Implementing source term

Hello everybody,

i am not really sure how to implement the source term of my problem. The problem is the RHS and it know about the issues with the regularity. It looks like the following:

(\sigma(\theta)\nabla \varphi) * \nabla \varphi.

$$ \sigma(\theta)$$ is the heat conductivity for an isotropic material. First of all i could just say it is a constant but my problem is the quadratic term after it.

Thank you and greetings

I want something like Expression(…). Or?