For very small particles (with a diameter less than 1 μm), scientists found that the settling velocity is faster than that predicted by the Stokes's law.
Why is that? See the animation below:
A critical assumption of Stokes's law is that the relative velocity of the gas right at the particle surface is 0. This is true when the medium is "continuous". As the particle gets smaller, the medium is no longer "continuous" to the particle and each molecule is no longer invisible to the particle. Gas molecules moving around the particle may miss the particle, which we call "slip". So, the velocity of the gas right at the surface is no longer 0 due to the missing collision. Since the collision is the source of drag. The particle's settling velocity becomes faster than previously theorized due to smaller-than-expected resistance.