  The Kelvin-Helmholtz (KH) instability is commonly found in many
astrophysical, laboratory, and space plasmas. It could mix plasma components of
different properties and convert dynamic fluid energy from large scale
structure to smaller ones. In this study, we combined the ground-based New
Vacuum Solar Telescope (NVST) and the Solar Dynamic Observatories (SDO) /
Atmospheric Imaging Assembly (AIA) to observe the plasma dynamics associated
with active region 12673 on 09 September 2017. In this multi-temperature view,
we identified three adjacent layers of plasma flowing at different speeds, and
detected KH instabilities at their interfaces. We could unambiguously track a
typical KH vortex and measure its motion. We found that the speed of this
vortex suddenly tripled at a certain stage. This acceleration was synchronized
with the enhancements in emission measure and average intensity of the 193
\AA{} data. We interpret this as evidence that KH instability triggers plasma
heating. The intriguing feature in this event is that the KH instability
observed in the NVST channel was nearly complementary to that in the AIA 193
\AA{}. Such a multi-thermal energy exchange process is easily overlooked in
previous studies, as the cold plasma component is usually not visible in the
extreme ultraviolet channels that are only sensitive to high temperature plasma
emissions. Our finding indicates that embedded cold layers could interact with
hot plasma as invisible matters. We speculate that this process could occur at
a variety of length scales and could contribute to plasma heating.
