel … They all have one thing in common: besides ‘real material’ they also consist of empty space: small cavities or continuous channels – sometimes even from one side of the material to the other. In other words: they are porous.
The presence of pores influences the materials properties to a large extent. Indeed, materials as aerogel consist almost entirely of cavities. Over 95% of the volume in these cavities consists of air, and the rest is ‘glass’: silicon dioxide spheres, a few (tens of) nanometers in size that form a 3D network, and hence the edges of the cavities. In this artifical way, aerogels are one of the lightest materials that we know. Since air as well as silicon dioxide is a poor heat conductor, the glass foam has an excellent thermal insulation. Moreover: since the air is trapped in numerous ‘nano-cavities’ that are not interconnected, no heat can flow away in the form of flowing air. Although the word ‘gel’ may suggest a viscous liquid, aerogels are indeed solids. Polystyrene foam, also known as expanded polystyrene, has in fact the same thermal insulating effect, but with cavities that are much larger. You can encounter polystyrene foam as insulating material within houses.
Closed pores as in aerogels and polystyrene foam are not connected to the outside world, while open pores are. In the extreme case, pores go throughout the entire material, i.e. from one end to the other. Filters or membranes utilise this in their filtering action. They are able to allow small particles (smaller than the pore size) to pass while rejecting larger particles.
You’ll find porous materials also in places where noise reduction is necessary. Streets that are covered with porous asphalt are usually more quiet than their ‘dense’ counterparts. Sound waves – vibrating air molecules – that hit porous materials are dampened in there. The large internal surface area of the material is responsible for that. Vibrating air molecules at the pore wall transfer a part of their vibration energy to the wall, that heats up a tiny bit. This is a passive way of noise reduction, which exists along active ways of generating anti-noise using piezo materials like PZT to reduce noise.