Despite decades of research, it remains to be established whether the
transformation of a liquid into a glass is fundamentally thermodynamic or
dynamic in origin. While observations of growing length scales are consistent
with thermodynamic perspectives like the Random First-Order Transition theory
(RFOT), the purely dynamic approach of the Dynamical Facilitation (DF) theory
lacks experimental validation. Further, for glass transitions induced by
randomly freezing a subset of particles in the liquid phase, simulations
support the predictions of RFOT, whereas the DF theory remains unexplored.
Here, using video microscopy and holographic optical tweezers, we show that
dynamical facilitation in a colloidal glass-forming liquid unambiguously grows
with density as well as the fraction of pinned particles. In addition, we show
that heterogeneous dynamics in the form of string-like cooperative motion,
which is believed to be consistent with RFOT, emerges naturally within the
framework of facilitation. Most importantly, our findings demonstrate that a
purely dynamic origin of the glass transition cannot be ruled out.Comment: 13 pages, 3 figures. Submitted to Nature Communications on the 17th
of March, 201
