(184d) Chiral Rheology of a Phospholipid Monolayer: Ductile Plasticity Vs. Brittle Fracture
The phospholipid DPPC contains two hydrocarbon tails attached to the headgroup in a chiral fashion, and biologically-synthesized DPPC is enantiomerically-pure (R). At moderate surface pressures, DPPC monolayers at the air-water interface exhibit phase coexistence between disordered liquid (LE) and condensed liquid (LC) phases, wherein crystals of LC-DPPC nucleate and grow, forming domains whose micron-scale shape belies the underlying molecular chirality. Using our microfabricated, ferromagnetic, amphiphilic "microbuttons" as rheological probes, we have previously shown LC-DPPC to behave as a concentrated 2D emulsion -- a viscoelastic solid with a finite, line-tension-mediated yield stress, and hour-long "healing" times following large-strain deformations.
Here, we reveal this single-component, uncrosslinked molecule to reveal a further surprise: its nonlinear rheological response shows a strong signature of its chirality. In particular, the nonlinear elastic response is stiffer when the microbutton is rotated counter-clockwise (CCW) than clockwise (CW), and the CCW yield stress is larger than the CW yield stress. This chiral rheology varies linearly with the enantiomeric excess of R vs. S. Direct visualization of the deforming domains shows qualitatively different nonlinear domain evolution for CCW and CW deformations. Furthermore, under applied CW stresses above the CW yield stress, domains exhibit ductile plasticity, deforming smoothly into long, thin domains. By contrast, domains exhibit brittle fracture under large CCW deformations. These properties can be understood in terms of simple mechanical models.