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In situ observation of colloidal monolayer nucleation driven by an alternating electric field


The nucleation of crystalline materials is a hotly debated subject in the physical sciences1. Despite the emergence of several theories in recent decades2,3,4,5,6,7, much confusion still surrounds the dynamic processes of nucleation5,6,7. This has been due in part to the limitations of existing experimental evidence. Charged colloidal suspensions have been used as experimental model systems for the study of crystal nucleation and structural phase transitions8,9,10,11,12, as their crystallization phase diagram is analogous to that of atomic and molecular systems, but they can be visualized using microscopy. Previously, three-dimensional imaging of colloidal nucleation dynamics was achieved using confocal microscopy13. However, the limited temporal resolution of the confocal microscope is of concern when trying to capture real-time colloidal crystal nucleation events. Moreover, as the thermodynamic driving force has remained undefined, data on key factors such as the critical nuclei size are at best semiquantitative13. Here we present real-time direct imaging and quantitative measurements of the pre- and post-nucleation processes of colloidal spheres, and the kinetics of nucleation driven by an alternating electric field, under well-defined thermodynamic driving forces. Our imaging approach could facilitate the observation of other rarely observed phenomena, such as defect and grain-boundary formation14 and the effects of foreign particles during crystallization15. Furthermore, it may prove useful in identifying optical and biological technologies based on colloids16,17,18,19,20,21.

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We thank D.W. Li for help with the imaging process, and for discussions. We also thank C. Strom for reading the draft. This work was supported by the Science and Engineering Research Council of Singapore.

Author information

Correspondence to Xiang Y. Liu.

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Competing interests

A provisional patent application (US application number, 60/519,573; filing date 12 November 2003) has been deposited for the technologically important development disclosed in this Letter, because it may have far-reaching industrial implications. Uncertain and indirect small-scale financial competing interests for the authors exist in the form of sharing in royalties provided, if the provisional patent application becomes regularized, and it results in licence agreements. Because the disclosed invention was done in the course of employment, the exclusive property might belong to the employer (NUS) who will be the assignee of a possible regular application.

Supplementary information

Supplementary Figure

Comparison of the critical size nc of nuclei from the experimental measurement, classical nucleation theory and nucleation theorem. (DOC 579 kb)

Supplementary Table

The measured kinetics parameters of nucleation of colloidal monolayer at the various values of the driving force. (DOC 70 kb)

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Further reading

Figure 1: Method of assembling a 2D colloidal monolayer by controlling the alternating electric field (AEF).
Figure 2: Snapshots of the pre-nucleation process at multiples of t0 = 0.294 s, where t0 is the timing interval of the images.
Figure 3: Statistical measurements of parameters of nucleation kinetics.
Figure 4: Comparison of the colloidal assemblies obtained under constant electric field and under an AEF.


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