What is the Radial Component of Velocity on the Surface of a Solid Sphere?

[Calu]

Homework Statement

In certain hydrodynamic flows the velocity of a liquid is given by v = -grad φ,
where φ is referred to as the velocity potential. The potential given by:
'equation'
corresponds to flow around a solid sphere of radius a. U is a constant.
Determine the velocity of the fluid throughout the flow. Confirm that the
radial component of velocity is zero on the surface of the sphere.

I've successfully found the velocity of the fluid, as my answer is in agreement with the answer given. However, I'm not sure this part of the question means: '
Confirm that the
radial component of velocity is zero on the surface of the sphere.'

Could anyone help me?

 

[mfb]

A radial component of the velocity would mean something flows into or out of the sphere. This should not happen, and you are supposed to verify this.
In other words, the flow at the surface has to be parallel to the surface.