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Duros001 t1_j2r5d2h wrote

Multiple factors all work in concert to make materials sticky, two main ones are Adhesion and Cohesion

Cohesion is the force that causes similar things to stick together, whereas adhesion causes different things to stick together.

Materials like pine tar (pitch etc) have high cohesion and adhesion. Water sticks to other materials (seen as surface tension in droplets on glass for example) so has a slightly high adhesion, but it is easy to pull water away from other water molecules.

Let’s use your fingers stuck together as an example. Pitch tar molecules don’t want to pull away from each other, and are quite closely packed together. This increased viscosity, density and cohesion makes it difficult when mechanically separated for air to be able to get into the material to displace the tar that coating your fingers, so as your fingers separate, it also causes a suction effect, as the air cannot get in to regulate pressure.

All these factors add together to apply mild forces that work together; Viscosity, Density, Adhesion, Cohesion and Suction

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graebot t1_j2rcrr5 wrote

So would tar be less sticky in a vacuum?

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Indemnity4 t1_j2usq4x wrote

The properties of tar will not change in a vacuum (depending on how strong is the vacuum).

Some glues are air drying, but pine tar is not. The adhesion and cohesion properties do not change with oxygen or gravity.

However, pine tar is a mixture of hydrocarbons, acids and bases. At low vapour pressure some of those will evaporate / outgas. All the volatile stuff is acting as a solvent for the non-volatile stuff. Too strong a vacuum or too long in vacuum and the pine tar will become hard and brittle.

Problems exist in space because of the vacuum but also it's really cold. Many adhesives are only active above the glass transition temperature or melting point.

There are test standards for minimal outgassing required for adhesives used in space craft. It must not lose too much mass, and that lost mass should not recondense in a harmful way.

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Triairius t1_j2ugj9b wrote

I am actually unsure, but I suspect it might be the same amount of sticky, as there is still no air getting in to regulate the pressure. However, the tar itself might react to a vacuum somehow.

These are just my thoughts, since your question intrigued me. Someone else will hopefully provide an informed answer.

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graebot t1_j30yk7p wrote

If there's no atmosphere, then there's no pressure to press things together. Suction cups, for instance, do not work in a vacuum. They require atmospheric pressure to press the cup to the surface. If part of tar's stickiness comes from the suction effect in an atmosphere, then at least some of that "stickiness" must disappear

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[deleted] t1_j2renk4 wrote

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l4mbch0ps t1_j2s9oua wrote

Nah, it would be less sticky in a vacuum because you wouldn't have air pressure working to close the vacuum bubbles formed in the stretching material. Suction doesn't work without air pressure.

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baggier t1_j2tab8w wrote

I would broadly agree except on the density bit. Tar is not much denser than water but the key attribute is the size of the molecules. Larger molecules have more area to interact with each other, resulting in more attraction and slower motion (e.g. higher viscosity). Tar molecules are about 10-20 times bigger than water.

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Duros001 t1_j2vp2y3 wrote

True, but the interstitial pressure of a compound or emulsion is also dependent of the presence of micelles, as I presume tar is made up of various length hydrocarbons, some of which will be relatively short chain oils, among a soup of other organic compounds

Plus there are several types of “Tar” Pitch/Coal Tar are even lumped together on databases: 1.1-1.4 g/cm^3 (https://echa.europa.eu/registration-dossier/-/registered-dossier/15300/4/5) The water content, hydrocarbon lengths etc will depend on so many factors, temp it was refined at, local or actual source of “tar” (pine, coal, peat etc)

It’s almost like we’re trying to generalise a cake mix, made by thousands of different bakers, it contains mostly the same stuff, but will cook totally different :)

Edit: But yes, you are correct that viscosity is greatly affected by particle/molecular size :D thanks for the correction :)

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Indemnity4 t1_j2us3j3 wrote

> what makes it adhere and resist pulling your fingers apart?

We call that effect "tackiness". In the link a bunch of scientists (including one from 3M) discuss it better than I can.

> Also, is the pitch molecularly homogenous?

Not quite.

It is a mixture of different molecules of varying length. It's roughly the equivalent of your junk drawer full of loose cables. Yes, they are all cables, but different connectors, sometimes more than two ends, different lengths, etc.

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