  We consider fluid wetting on a corrugated substrate using effective
interfacial Hamiltonian theory and show that breaking the translational
invariance along the wall can induce an 'unbending' phase transition in
addition to unbinding. Both first order and second order unbending transitions
can occur at and out of coexistence. Results for systems with short-ranged and
long-ranged forces establish that the unbending critical point is characterised
by hyperuniversal scaling behaviour. We show that, at bulk coexistence, the
adsorption at the unbending critical point is a universal multiple of the
adsorption for the correspondent planar system.
