The effect of set point ratio and surface Young’s modulus on maximum tapping forces in fluid tapping mode atomic force microscopy

Abstract

There is great interest in using proximal probe techniques to simultaneously image and measure physical properties of surfaces with nanoscale spatial resolution. In this regard, there have been recent innovations in generating time-resolved force interaction between the tip and surface during regular operation of tapping mode atomic force microscopy ͑TMAFM͒. These tip/sample forces can be used to measure physical material properties of surface in an analogous fashion to the well-established static force curve experiment. Since its inception, it has been recognized that operation of TMAFM in fluids differs significantly from that in air, with one of the major differences manifested in the quality factor ͑Q͒ of the cantilever. In air, Q is normally on the order of 200-400, whereas in fluids, it is of the order of approximately 1-5. In this study, we explore the impact of imaging parameters, i.e., set point ratio and free cantilever oscillation amplitude, on time varying tip-sample force interactions in fluid TMAFM via simulation and experiment. The numerical AFM model contains a feedback loop, allowing for the simulation of the entire scanning process. In this way, we explore the impact of varying the Young's modulus of the surface on the maximum tapping force.

Investigation of temperature induced mechanical changes in supported bilayers by variants of tapping mode atomic force microscopy

“…At 28 °C, the lipid patch had a height of 3.6 ± 0.1 nm (Fig. ), consistent with previous reports (Kumar et al ., ; Shamitko‐Klingensmith et al ., ). However, this bilayer height was slightly smaller than the theoretical height of the lipid bilayers (∼4.5–5 nm) due to the normal force applied by the cantilever tip compressing the supported bilayer (Kumar et al ., ).…”

“…), consistent with previous reports (Kumar et al ., ; Shamitko‐Klingensmith et al ., ). However, this bilayer height was slightly smaller than the theoretical height of the lipid bilayers (∼4.5–5 nm) due to the normal force applied by the cantilever tip compressing the supported bilayer (Kumar et al ., ). This observed compression was consistent with the bilayer being a more compliant surface compared to the mica substrate, as has been demonstrated by simulations of complete tapping mode AFM experiments (Shamitko‐Klingensmith et al ., ).…”

“…The experiments presented here were performed with a set point ratio of 0.8. This set point ratio was chosen based on previous studies indicating that this setting was optimal for imaging TBLE bilayer patches on mica (Kumar et al, 2010;Shamitko-Klingensmith et al, 2012). Furthermore, the exposed mica acts as an internal control to verify that F total remained constant and to facilitate direct comparison between images taken at different temperatures (Kumar et al, 2010), and all results are reported relative to this control.…”

“…This set point ratio was chosen based on previous studies indicating that this setting was optimal for imaging TBLE bilayer patches on mica (Kumar et al, 2010;Shamitko-Klingensmith et al, 2012). Furthermore, the exposed mica acts as an internal control to verify that F total remained constant and to facilitate direct comparison between images taken at different temperatures (Kumar et al, 2010), and all results are reported relative to this control. While the absolute values of bilayer height, phase shifts, amplitude responses of specific harmonics, and tapping forces from experiment to experiment could vary due to the difficulties in controlling all of the previously mentioned imaging parameters, the relative shifts in these values compared to the internal mica control were consistently reproduced, except in phase imaging as will be discussed later.…”

“…For example, increasing the spring constant of the cantilever results in a larger magnitude of imaging force, but the power law dependence of F max with the elastic modulus is preserved (Payam et al, 2012;Chaibva et al, 2014). Furthermore, the relative magnitude of F max is also preserved with changes in the set point ratio and cantilever free amplitude (Kumar et al, 2010;Shamitko-Klingensmith et al, 2012). To evaluate the ability of F max images to detect temperature induced mechanical changes in lipid bilayers associated with a phase transition, SPAM was used to invert the cantilever deflection signal into time-resolved tip/sample forces.…”