Well, first of all, because this test tube is being rotated in a circular motion, what that means is we know from physics this particle will have a certain tangential velocity. And the velocity here, the vector, will point tangent with respect to the circle. So this is why the velocity basically points this way for this particular particle. Now, even though the tangential velocity points this way, because this particle is suspended inside a fluid, that fluid will create a force, will exert a force on that particle. And this is the force that creates the centripetal acceleration of that object, of the particle that makes it move in a nearly circular fashion. Now, the reason the particle doesn't actually move in a perfect circle is because the particle has inertia.