The nanomechanics of molecularly thin films studied by force spectroscopy AFM

Abstract

Most materials break through the extension of their most prominent
crack, as Griffith predicted over a century ago. Despite the fact that the
extension of a crack occurs in the nanometre-sized area located at the crack
tip end, we still know little about the crucial role that forces play at this scale.
With the advent of the Atomic Force Microscopy (AFM), we have been able
to apply small calibrated forces at the nanoscale. Until now, AFM has been
most successful at unveiling the mechanical properties of biological materials
while pulling. Investigating the mechanics of materials while pushing,
however, has been less successful. Until now, most indentation experiments
were performed at a constant pushing velocity, which precluded measuring
the detailed rupture kinetics of the material. In this vein, we have developed
an AFM capable of applying a more complex indentation protocol, called
force-clamp, which expands the time window of experimentation and allows
mapping out the energy landscape of the rupture mechanism. Then, we have
investigated the rupture kinetics of an Angstrom-scale simple 2D material –
confined solvation layers. By applying force-clamp, we have discovered that
the rupture (and reformation) of these solid-like layers occurs through the
disruption of a single molecule, contrary to currently accepted mechanical
contact models. Secondly, we have investigated the more complex mechanism
of lipid membrane rupture, which involves the displacement of tens to
hundreds of molecules. In this case, we have developed a pore nucleation
model to fit the complex rupture kinetics, which is far from the currently used
two-state model. Finally, we have indented whole live cells. As a result, we
have measured that lipid membrane lateral interactions ultimately define cell
membrane integrity. Altogether, these experiments point out the key role that
intermolecular forces play to define the mechanical strength of materials from
a fraction of nanometre to several micrometres.

Date of Award	2016
Original language	English
Awarding Institution	King's College London
Supervisor	David Richards (Supervisor) & Sergi Garcia-Manyes (Supervisor)