arXiv subjects

Ashley L. Nord

Publications and source records attributed to Ashley L. Nord.

4 recordsLinked to original sources

arXiv2025

Understanding and improving axial detection in optical tweezers based on the interference of forward- and backward- scattered light

Fast and accurate 3D position detection in optical tweezers (OT) is essential for quantitatively monitoring subtle variations in the mechanical properties of microscopic systems ranging from biomolecules to cells and colloids. Because standard OT configurations do not provide direct access to the axial position, axial detection typically relies on temporal fluctuations in forward-scattered optical power to infer the position of the particle. This approach generally assumes a linear-response regime in which the signal arises from the interference between the forward scattered and the nonscattered optical fields; however, under certain conditions, the backward-scattered contribution becomes non-negligible, leading to deviations from the linear response. Here, we present a simple yet comprehensive model for axial detection in standard OT while explicitly accounting for the backward-scattered field. Together with experimental validation, this framework neatly explains the standing-wave response observed when the backward-scattered field interferes with the nonscattered and the forward-scattered components, enabling accurate estimation of trap stiffness and particle diffusion under more general conditions. This work deepens our understanding of the phenomenology observed in real optical-tweezers measurements and extends their capabilities to conditions where standard approaches fail.

physics.optics↗

arXiv2021

Dynamic stiffening of the flagellar hook

Many bacteria are motile by means of one or more rotating rigid helical flagella, making them the only known organism to use rotation as a means of propulsion. The rotation is supplied by the bacterial flagellar motor, a particularly powerful rotary molecular machine. At the base of each flagellum is the hook, a soft helical polymer that acts as a universal joint, coupling rotation of the rigid membrane-spanning rotor to rotation of the rigid extra-cellular flagellum. In multi-flagellated bacterial species, where thrust is provided by a hydrodynamically coordinated bundle of flagella, the flexibility of the hook is particularly crucial, as many of the flagella within the bundle rotate significantly off-axis from their motor. But, consequently, the thrust produced by a single rotating flagellum applies a significant bending moment to the hook. So, the hook needs to simultaneously provide the compliance necessary for off-axis bundle formation and the rigidity necessary to withstand the large hydrodynamical forces of swimming. To elucidate how the hook can fulfill this double functionality, measurements of the mechanical behavior of individual hooks under dynamical conditions are needed. Here, via new high-resolution measurements and a novel analysis of hook fluctuations during in vivo motor rotation in bead assays, we resolve the elastic response of single hooks under increasing torsional stress, revealing a clear dynamic increase in their bending stiffness. Accordingly, the persistence length of the hook increases by more than one order of magnitude with applied torque. Such strain-stiffening allows the system to be flexible when needed yet reduce deformation under high loads, allowing cellular motility at high speed.

physics.bio-ph↗

arXiv2020

High resolution photonic force microscopy based on sharp nano-fabricated tips

Sub-nm resolution images can be achieved by Atomic Force Microscopy (AFM) on samples that are deposited on hard substrates. However, it is still extremely challenging to image soft interfaces, such as biological membranes, due to the deformations induced by the tip. Photonic Force Microscopy (PhFM), based on optical tweezers (OT), represents an interesting alternative for soft scanning-probe microscopy. Using light instead of a physical cantilever to hold the scanning probe results in a stiffness ($k_{OT}\sim0.1-0.001$ pN/nm) which can be 2-3 orders of magnitude lower than that of standard cantilevers ($k_{AFM}\sim 10$ pN/nm). Combined with nm resolution of displacement measurements of the trapped probe, this allows for imaging soft materials without force-induced artefacts. However, the size of the optically trapped probe, often chosen as a $\sim μ$m-size sphere, has so far limited the resolution of PhFM. Here we show a novel and simple nanofabrication protocol to massively produce optically trappable quartz particles which mimic the sharp tips of AFM. We demonstrate and quantify the stable trapping of particles with tips as sharp as 35 nm, the smallest used in PhFM to date. Raster scan images of rigid nanostructures with features smaller than 80 nm obtained with our tips compare well with AFM images of the same samples. Imaging the membrane of living malaria-infected red blood cells produces no visible artefacts and reveals the sub-micron structural features termed knobs, related to the parasite activity within the cell. The use of nano-engineered particles in PhFM opens the way to imaging soft and biological samples at high resolution.

physics.ins-det↗

arXiv2019

Load-dependent adaptation near zero load in the bacterial flagellar motor

The bacterial flagellar motor is an ion-powered transmembrane protein complex which drives swimming in many bacterial species. The motor consists of a cytoplasmic 'rotor' ring and a number of 'stator' units, which are bound to the cell wall of the bacterium. Recently, it has been shown that the number of functional torque-generating stator units in the motor depends on the external load, and suggested that mechanosensing in the flagellar motor is driven via a 'catch bond' mechanism in the motor's stator units. We present a method that allows us to measure -- on a single motor -- stator unit dynamics across a large range of external loads, including near the zero-torque limit. By attaching superparamagnetic beads to the flagellar hook, we can control the motor's speed via a rotating magnetic field. We manipulate the motor to four different speed levels in two different ion-motive force (IMF) conditions. This framework allows for a deeper exploration into the mechanism behind load-dependent remodelling by separating out motor properties, such as rotation speed and energy availability in the form of IMF, that affect the motor torque.

physics.bio-ph↗