Research Group Peter Schall

Institute of Physics, University of Amsterdam

Project: Colloidal Molecules

Colloidal particles offer fascinating insight into the statistical mechanics and assembly behaviour of atoms. The particles, about a micrometer in size, have thermal energy and attractive/repulsive interactions similar to atoms, making them form phases very similar to their atomic counterpart. Yet, as these colloidal particles are larger and move more slowly, they can be easily observed in real space and time. Besides being atomic models, these particles serve as building blocks for new micro- and nanoscale materials that are used, e.g. in photonics and optoelectronics.

Recently, we succeeded in  assembling analogues of molecules using “patchy” colloidal particles. These particles interact via attractive patches in specific directions only, making them form structures known from molecular compounds. The movie below (Fig. 1) shows a pair of particles with four patches, clearly bonded via one of those patches (bright). Excitingly, such 4-patch particles form structures analogous to carbon atoms in their sp3 hybridized state, such as carbon rings C5 and C6, and more complex molecules such as butane and butene, see Fig. 2. Using these “colloidal molecules”, we obtain unique insight into their atomic counterpart by 3D reconstruction of their structure (see Fig. 3 for a carbon-like C5 ring). We analyse their vibration spectrum, conformations and relaxation, and how this depends on the patch attraction, which we can vary continuously. In this project, the student can explore assembling different molecular compounds, and study their formation and relaxation, thus obtaining insight into the molecular dynamics.

Fig. 1: Bonded tetrapatch particles.
The bonded particle pair exhibits Brownian motion, making it exhibit similar dynamics as thermally excited atomic molecules.

Fig. 2: Colloidal Molecules
Patchy particle structures observed by microscopy (left) and their molecular analogues (right). These 4-patch (tetramer) particles form molecular compounds known from carbon chemistry.

Fig. 3 Conformations of Colloidal Cyclopentane
Three-dimensional reconstruction of colloidal cyclopentane (A), and their molecular counterpart (B). The colloidal molecule shows planar, half-chair and envelope conformations, analogous to molecular cyclopentane. The larger (micrometer) size of colloidal cyclopentane, allows us to directly observe its dynamics in three dimensions.

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