Magnetic Rotational Spectroscopy for Probing Rheology of Nanoliter Droplets and Thin Films

Abstract

As one of the characterization tools for nanoscience & nanotechnology, magnetic rotational spectroscopy (MRS) with magnetic nanoprobes is a powerful method for in-situ characterization of minute amounts of complex fluids and for study of nanoscale structures in fluids. In MRS, a uniformly rotating magnetic field rotates magnetic micro-or nano-probes in the liquid and one studies rheological properties by analyzing the features of the probe rotation. The technique relies only on full rotations (rather than oscillations) of the magnetic moment of the probes. AbstractIn-situ characterization of minute amounts of complex fluids is a challenge. Magnetic Rotational Spectroscopy (MRS) with submicron probes offers flexibility and accuracy providing desired spatial and temporal resolution in characterization of nanoliter droplets and thin films when other methods fall short. MRS analyzes distinct features of the in-plane rotation of a magnetic probe, when its magnetic moment makes full revolution following an external rotating magnetic field. The probe demonstrates a distinguishable movement which changes from rotation to tumbling to trembling as the frequency of rotation of the driving magnetic field changes. In practice, MRS has been used in analysis of gelation of thin polymer films, ceramic precursors, and nanoliter droplets of insect biofluids. MRS is a young field, but it has many potential applications requiring rheological characterization of scarcely available, chemically reacting complex fluids.

Synthesis and characterization of nanorods for magnetic rotational spectroscopy

“…This dependence for nickel nanorods has been studied by our group and documented in Ref. 37. During the synthesis of Co nanorods, the applied voltage was kept constant and only the deposition time was varied.…”

“…The viscous torque depends on the geometry and angular velocity of the rod _ u. The torque balance reads [35][36][37] c _ ue ¼ m  B;…”

“…1(b)). [35][36][37] This theory assumes that the magnetic moment associated with the nanorod does not change its magnitude or direction relative to the nanorod and is perfectly aligned with the geometrical long axis of the nanorod. This puts constraints on the materials that can be used as MRS probes.…”

Magnetic Submicron Mullite Coatings with Oriented SiC Whiskers

“…The viscous drag from the polymer solution opposes the nanorod rotation. Balancing the magnetic torque by the viscous torque, Γ η = γ φ̇ , where γ is the drag coefficient defined in the Supporting Information, one arrives at the following equation − where the characteristic frequency ω c SP is introduced as , where d and l are the diameter and length of the nanorod, respectively, η is the solution viscosity, μ 0 is the permeability of vacuum, and B is the applied magnetic field.…”

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

“…The optical cell equipped with the environment control circuit is fit under the Olympus BX51 upright microscope. We illustrate robustness of this stage by studying viscous properties of liquids with a high-speed rheological technique based on Magnetic Rotational Spectroscopy [4,36,54,55], using magnetic nano-and microrods as the probes.…”