APS March Meeting 2014
March 3-4 2014, Denver
Talk by Dr. Jamie Hobbs entitled "Torsional Tapping atomic force microscopy for molecular resolution imaging of semicrystalline polymers", and by Richard Bailey entitled "Atomic Force Microscopy Measurements of the Mechanical Properties of Cell Walls on Living Bacterial Cells"
Torsional tapping atomic force mi-croscopy (TTAFM) provides a considerable improvement in signal-to-noise when compared with conventional AFM imaging approaches. This enables the routine use of ultra-sharp whisker tips and leads to true molecular resolution imaging in the crystalline and crystal-amorphous interface zones in semi-crystalline samples. Peak-to-peak resolution below 0.4 nm is obtainable even on topographically rough samples. Here we will present the result of recent studies showing the molecule by molecule chain structure of various polymer samples including polyethylene and polypropylene, showing how chain conformation within the crystal and at the crystal-amorphous and crystal-air interface is influenced by processing conditions. Of particular interest are observations of the roughness of the crystal fold surface at the nanometer level even on samples that have been annealed for long times. It is also clear that the crystal surface that is presented is not always dominated by the chain like nature of the molecules, but in some cases can have a more complex character that might strongly influence how the process of crystallization should be modelled. Data on the chain level internal structure of bulk samples as revealed by cryo-microtoming, will also be discussed.
Staphylococcus aureus is a major cause of infection in humans, including the Methicillin resistant strain, MRSA. However, very little is known about the mechanical properties of these cells. Our investigations use AFM to examine live S. aureus cells to quantify mechanical properties. These were explored using force spectroscopy with different trigger forces, allowing the properties to be extracted at different indentation depths. A value for the cell wall stiffness has been extracted, along with a second, higher value which is found upon indenting at higher forces. This higher value drops as the cells are exposed to high salt, sugar and detergent concentrations, implying that this measurement contains a contribution from the internal turgor pressure. We have monitored these properties as the cells progress through the cell cycle. Force maps were taken over the cells at different stages of the growth process to identify changes in the mechanics throughout the progression of growth and division. The effect of Oxacillin has also been studied, to better understand its mechanism of action. Finally mutant strains of S. aureus and a second species Bacillus subtilis have been used to link the mechanical properties of the cell walls with the chain lengths and substructures involved.