  Various soft materials share some common features, such as significant
entropic effect, large fluctuations, sensitivity to thermodynamic conditions,
and mesoscopic characteristic spatial and temporal scales. However, no
quantitative definitions have yet been provided for soft matter, and the
intrinsic mechanisms leading to their common features are unclear. In this
work, from the viewpoint of statistical mechanics, we show that soft matter
works in the vicinity of a specific thermodynamic state named moderate point,
at which entropy and enthalpy contributions among substates along a certain
order parameter are well balanced or have a minimal difference. Around the
moderate point, the order parameter fluctuation, the associated response
function, and the spatial correlation length maximize, which explains the large
fluctuation, the sensitivity to thermodynamic conditions, and mesoscopic
spatial and temporal scales of soft matter, respectively. Possible applications
to switching chemical bonds or allosteric biomachines determining their best
working temperatures are also discussed.
