Development of a Pulsed-Field Differential Mobility Analyzer: A Method for Measuring Shape Parameters for Nonspherical Particles

Abstract

For a nonspherical particle, a standard differential mobility analyzer (DMA) measurement yields a mobility-equivalent spherical diameter, but provides no information about the degree of sphericity. However, given that the electrical mobility for nonspheres is orientation-dependent, and that orientation can be manipulated using electric fields of varying strength, one can, in principle, extract some type of shape information through a systematic measurement of mobility as a function of particle orientation. Here, we describe the development of a pulsed-field differential mobility analyzer (PFDMA) which enables one to change the peak E-field experienced by the particle to induce orientation, while still maintaining the same time-averaged field strength as a standard DMA experiment. The instrument is validated with polystyrene latex (PSL) spheres with accurately known size, and gold rods with dimensions accurately determined by transmission electron microscopy (TEM). We demonstrate how the instrument can be used for particle separation and extraction of shape information. In particular, we show how one can extract both length and diameter information for rod-like particles. This generic approach can be used to obtain dynamic shape factors or other multivariate dimensional information (e.g., length and diameter).

Shaping the future of nanomedicine: anisotropy in polymeric nanoparticle design

“…In addition, this method was utilized to characterize the content and anisotropy of spherical Janus nanoparticles of size 900 nm . Methods based on light scattering of gold nanorods at different wavelengths of light and light scattering detecting differences of electrophoretic mobility of nanorods versus nanospheres have also been reported.…”

Optimizing Filtration Experiments for Length and Fractal Dimension Characterization of Non-Spherical Particles

“…For separating non-spherical particles and spheres, Li et al (2012Li et al ( , 2013Li et al ( , 2014 found that the electrical mobility of nonspherical particles was dependent on the particle orientation in the DMA. However, the method only identified non-spherical particles with aspect ratios at least 10, such as nanorods.…”

“…For example, the scanning mobility particle sizer (SMPS, Model 3936; TSI Inc., Shoreview, MN, USA) measures the size distribution of nanoparticles and the nanoparticle surface area monitor (NSAM, Model 3550; TSI Inc., Shoreview, MN, USA) measures the human lung-deposited surface area of particles. To date, very few online instruments or methods are designed for measuring particle length, which is an important parameter responsible for the respiratory deposition due to interception, e.g., the Real-Time Fiber Monitor (M7400AD; MSP Corp., Shoreview, MN, USA), the pulse-field voltage differential mobility analyzer, DMA (Li et al 2014), interception equivalent diameter of spark discharge carbon agglomerates by filtration (Lange et al 2000), and our previous work on carbon nanotube (CNT) length also by filtration (Bahk et al 2013). In addition to the particle surface area and length, mass-mobility fractal dimension of non-spherical particles are also of high interest in nanomaterial manufacturing industries (e.g., carbon black, titania, silica, and alumina by flame reactors).…”

The effect of electric-field-induced alignment on the electrical mobility of fractal aggregates

“…The transition occurs at decreasing fields for increasing particle size (both in terms of primary sphere size and the number of primaries), as expected since polarizability is proportional to particle volume. This behavior has prompted some researchers to propose methods to separate particles with different shapes by exploiting the changes in mobility at different electric fields, such as by size-selecting particles in a DMA, followed by separation with second DMA operated at a different field strength (Zelenyuk and Imre 2007;Li et al 2014b). Using this method, one can distinguish between spheres (or aggregates with fractal dimension near 3) and more elongated particles like rods, chains, prolate ellipsoids, or soot-like aggregates, since the mobility of a sphere does not change with field strength.…”

“…Researchers have proposed experimental methods for obtaining shape information or separating particles with different shapes by exploiting the dependence of particle mobility on orientation in a DMA (Zelenyuk and Imre 2007;Li et al 2014bLi et al , 2016. Such procedures involve size-selecting particles in consecutive DMAs operated at different field strengths (or, equivalently, at different sheath flow rates).…”