Boundary conditions that depend on gradients of the field

[JonasVeenstra]

Dear Dr. Zwicker,

Thanks for developing an extremely useful package!

I am looking to implement boundary conditions that are functions of the field u and its gradients.
I am aware that we can use bc={"value_expression": bc_value} to set the field and its derivative equal to a function of time, space and the fields.
Is it possible to construct a boundary condition that contains gradients and laplacians?
For example, can I integrate the wave equation while constraining the laplacian of the field to be zero at the boundary?

[david-zwicker]

I'm not a 100% sure what you're trying to achieve, but it is possible to control the second derivative in normal direction at the boundary using the curvature keyword; see documentation of the boundary conditions. You might thus achieve what you want by simply using pde.WavePDE(bc={"curvature": 0}).