Soft matter physics – colloidal science
1. introduction
I will give a brief introduction to soft matter, then focus on one important class of soft
material – colloid, including their interactions, phase behaviors, structures, dynamics
transport and rheological properties. Experimental techniques about colloids will be
discussed.
2. configurational temperature
k BT 
FF
 F
Temperature reflects how strong the molecular motions are.
Without knowing how fast the particles move, the thermodynamic temperature can
still be calculated form their static configurations. This configurational temperature
proposed in 1998 was confirmed in experiment for the first time in our colloid system.
We generalized the configurational temperature to an infinite series of
hyperconfigurational temperature and their applications will be discussed.
3. Brownian motions
I will talk about the history of Brownian motion and the recent
research on this topic, including our experiment about the
Brownian motion of a single ellipsoid. The translation-rotation
coupling and the non-Gaussian displacement distribution will be
discussed.
4. pattern formation
Colloids under electric fields can self-assemble into a rich set of
patterns. Some of them are analogous to the Rayleigh-Benard
convection, and some of them can be explained by the chargeinjection effect.
5. colloidal crystal melting
Despite a long history of study on crystal melting, there
are still many open questions yet to be answer. Colloids
have been used as outstanding model systems for the
study of phase transitions because the trajectories of
individual particles can be measured under microscope.
We use diameter-tunable microgel colloidal spheres to
study the melting of colloidal crystal in three dimensions, two dimensions and thin
films. Nucleation kinetics and nucleation precursor in 3D, hexatic phase in 2D,
superheated crystal, defects and boundary effects will be discussed.
6. solid-solid transitions
Solid-solid transitions widely exist in nature, but their
microscopic mechanism is poorly understood because it
is difficult in theory, simulation and experiment. We
performed the first experiment on nucleation during solid-solid transitions with
single-particle dynamics by using colloids and discovered two types of novel
nucleation processes.
7. glass transitions
The nature of glass transition is one of the major challenge is
physics. When a liquid is supercooled towards the glass
transition, dynamics drastically slows down and becomes
progressively more heterogeneous, while the static structure
changes little. Finding a relationship between the dynamics
and static structural properties is a long-standing challenge.
We report the first experiment about colloidal glass composed
of non-spherical particles and two structural signatures of dynamic heterogeneities.
The phase behavior of ellipsoids will be discussed.
8. geometrical frustration
We use colloidal spheres to mimic Ising spins on a triangular
lattice and achieved the first geometrically frustrated system
with single-‘spin’ dynamics. It connects the two research fields
of soft matter and magnetism.
9. phase-space network (statistical mechanics/math)
Phase space is a key concept in statistical physics,
but often too large and complicated to be exactly
constructed and measured. We propose to use
complex-network analysis to understand phase
spaces of some systems with discrete degree of
freedom. The phase-space networks of some spin
systems and lattice gases share some common features and establish a new class of
complex networks that have different topology compared with all the previously
known networks. An interesting mathematical by-product is the one-to-one mapping
between the 2D jigsaw-puzzle tiling and 3D sphere stacking. This mapping casts new
light on some combinatorial problems.