Chiral objects, which break mirror symmetry, range in scale from spiral galaxies to hurricanes to DNA. Macroscopic examples include snail shells , flowers and bacterial flagella. The molecular building blocks of biology are chiral, including 19 of 20 amino acids, nucleotides, and many sugars and lipids. Natural phospholipids have a chiral carbon in the sn-2 position and are exclusively R-enantiomers. This chirality is manifested in the counter-clockwise spiral patterns of condensed-phase domains in monolayers and bilayers, in which the chiral orientational order extends tens of microns. Here, we demonstrate that the mechanical response of two-dimensional, condensed-phase monolayers of 1,2-dipalmitoyl-sn-glycero-3-sn-phosphocholine (R-DPPC) is chiral as well, with nonlinear rheological properties that differ between monolayers sheared clockwise (C) or counter-clockwise (CC). The nonlinear elastic modulus and yield stress of R-DPPC monolayers are greater when sheared clockwise, i.e. against the natural CC winding of individual DPPC domains, than when sheared counter-clockwise. These differences disappear for racemic mixtures. Direct visualization of the deforming domains reveals the dramatic consequences of this chiral rheology. Domains fracture when sheared in the clockwise direction, but deform in a ductile, plastic fashion under strong counter-clockwise shearing. The macroscopic consequences of the underlying molecular chirality are remarkable given the single-component, non-cross-linked nature of DPPC monolayers.
Monolayer viscous and elastic properties are key to the stability, function and dynamics of lung surfactant monolayers. Normal breathing necessitates that the lung surfactant monolayer maintain a low surface tension as alveoli are inflated every few seconds during normal respiration, requiring that the surfactant rapidly spread to cover the expanding interface. At the same time, the lung surfactant monolayer must remain within the deep lung, despite the large surface tension gradients acting to pull it toward the trachea where the surface tension is higher. In technology, applications of foams, emulsions and dispersions can depend critically on the rheology of the surfactant monolayers used to stabilize them. On a more fundamental level, surface viscosity and elasticity can be exquisitely sensitive to molecular packing, correlation lengths, and interactions in monolayers that are not easily resolved in pressure-area isotherms or fluorescence images as we show here.
