Multifragmentation resulting from an expanding nuclear matter is investigated on the basis of quantum molecular dynamics. In particular, the dependence of the fragment mass distribution on the initial temperature (T init ) and the radial flow velocity (h) is studied. When h is large, the distribution shows exponential shape, whereas for small h, it obeys the power law. Although the power law is what Fisher's droplet model predicts, the fragmentation mechanism in an expanding system is found to be different from the one in a thermally equilibrated system.

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Using an expanding matter model with a Lennard-Jones potential, the instability of the expanding system is investigated. The pressure, temperature, and density fluctuations are calculated as functions of density during the time evolution of the expanding matter, which are compared to the coexistence curve calculated by the Gibbs ensemble. The expanding matter undergoes the first order phase transition in the limit of the quasistatic expansion. The resultant fragment mass distributions are also investigated.

To seek for a possible origin of fractal pattern in nature, we perform a molecular dynamics simulation for a fragmentation of an infinite fcc lattice. The fragmentation is induced by the initial condition of the model that the lattice particles have the Hubble-type radial expansion velocities. As time proceeds, the average density decreases and density fluctuation develops. By using the box counting method, it is found that the frequency-size plot of the density follows instantaneously a universal power-law for each Hubble constant up to the size of a cross-over. This cross-over size corresponds to the maximum size of fluctuation and is found to obey a dynamical scaling law as a function of time. This instantaneous generation of a nascent fractal is purely of dynamical origin and it shows us a new formation mechanism of a fractal patterns different from the traditional criticality concept.Key words: fractal, power law, molecular dynamics, fragmentation PACS: 05.45. Df, 05.65.+b, 62.20.Mk Since the pioneering work of Mandelbrot [1] a large effort has been directed towards understanding the ubiquitous manifestation of fractal structure in nature. Since fractal structure is intimately related to a scaling property, a customary way of identifying the fractal is to check if the power-law relation of the frequency as functions of the size holds. Malcai et al. [2] in this way have listed the fractal dimensions and the corresponding scaling ranges for a variety of physical systems. They pointed out that the finiteness of the fractal

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