Why don't objects fuse to my finger when I touch them?

Category: Chemistry      Published: December 20, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

This child has definitely bonded with her environment. There is always bonding involved when you touch an object, but the effects are often too weak to notice. Public Domain Image, source: Christopher S. Baird.

Objects always fuse to your finger when you touch them. You often don't notice this for two reasons: 1) Your finger is stronger than the object you are touching. When you pull away, you rip atoms off the object and these atoms remain fused to your finger as a microscopic residue. 2) Your finger is stronger than the bonds fusing you to the object. When you pull away, you simply break the bonds holding you to the object. If you notice this at all, you experience it as a sticky feeling when you pull your finger off of a substance.

At the atomic level, the fusing of your finger occurs because of chemical bonding. There are several different types of chemical bonds:

• Covalent bonding is a very strong bonding between atoms caused by sharing electrons. Covalent bonding will happen if you touch a highly reactive substance. Touch your finger to hydrochloric acid and your finger "fuses" just fine with the acid, causing damage. Touch certain glues, and covalent bonding causes the glue to fuse with your finger. Pull your finger way (if possible) and some glue is still bonded to your finger.
• Ionic bonding is caused by the giving and taking of electrons. Ionic bonding is responsible for the stickiness of many glues and paints.
• Hydrogen bonding is when a partially bare hydrogen atom in one molecule is attracted to another molecule (or another part of itself). Hydrogen bonding occurs often between molecules in liquids. Stick your finger in water and pull it out. Your finger is wet because some of the water has hydrogen bonded with your finger. If you lick a metal pole on a cold day, the fusing power of hydrogen bonds becomes very evident.
• Metallic bonding is what happens between a large number of metallic atoms. Since your finger is not made of metal, it does not bond in this way. Metals will instead bond to your finger in other ways.
• Bulk electrostatic bonding is the static cling between large pockets of excess charge on the surface of objects. When you touch a sock just out of the dryer and it sticks to you, it is because it has bonded with you via bulk electrostatic bonding.
• Mechanical bonding is what happens when large groups of molecules intertwine. Touch a fishing hook so that it spears your finger and it has mechanically bonded to you. Many machines use mechanical bonds (screws, bolts, latches, ties) so that they can be easily disassembled without changing or damaging the material. At the atomic level, mechanical bonding is due to atomic repulsion between the two objects that are intertwined, keeping them from passing through each other.
• Van der Waals bonding happens all the time, but it is most important when none of the above bonding types are available. This bonding is very weak so you often don't notice it. When you pull Saran Wrap out and it sticks to your hand, it is doing this because it has bonded with you via Van der Waals forces.

Note that bonding occurs at the surface of the objects that you are touching. Because of this fact, the act of bonding to every object that you touch does not really change who you are. It just makes your hands dirty, takes off some dead skin cells, or gives you a (sometimes imperceptible) sticky feeling when you pull away. Additionally, every time you experience friction, you are feeling the bonding between you and the rubbing surface slowing you down.

Examples of objects and substances that will bond strongly enough to your hands for you to notice include:

• water
• peanut butter
• honey
• milk
• eggs
• corn syrup
• jelly
• just about anything liquid
• spices (you can often still smell the residue on your hands even after washing)
• glue
• paste
• epoxy
• wet paint
• resins
• melted wax
• strong acids
• strong bases
• raw alkalis
• raw halogens
• statically charged balloons
• statically charged socks
• sticky tape
• stickers
• ice pressed against cold wet hands
• candy
• dirt
• mud
• body fluids
• pen ink
• pencil graphite
• charcoal
• splinters

Topics: Van der Waals, bond, bonds, chemical bond, covalent, electromagnetism, interaction, ionic, sticky