A new twist on interfacial tension

In a mixture of two molecular components, the surface tension is defined as the energetic cost per unit area of moving molecules from the bulk and bringing them to the interface. The higher the magnitude of the surface tension, the greater the tendency of two components to demix. Surface tension allows trees to carry nutrients from the roots out to the branches, and water striders to walk on the surface of water.

The interface between hydrophobic and hydrophilic components has very high interfacial tension. A common way to adjust the magnitude of surface tension is to add amphiphilic molecules (like soaps), which contain both hydrophilic and hydrophobic components. These amphiphilic molecules prefer to be at the interface between the two components, and effectively lower the interfacial tension, allowing the components to mix more easily. This is how detergent causes oily stains to dissolve in water.

In a recently published article in Nature, an interdisciplinary team of researchers at Brandeis headed by Zvonimir Dogic, and consisting of experimental, theoretical, and computational physicists as well as biologists, has demonstrated a new way of controlling interfacial tension using a molecular property called chirality, or lack of mirror symmetry. The study was performed on a model system of two-dimensional colloidal membranes composed of the rod-like bacteriophage virus fd, which are about one micrometer in length and 7 nanometers in diameter. The electrostatically repulsive virus particles are condensed into membranes through the depletion mechanism by adding non-adsorbing polymer to a virus suspension. Because the fd rods are chiral, they tend to twist by a small angle with respect to neighboring rods. However, the geometry of the membrane prevents twisting in the structure’s interior; only along the perimeter can the rods twist. Thus, increasing the strength of chirality of the rods both lowers the energy of the rods along the membrane’s edge and increases the frustration of untwisted rods in the bulk, lowering the interfacial tension. This contrasts the standard method of controlling interfacial tension using amphiphilic molecules, since the rod-like particles are completely homogenous, and do not contain any hydrophilic components.

The strength of chiral interactions in fd is temperature sensitive; the rods are achiral at 60o C, and the strength of chirality increases with decreasing temperature. By increasing the strength of chiral interactions in-situ, the team of researchers was able to dynamically vary the membrane’s interfacial tension in order to drive a dramatic transition from a membrane to several twisted ribbon structures (Movie 1). The twisted ribbons have much more interfacial area than the membranes, but are much “twistier” structures, and are therefore favored when the strength of chirality is relatively high. Additionally, the team was able to drive the same membrane-to-ribbon transition using optical tweezers, as shown in Movie 2. Membranes and ribbons are only two of a myriad of structures that were observed in the fd system. This work presents a powerful new method to control the assembly of materials by tuning interfacial tension with chirality.

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