Have you ever wondered that why it is that small objects can stick to your fingers easily if you push on them hard enough? You can try it with a penny by squeezing down on one side with your finger. The penny should stick to your skin. Try and lift the penny in the air and see how long you can hold it. As long as you don’t sweat or move too much, you should be able to hold for a few minutes, at least. Why does this work with a penny and not with, let’s say, your phone?
When it comes to the correct answer, there are actually two explanations for this phenomenon. The first one is related to the chemical properties of your skin and the object.
Whenever any two surfaces are brought together, there’s a weak chemical bonding between them, which is usually the source of adhesion. The real question is, why don’t our fingers pick up everything they touch? They do! Try touching some fine powder, or some liquid. The surface of powder-grains will weakly bond to your fingers.Similarly, the surface of everything weakly bonds to your fingers. However, large objects usually weigh too much to be lifted.
So, how is it possible that we can sometimes pick up much larger objects, like a penny? That’s simply a matter of the size of the surface area in contact. Ordinarily, our fingers don’t match the shape of an object’s surface. Our fingertips are not smooth. If we touch an object, the total area of direct contact at the micro-scale is very, very small. With such a small contact area, the total bonding force between the surfaces is also small.
If instead, your fingers were flat and polished, then they could pick up a much heavier object, which also had a flat and polished surface. Or, by pushing our fingers hard against a non-flat, non-polished surface, we can cause our skin to partly conform to that object’s shape, at least temporarily. In that case, the total area of contact will be much larger, so the total force of the weak chemical bond between the surfaces will also be much larger.
Try pushing your finger against a staple, or against hair strands and large dust-grains. You’ll see that they can be lifted easily.
Which common surface employs this trick for sticking to objects? Adhesive tape. The surface of the tape is not rigid. It is much more like a liquid and can easily change shape, especially down at the micro-scale. And because of this, whenever an object is placed against it, an enormous amount of the tape’s surface actually touches the object. The tape surface-molecules don’t necessarily have a larger bonding force. It’s just that a lot of the molecules are flexible enough to fit into the tiny gaps on the surface of the object.
The second explanation is more related to Physics and the directions of air pressure. Air has a unique property due to which it tends to go wherever its quantity is the lowest. Or, it always goes from an area of higher concentration to an area of a lower concentration. It is the reason why, when you spray perfume at one corner of a room, the smell gradually spreads to the whole room. The air (perfume particles) moves in a direction where its concentration is the lowest.
This is also how straws work. When you suck from a straw, you force the air out of it into your mouth. This leads to an empty space in the straw. To compensate, the air present around the liquid pushes it upwards into the straw. Hence, when the pressure gets low at one point, air rushes towards that point to increase the pressure.
In case of a coin, when it stick to your finger due to chemical bonding, all the air between your finger and the coin has been pushed out, and there is almost no space between the two (there are tiny pores in your skin that become closed when your finger presses against the coin). This causes the air from the surrounding to push upward against the coin, making it a contributory factor in keeping the coin attached. In the case of a heavier object, the air pressing upward can’t support the weight due to which the object tends to fall downward.