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Suppose we let a ball move in a fluid with initial velocity $u_i$. Then as time goes on its velocity goes down and tends to $0$ after infinite time. I understand viscosity in the following way:

In order for the ball to move it must "create space", i.e. pull aside the molecules of the fluid. But to move them aside, it must break their intermolecular (attractive) interactions. The energy for breaking these interactions comes from the kinetic energy of the ball and is converted into potential energy that turns into heat.

This explanation though doesn't address what happens when the repulsive intermolecular forces are dominant. How does viscosity depend on these forces?

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  • $\begingroup$ Note also that viscosity is not a single quantity -- it can vary with rate, as in thixotropic, such as mayonnaise and clay/water mixtures, or dilatant, such as oobleck and Slip Lock limited-slip lubricant. So rate should be a consideration in any theory. $\endgroup$
    – DrMoishe Pippik
    Commented Feb 27, 2022 at 21:52
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    $\begingroup$ In a liquid, the overall forces are attractive. If these forces diminish, or become repulsive, the liquid vaporizes and the viscosity drops (but gases have viscosity too!). $\endgroup$
    – James Gaidis
    Commented Feb 28, 2022 at 14:43

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Intermolecular forces are attractive for 'big' distances between the molecules and repulsive for 'small' distances between the molecules.

In order for the ball to move it must "create space", i.e. pull aside the molecules of the fluid. But to move them aside, it must break their intermolecular (attractive) interactions.

The water molecules have a preferred distance between each other.At this distance the potential energy of the system is at its lowest.When a ball comes , the distance between 2 water molecules becomes shorter and the repulsive forces start to dominate.

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