Acoustic spectroscopy for evaluating dimensions of cylindrical carbon nano-objects in colloidal systems

- A method for evaluating dimensions of cylindrical objects in colloids is proposed.
- The mechanism relies on orienting nano-objects along accelerated flow of colloid.
- Alignment of nano-objects is controlled by measuring viscosity of colloid.
- A diameter is calculated for cylindrical objects oriented across ultrasonic wave.
- An aspect ratio is evaluated from spectra of oriented and non-oriented objects.

Ultrasound attenuation spectroscopy was applied to develop an express method for determining two dimensions of cylindrical nano-objects in colloids. For this we offered to perform measurements of attenuation spectra for two independent states of colloids: with the isotropic distribution of nano-objects and with orientation of nano-objects across the ultrasound beam. The oriented state of the nano-objects is achieved in the flowing colloid due to flow acceleration in the constricted channel. As a criterion of maximum orientation of nano-objects along the flow we used the decreasing of longitudinal viscosity close to the asymptotic value. A significant difference was found between attenuation spectra measured in two states for three aqueous colloids of carbon nanotubes and nanofibers with aspect ratio from 27 to 415. The diameter values from 5 to 21 nm of the nano-objects, calculated from attenuation spectra of oriented state, corresponded to TEM and AFM data. We suggested the methodology for evaluation of the aspect ratio of the cylindrical nano-objects in colloids based on the attenuation spectra in two states of colloids and the calibration function which should be preliminary defined using acoustic and microscopy tests of reference colloids.

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