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In Hydrostatic, if the fluid is a perfect fluid that is non-viscous, it doesn't experience any shear force because if it would experience so it might start flowing so something might be working to make it stay at equilibrium.

Pressure exerted on the walls of a vessel is the force exerted per unit area by the Brownian Motion of the fluid particles. Since shear force on the fluid is zero, therefore reaction force exerted by the vessel particles on the fluid is perpendicular to the surface area of vessel which implies the fluid particles are making contact perpendicularly with the surface of the vessel.

But this is not true as in Brownian Motion, particles vibrate in every direction. So there will be possibilities where they will hit the surface of the vessel at an angle. So the reaction force from the vessel to the fluid will also be at an angle, which will have a component parallel to the surface area of the vessel.

So ideally, the fluid should experience a shear force? If not, an explanation of how this works at a microscopic level would be beneficial.

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  • $\begingroup$ I found a comment of @Chemomechanics on this answer. It states that the molecules rearrange to counter those shear forces in the expense of breaking some bonds link $\endgroup$
    – soumyadip_poddar
    Commented Jul 16 at 16:10

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The force being always perpendicular to the walls is not contradictory with the impact being not perpendicular. Think of a billiard table without friction. The angle of incidence will be equal to the angle of reflection of the balls. The impact force is always normal to the table sides, there is no shear forces.

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  • $\begingroup$ Got it. I was assuming that if the impact is at an angle then the fluid particle will travel in the same direction as it was incident until the billiard table where the surface will exert force normally only hence the reflected direction will not be equal to the incident direction. I was excluding the fact that the force exerted by walls is actually a normal force exerted to the fluid particles. And normal force acts perpendicular to the surface of the body whatever might be the impact. $\endgroup$
    – soumyadip_poddar
    Commented Jul 17 at 7:27

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