  We numerically investigate and experimentally demonstrate an in-situ
topological band transition in a highly tunable mechanical system made of
cylindrical granular particles. This system allows us to tune its
inter-particle stiffness in a controllable way, simply by changing the contact
angles between the cylinders. The spatial variation of particles' stiffness
results in an in-situ transition of the system's topology. This manifests as
the emergence of a boundary mode in the finite system, which we observe
experimentally via laser Doppler vibrometry. When two topologically different
systems are placed adjacently, we analytically predict and computationally and
experimentally demonstrate the existence of a finite-frequency topologically
protected mode at their interface.
