Different to simple solids and liquids, soft matter is typically composed of macromolecules or molecular aggregates with relatively weak interactions. It is a large family of materials. Common soft matter includes polymers, liquid crystals, colloids, biomacromolecules, and biological assembles, which are all indispensable to our everyday life. Soft matter exhibits rich self-assembly behaviors and unique physical properties, such as hierarchical structures, large non-linear response to external stimuli, and complex internal dynamics. Soft matter physics is an important branch of condensed matter physics, focusing on the physical principles and properties of soft matter. Some fundamental questions in soft matter physics include:

(1) How do specific interactions guide simple building blocks to form complex and ordered structures?

(2) What is the nature of complex diffusion, relaxation, and, glass transition in soft matter?

(3) What drives or hinders proteins to transport and organize in cell membranes and protein droplets?

All these questions are critical and meanwhile challenging. Advanced characterization techniques including synchrotron techniques are playing important roles in resolving these puzzles.

Synchrotron radiation techniques are widely used for the characterization of soft matter, revealing soft matter systems' multi-scale structure, dynamic behaviors, electronic structures, and beyond. Compared with other synchrotron facilities in China, Hefei Light Source (HLS) and the Hefei Advanced Light Facility (HALF) provide relatively low X-ray energy. Multiple beamlines at both facilities cover the soft X-ray energy range, whereas three beamlines of HALF cover the tender and medium-energy X-ray regime. Such a relatively low X-ray energy is highly suitable for soft matter characterization due to two key factors. First, lower energy provides high scattering cross section, which greatly benefits scattering-based techniques. Secondly, the abundant resonance edges in the soft to medium-energy X-ray regime enable fine characterization of elemental distribution, valence bonds, and oriented self-assemblies in soft matter.

Synchrotron characterization of soft matter largely relies on scattering-based techniques, such as small-angle and wide-angle X-ray scattering, X-ray diffraction, full-field and ptychographic X-ray imaging, which can provide structural information at nano- to micro-meter scales, such as particle sizes, polymer chain conformations, liquid crystal phases, and internal structures of cells. X-ray photon correlation spectroscopy can study the kinetic behavior of soft matter in situ, such as molecular diffusion, self-assembly behavior, phase transitions, and domain motion. At HLS and HALF, all the techniques above can be performed at multiple resonant edges for heterogeneous characterization. In addition to scattering-based techniques, spectroscopic techniques such as X-ray absorption spectroscopy can probe the chemical environment and electronic structures of specific elements, which finds its applications in many soft matter systems such as doped organic photovoltaic and metalloproteins.