Ultrasonic Trap To Monitor Morphology and Stability of Developing Microparticle Aggregates

Abstract

Aggregation and flocculation of latex particles have been examined in an ultrasonic standing wave trap that draws microparticles from suspensions into a light-microscope prefocused central region of the pressure node plane. The ultrasound chamber had a single half-wavelength path length (λ/2 ) 0.25 mm) at its resonant frequency of 3.17 MHz. The full course of aggregate growth, in water or in 10 mM CaCl2, of 25 µm diameter latex particles was monitored by video microscopy. The particles formed two-dimensional aggregates in the nodal plane. Particles suspended in deionized water joined the aggregate both as single particles and as clusters. Incoming particles rolled over particles at the edge of the aggregate. The aggregate then adjusted its internal order to maintain a regular array of particles in apparent close contact. In contrast, particles in salt solutions interacted strongly on contact so that the aggregates grew as dendritic structures. The perimeter fractal dimensions of 1.15 and 1.59 in water and in salt, respectively, were typical of transport-limited and reaction-limited aggregation. When the ultrasonic exposure was terminated, the compact, low fractal dimension flocs disaggregated as they sedimented while the dendritic aggregates remained intact. It is shown that at surface separations of the order of 10 nm the acoustic particle interaction forces (dependent on the separation of centers of mass) are small compared to the van der Waals and electrostatic interaction forces (dependent on surface-surface separation) that normally control microparticle aggregation. It is concluded that the ultrasound did not significantly influence the net interactions that determine aggregate morphology and stability and that the ultrasonic trap technique holds promise for elucidating patterns of aggregate growth.

Molecular adhesion development in a neural cell monolayer forming in an ultrasound trap

“…The associated partitioning of particles into the air-water interface is not appropriate for study of animal cell surface interactions. Optical [15], dielectrophoretic [16] and ultrasonic [17,18] traps can, on the other hand, hold cells in suspension for microscopic observations. Optical traps capable of manipulating a number of cells at once have recently been created by forming light intensity patterns in a sample [15].…”

“…An ultrasound standing wave trap (USWT) to form and levitate a 2-D aggregate of particles or cells suspended away from the influence of substrata in the focal plane of a light microscope has recently been described [17,18]. The trap is a single half wavelength resonator with a pressure node plane at its centre.…”

“…However, the resultant of those forces (electrostatic, Van der Waals, steric and hydration) that normally (i.e., in suspensions without ultrasound) determines the outcome of particle interaction is a function of the even shorter surface-surface separation distance h (nm scale (Figure 1bii)). Consequently, the USWT forces operate on two scales i.e., the long range F NP driving particles to a region where they are retained by acoustic forces and where they then interact in a manner reflecting only the very short range non-acoustic force F Nh [17,18]. A schematic diagram of the USWT and particle microscopy system is shown in Figure 2.…”

Flocculation and Separation of Oil Droplets in Ultrasonic Standing Wave Field

“…It has long been established that in a nonviscous fluid ultrasonic standing wave plane, that is generated by superposition of two waves with the equal amplitude and frequency traveling in opposite directions, provides a potential energy minimum toward which rigid particles and compressible droplets as well as small aggregates of diameter lass than the wavelength move and there grow to form larger aggregates (8)(9)(10). The driving force, F ac , is axial direct ultrasonic radiation pressure that makes the particles accumulate and come into close proximity to one another at nodal or antinodal pressure planes (11):…”

Rapid Molecular and Morphological Responses of Prostate Cell Lines to Cell–Cell Contact

“…In an appropriately designed field (Bazou et al 2005b) they then move, within that plane, to accumulate at the center of the field. The technique has provided insight to the role of particle/cell surface properties in determining aggregate morphology for latex particles (Spengler and Coakley 2003) and for cells Bazou et al 2005a;Bazou et al 2006). Application of the trap has enabled data, such as the intracellular temporal progression of F-actin, N-cadherin, and NCAM formation in neural cells to be extracted from a large (ca.…”