Keywords: nanoparticle, particle size distribution, particle characterization
Summary:Ultrasonic Pulsed Doppler (USPD) is a powerful and unique ultrasonic backscatter technology for the characterization of nanoparticles. While conventional wisdom suggests that backscatter is too small to be measured in nanometer range, this apparent limitation has been overcome by measuring particles in motion so that the backscattered energy is Doppler shifted and received at frequencies differing from the interrogating. This avoids measurement clutter and has allowed backscatter detection down to the sub-10 nm range. Particle sizing by USPD is based on a novel principle: With adequately high intensity, the particles are set into motion by the interrogating signal with velocities that are correlated with their size. Consequently the spectrum of the backscatter contains the particle size information. The USPD system consists of a single transducer that both transmits a narrowband interrogating signal and receives the backscattered signal from the particles that have been set into motion by the interrogating signal. The shape of the received spectrum at the Doppler shifted frequencies can be simply transformed into a particle size distribution. This is in contrast to forward-scattering ultrasonic characterization systems that require complicated processing. Since only a single transducer is required, the nature of the vessel holding the suspension is of secondary importance so that in addition to being suited for developed as a standalone instrument, USPD can also be incorporated into, for example, existing process equipment such as PAT. Other advantages of USPD include the ability to measure in high concentration and/or opaque solutions with lessor requirements for dilution than with optical methods, better size resolution than achieved with many conventional instruments, and, as the strength of the backscatter depends on the density and compressibility of the particles, it may be possible to measure or monitor these physical properties. The overall strength of the backscatter can also be used to measure concentration. USPD can also be used to measure the flow of existing streams of particles. In this regard, as the system only interrogates very small fluid volumes it can be used to probe flow fields with high spatial resolution. Examples of USPD measurements with polystyrene, silica and metallic nanoparticles will be shown.