Does the presence of water molecules create more gaps between 2 rough surfaces so there is less friction or most of the friction are absorbed by the water molecules?

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    $\begingroup$ If water fills all the 'gaps' resulting in a layer of water between you and the surface, then you can shear the water layer. One might suggest there is no longer 'friction' between you and the other solid surface. $\endgroup$
    – Jon Custer
    Commented Feb 17, 2022 at 16:14
  • $\begingroup$ Have you never wondered how oil act as a lubricant? $\endgroup$
    – Farcher
    Commented Feb 17, 2022 at 17:21

2 Answers 2

  • Wet surface is slippery because water molecules decrease the amount of friction actually available during the direct contact with surface.
  • Water layer acts as a lubricant because of the shearing effect on the layer of water molecules as water molecules don't hold themselves tight enough to create enough friction to walk on them.
  • Water molecules get displaced when we try to walk on them, thus decreasing the amount of friction between our shoe and the surface.

Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface. This is called wetting and it will be specific for different kind of matterial surface and also dependant on the water(soft water = weak cohesive forces, hard water = strong cohesive forces) and you did not specified what kind of materials and what is the force that is pushing this two surfaces together -> with small forces pushing the sirfaces together this can actualy have a huge influence on this interaction. You said "wet" so that indicates a good degree of wetting but it is also posible that it was used by you unintentionaly. "2 rough surfaces" that part is also making the question tricky to answer because 2 very rought surfaces can be so rought that friction can be not affected by the small amount of water betwen them.

But ignoring all the unknown factors specified above the answer would be that water as a liquid fills in irregularities and by doing so reduces the solid vs. solid type of interaction. I may also dare to sugest to read something about "angle of repose" as it may provide some deeper understanding of the interaction betwen two rough surfaces.


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