Between two extremely close metal plates, there is a force of attraction because the metal plates have to be nodes of any electromagnetic quantum fluctuations and so there are less virtual particles between the two plates than there are outside. This causes a difference in radiation pressure that pushes the two plates together.
I don't understand why there won't be a similar force would be generated by a metal plate in the shape of a 'u', because the shape would cause electromagnetic quantum fluctuations to be a node at the closed end and an antinode at the open end. this would reduce the virtual particles inside the tube and so there would be a radiation pressure pushing forwards.
And if this would accelerate this object would it break conservation of energy.