  In the iron pnictides and chalcogenides, multiple orbitals participate in the
superconducting state, enabling different gap structures to be realized in
distinct materials. Here we argue that the spectral weights of these orbitals
can in principle be controlled by a tetragonal symmetry-breaking uniaxial
strain, due to the enhanced nematic susceptibility of many iron-based
superconductors. By investigating multi-orbital microscopic models in the
presence of orbital order, we show that not only $T_{c}$ can be enhanced, but
pairs of accidental gap nodes can be annihilated and created in the Fermi
surface by an increasing external strain. We explain our results as a mixture
of nearly-degenerate superconducting states promoted by strain, and show that
the annihilation and creation of nodes can be detected experimentally via
anisotropic penetration depth measurements. Our results provide a promising
framework to externally control the superconducting properties of iron-based
materials.
