Nucleation: What Happens at the Initial Stage?This research is supported by ARF Project R-144-000-148-112. We are much indebted to Dr. C. Strom for her valuable suggestions and critical reading of the manuscript

Nucleation, a process taking place on a nanoor subnanoscale in typical atomic and molecular systems, creates original crystal embryos from the mother phase. It is one of the most important aspects in our daily life. It is important in situations ranging from the condensation and freezing of water (e.g. rain, snow, freezing), the formation of bones and teeth and the correlation between synergetic structures and their superior properties, to semiconductor/IT and nanotechnologies, because in most cases nucleation determines whether a phase will form, how the formation will occur, and what structure will be formed. Therefore, it is the cornerstone for many very important modern technologies, including life/nanosciences and engineering. Although many different nucleation theories, both classical and nonclassical, have been developed, nucleation, in particular its mechanism, continues to be one of the most poorly understood and disputable phenomena in the past half century. The formation of nuclei is essentially a dynamic process dominated by the competition between the reduced bulk free energy and the increased surface free energy. This competition gives rise to the so-called nucleation barrier and the critical size of nuclei. One of the major issues in the theoretical study of nucleation is to determine the nucleation barrier. However, the nucleation barrier is dependent on the structure of nuclei as well as on supersaturation. The so-called classical nucleation theory (CNT), which is themost widely used theory about nucleation, assumes that the structure and hence the surface tension of nuclei are the same as that of the new stable phase. Nevertheless, this assumption has been challenged by evidence from simulations and experiments. The key point of this evidence is that in crystallization, nucleation begins with a metastable structure and the stable structure is reached subsequently. This picture is consistent with the prediction of the Ostwald s rule. However, the kinetics of the transition from the metastable structure to the stable structure has so far been unclear because of the absence of direct observation of the transition process in real space. This is because in typical atomic systems, atoms are too small to be observed directly. As an alternative approach, colloids have been employed as a model system to study phase transitions, because colloidal particles in solutions behave like large “atoms”, and the phase behavior of colloidal suspensions is similar to that of atomic and molecular systems. The advantage of this kind of model system is that colloidal particles, and thus the nucleation process, can be observed directly by a normal optical microscope. Furthermore, the thermodynamic driving force for the nucleation can be controlled precisely in colloids, so that quantitative measurement and data interpretation become possible. In this study, the structure evolution of nucleating clusters at the initial stage of nucleation was examined in a twodimensional colloidal model system driven by an alternating electric field. It was found that distinct from the assumption of the CNT, nuclei are created with a metastable liquid-like structure, and the final stable crystal-like structure is approached through a continuous structure transition. This study offers the first experimental observation of the structure transition. Furthermore, the impact of the metastable structure on nucleation barrier was investigated in a quantitative manner. As two-dimensional (2D) and three-dimensional (3D) nucleation kinetics are very similar, the knowledge acquired in this study will significantly advance our understanding of nucleation in general. Figure 1a shows the experimental setup, and Figure 1b shows the phase diagram of the solution used in this study. Three different phases were identified: three-dimensional liquid (3DL), two-dimensional crystal (2DC), and threedimensional disordered aggregate (3DDA). In the 2DC region, the attractive force between the colloidal particles will be enhanced by increasing the amplitude or decreasing the frequency of the alternating electric field (AEF). To quantify the ordering of the two-dimensional crystal nuclei, a local two-dimensional bond-order parameter was defined by Equation (1).