Magnetic stage with environmental control for optical microscopy and high-speed nano- and microrheology

Abstract

A novel design of a low-field magnetic stage for optical microscopy of droplets and films within a controlled environment is described. The stage consists of five magnetic coils with a 3D magnetic sensor in a feedback control loop, which allows one to manipulate magnetic nano and microprobes with microTesla fields. A locally uniform time-dependent field within the focal plane of the microscope objective enables one to rotate the probes in a precisely set manner and observe their motion. The probe tracking protocol was developed to follow the probe rotation in real time and relate it with the viscosity of the host liquid. Using this magnetic stage, a method for measuring mPa•s-level viscosity of nanoliter droplets and micron thick films in a 10-20 second timeframe is presented and validated. The viscosity of a rapidly changing liquid can be tracked by using only a few visible probes rotating simultaneously. Vapor pressure and temperature around the sample can be controlled to directly measure viscosity as a function of equilibrium vapor pressure; this addresses a significant challenge in characterization of volatile nanodroplets and thin films. Thin films of surfactant solutions undergoing phase transitions upon solvent evaporation were studied and their rheological properties were related to morphological changes in the material.

3D motion of flexible ferromagnetic filaments under a rotating magnetic field

“…An out of plane motionprecession -was predicted for soft magnetic particles in a rotating field 14 . Later this was observed experimentally for ferromagnetic nanorods 15 and recently elaborated in detail both experimentally and theoretically by Palkar et al 6 , where they show how a ferromagnetic rod beyond a critical frequency can have two possible regimes: unstable in-plane asynchronous back and forth motion and stable synchronous precession. It is worth noting that the critical frequency f c both for ferromagnetic rods and filaments is proportional to the magnetic field H and inversely proportional to the length squared L 2 .…”

Haemolymph viscosity in hawkmoths and its implications for hovering flight

“…Analyses of the viscous drag on nanorods exerted by haemolymph of adult Lepidoptera, with its small concentration of haemocytes, indicate that the haemolymph behaves as a Newtonian fluid with constant viscosity [7,27,28,42,54]. Haemolymph viscosity of all butterflies and hawkmoths varied in the range 1.3±0.25 mPa s<η<2.2±0.5 mPa s and was always greater than that of deionized water, η=0.9±0.1 mPa s (electronic supplementary material, table S1).…”

“…The rotation was recorded and the nanorod length and direction were analysed using Labview and MatLab algorithms that allowed us to track the nanorods and fit their trajectories, using the models of two-dimensional and three-dimensional rotations of nanorods [27,28]. Both two-dimensional and three-dimensional motions of nanorods were analysed, and a system of differential equations describing nanorod rotation [28] was numerically solved to extract the fitting parameter ωnormalc=false(πd2L0⋅Mnormalr⋅Bfalse)/ false(4normalΓ⋅ηfalse), where η is the haemolymph viscosity and Γ=false(πL03false)/ false(3ln⁡false(L0/dfalse)−2.4false) is the nanorod form-factor.…”

“…Haemolymph was placed on the dried nanorod-methanol residue and stirred with a glass rod to disperse the nanorods. Pure nitrogen gas pre-bubbled through deionized water to saturate it with water vapour was passed through an environmental chamber [27] and over the surface of the sample. Water evaporation was significantly slowed, and no oxidation occurred, which was verified by the absence of colour royalsocietypublishing.org/journal/rspb Proc.…”

“…The lack of viscosity data hinders rigorous analysis of the energy demands for flying insects. To fill this gap, we characterized haemolymph viscosity of the adults of 14 species of Lepidoptera by applying magnetic rotational spectroscopy (MRS) with magnetic nanorods [7,27,28]. To put the viscosity data into the context of haemolymph circulation, we used micro-computed tomography scans to characterize the structure of the thoracic haemolymph pathways of hawkmoths.…”