$$\tau_{xx} = \lambda (\vec{\nabla}\cdot\vec{V})+2\mu\frac{\partial u}{\partial x}$$
You can see the velocity gradient term in there, with the leading coefficient being the proportional part. Do note that $$\lambda$$ is hard to measure, and this is where Stoke's Hypothesis (see number fudge so the equations can be solved) comes into play, where we just assume that:
$$\lambda = -\frac{2}{3}\mu$$