Study of symmetries in colloidal monolayers
Nature likes some symmetries, but dislikes others. Ordered solids often display a so-called 6-fold rotation symmetry. To achieve this kind of symmetry, the atoms in a plane surround themselves with six neighbours in an arrangement similar to that found in a honeycomb. As opposed to this, ordered materials with 7-fold, 9-fold or 11-fold symmetries do not appear to arise in nature.
Researchers from the Max Planck Institute for Metals Research, the University of Stuttgart and the Technische Universität Berlin discovered the reason for this when they tried to impose a 7-fold symmetry on a layer of charged colloid particles using strong laser fields: the emergence of ordered structures requires the presence of nuclei to which the atoms with the corresponding symmetry can attach. Such nuclei can be found in large numbers in the symmetries for which nature shows a preference. However, they only arise sporadically in patterns with 7-fold symmetry.
The researchers generate light patterns like the ones shown in the picture above by superimposing several laser beams (Image: Jules Mikhael, University of Stuttgart). Flower-shaped structures form in the laser reliefs which act as a nucleus for the order (top left: 5-fold; top right: 6-fold; bottom left: 7-fold; bottom right: 8-fold).
Lock and key mecanism for particle self-assembly
Physicists at the New York University Center for Soft Matter Research have created “handshaking” colloid particles that link together based on their shape rather than randomly. Their work, reported in Nature, marks the first time scientists have succeeded in programming colloid particles to join in this manner and offers a type of architecture that could enhance the creation of synthetic materials.
The graphic above shows how the researchers developed a “lock and key” mechanism that allows specific particles to join together (image courtesy of Nature). “We expect these interactions to offer unprecedented opportunities for engineering smart composite particles, new functional materials, and microscopic machinery with mobile parts,” wrote the researchers.
Nanopatterning with dendrimers
Presentation
The following section is a short introduction to my most recent research (2007 to 2009) at the University of Geneva in the laboratory for colloids and surface chemistry.
The adsorption of macromolecules at the solid-liquid interface is a very common yet complicated phenomenon. One has simply to think of what happens when a drop of blood falls on a surface. Blood is a complex colloidal system. The fluid contains several macromolecules (proteins, biopolymers) as well as ions (Fe, K, Cl, Na etc.) and other cells all interacting with each other and with the surface. Preventing or promoting the adsorption of blood (or its specific constituents) to a surface may be critical in some biomedical applications. This example illustrates how macromolecular adsorption is ubiquitous, yet can be of great importance for both industry and fundamental science.
In terms of nano-technological applications, charged macromolecules from solutions can be used to pattern mineral surfaces. Typically, an interface will acquire a charge when immersed in a solution, water in this case. Similarly, the macromolecule’s ionizable functional groups can take or release protons, thus have a pH-dependent charge. When the charges of both molecule and surface have opposite signs and are of sufficient magnitude, adsorption via electrostatic interactions occurs.
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