Crystal melting and crystallization is a common phenomenon in nature, usually a typical first-order phase change. A phase change has so far not been a fundamental theory, because it involves a large number of particle linkage, and the lack of universal class. In contrast, the phenomenological theory of continuous phase change is given by Landau half a century ago. In the crystallization and melting, the initial and final equilibrium states and some intermediate states can be thermodynamically calculated, but the kinetic process of phase change is difficult to predict. Especially in the initial stages of melting and crystallization nucleation process, due to their complexity and molecular is too small, too fast movement, single molecules in the three-dimensional objects can not be observed, resulting in its micro-process and essential mechanism There are still many unclear questions.
As early as the late 1960s, the Colloidal Particle system was found to be analogous to atomic and molecular systems. The so-called Colloidal Particle usually refers to the solution containing 10nm to 10μm particles, so that small particles have a strong Brownian motion, not precipitation, such as milk, ink, dyes and so on. By the eighties and nineties, the rise of computer image processing has led to the development of a set of methods in the field of soft matter physics, with uniform micro-Colloidal Particleal particles as a "large atom" to simulate the condensed matter physics difficult to see the atomic scale of the movement , The thermal motion of the particles can be observed directly through the optical microscope, and the real-time trajectories of the particles are obtained by image processing , and various quantitative measurements are made. When the particle diameter is greater than the visible light half-wavelength 0.3μm, can be directly seen by the optical microscope particles. The micron particles accumulate in the air in the form of powder, with no significant thermal motion. Therefore, the particles must be suspended in the solution to form a so-called Colloidal Particle, uniform size Colloidal Particleal beads can form gas, liquid, crystal, glass phase, thus providing a platform for the study of phase change. Opal gem is a Colloidal Particleal crystal in nature. Colloidal Particleal particles with diameters less than 5 μm have strong Brownian motion in the water and can achieve thermal equilibrium. Too large particles will lose significant thermal motion and are not suitable for studying thermodynamic phase transitions. After 20 years of research, Colloidal Particle as a model of atomic and molecular models has been widely recognized. Although the Colloidal Particle system and molecular system phase change is not the same, but the two can be a mutual reference. While the Colloidal Particle itself as a class of natural substances, its phase behavior is also worth studying.
Colloidal Particleal systems and molecular systems have the following differences: (1) Colloidal Particleal phase behavior is mainly determined by the ratio of the total volume of particles to the total volume of the space, that is, the volume fraction, rather than the temperature. The volume fraction plays the role of temperature in the Colloidal Particle in the molecular system. (See Figure 2 (c)), as long as the gap between the small ball is not enough to make the ball out of its lattice position, regardless of the temperature rose to the temperature of the ball into the crystal How high the crystal is difficult to melt. For hardballs, the potential energy between particles is U = 0, and its free energy F = U-TS = -TS is dominated by entropy S. The entropy is mainly determined by the free volume of the particles being movable. The entropy is the largest in the equilibrium state and the free energy is the lowest. And for atoms or molecules is the first interaction potential play a leading role, the entropy effect can be ignored. (2) Unlike the molecules in vacuum, the Colloidal Particleal particles are in solution and the motion is overdamped. In the Brownian motion of the micro-particles, each ballistic motion caused by the collision of water molecules is on the scale of 0.1μs, so in the normal 30 frames per second video frequency, see the micro-particles in the adjacent two The displacement is the diffusion movement of many steps after the random walk, and the square of the mean displacement is proportional to the time, not the average displacement is proportional to the time, so the velocity is not defined. (3) the fluid between the Colloidal Particleal particles will bring complex interactions to the particles. Considering that the viscous coefficient is a kinetic quantity, these fluids interact with each other in the equilibrium state without affecting the structure of static forces such as the equilibrium state, but have little effect on single particle motion and system dynamics. (4) the same kind of molecules are the same, and Colloidal Particleal particles somewhat uneven. (5) thermal kinetic energy kBT and entropy play a major role in the Colloidal Particle, basically no quantum effect, which is also a common feature of soft matter. Both the Colloidal Particleal particles and the molecules, the energy of each particle is in the order of kBT (kB is Boltzmann constant), but the Colloidal Particleal particle diameter 103 times larger than the number, the number density is 109 times smaller than the molecule, heat capacity and phase Latent heat is also 109 times smaller than the molecular system. Another elastic modulus is also 109 times smaller than the molecular system, so called "soft" material.