Sub-micron particle behaviour and capture at an immuno-sensor surface in an ultrasonic standing wave

Spin reversal and orbital torques on a viscous fluid Rayleigh sphere located arbitrarily in acoustical Bessel vortex (spiraling) beams

The goal of this work is to demonstrate the emergence of a spin torque singularity (i.e. zero spin torque) and a spin rotation reversal of a small Rayleigh lipid/fat viscous fluid sphere located arbitrarily in space in the field of an acoustical Bessel vortex beam. This counter-intuitive property of negative spin torque generation suggests a direction of spin rotation in opposite handedness of the angular momentum carried by the incident beam. Such effects may open new capabilities in methods of quantitative characterization to determine physical properties such as viscosity, viscoelasticity, compressibility, stiffness, etc., and other techniques for the rotation and positioning using acoustical tractor beams and tweezers, invisibility cloaks, and acoustically-engineered composite metamaterials to name a few examples. Based on the descriptions for the velocity potential of the incident beam and the scattering coefficients of the sphere in the long-wavelength approximation limit, simplified expressions for the spin and orbital radiation torque components are derived. For beams with (positive or negative) unit topological charge (m=±1), the axial spin torque component for a Rayleigh absorptive sphere is maximal at the center of the beam, while it vanishes for |m|>1 therein. Moreover, the longitudinal orbital torque component, causing the sphere to rotate around the center of the beam is evaluated based on the mathematical decomposition using the gradient, scattering and absorption transverse radiation force vector components. It is shown that there is no contribution of the gradient transverse force to the orbital torque, which is only caused by the scattering and absorption transverse force components. Though the incident acoustical vortex beam carrying angular momentum causes the sphere to rotate in the same orbital direction of the beam handedness, it induces a spin torque singularity (i.e. zero spin torque) and subsequent sign reversal. This phenomenon of negative spin torque generation may be exploited from the standpoint of particle sizing, and possibly other applications in particle manipulation and rotation. In addition, an application of the spin and orbital radiation torque formulations derived here in the Rayleigh limit concerns the inverse determination of the host fluid viscosity from the induced sphere spinning and/or orbital rotation.

Temperature and trapping characterization of an acoustic trap with miniaturized integrated transducers--towards in-trap temperature regulation

An acoustic trap with miniaturized integrated transducers (MITs) for applications in non-contact trapping of cells or particles in a microfluidic channel was characterized by measuring the temperature increase and trapping strength. The fluid temperature was measured by the fluorescent response of Rhodamine B in the microchannel. The trapping strength was measured by the area of a trapped particle cluster counter-balanced by the hydrodynamic force. One of the main objectives was to obtain quantitative values of the temperature in the fluidic channel to ensure safe handling of cells and proteins. Another objective was to evaluate the trapping-to-temperature efficiency for the trap as a function of drive frequency. Thirdly, trapping-to-temperature efficiency data enables identifying frequencies and voltage values to use for in-trap temperature regulation. It is envisioned that operation with only in-trap temperature regulation enables the realization of small, simple and fast temperature-controlled trap systems. The significance of potential gradients at the trap edges due to the finite size of the miniaturized transducers for the operation was emphasized and expressed analytically. The influence of the acoustic near field was evaluated in FEM-simulation and compared with a more ideal 1D standing wave. The working principle of the trap was examined by comparing measurements of impedance, temperature increase and trapping strength with impedance transfer calculations of fluid-reflector resonances and frequencies of high reflectance at the fluid-reflector boundary. The temperature increase was found to be moderate, 7°C for a high trapping strength, at a fluid flow of 0.5mms(-1) for the optimal driving frequency. A fast temperature response with a fall time of 8s and a rise time of 11s was observed. The results emphasize the importance of selecting the proper drive frequency for long term handling of cells, as opposed to the more pragmatic way of selecting the frequency of the highest acoustic output. Trapping was demonstrated in a large interval between 9 and 11.5MHz, while the main trapping peak displayed FWHM of 0.5MHz. A large bandwidth enables a more robust manufacturing and operation while allowing the trapping platform to be used in applications where the fluid wavelength varies due to external variations in fluid temperature, density and pressure.

Controlling the acoustic streaming by pulsed ultrasounds

We propose a technique based on pulsed ultrasounds for controlling, reducing to a minimum observable value the acoustic streaming in closed ultrasonic standing wave fluidic resonators. By modifying the number of pulses and the repetition time it is possible to reduce the velocity of the acoustic streaming with respect to the velocity generated by the continuous ultrasound mode of operation. The acoustic streaming is observed at the nodal plane where a suspension of 800nm latex particles was focused by primary radiation force. A mixture of 800nm and 15μm latex particles has been also used for showing that the acoustic streaming is hardly reduced while primary and secondary forces continue to operate. The parameter we call "pulse mode factor" i.e. the time of applied ultrasound divided by the duty cycle, is found to be the adequate parameter that controls the acoustic streaming. We demonstrate that pulsed ultrasound is more efficient for controlling the acoustic streaming than the variation of the amplitude of the standing waves.

Seed particle-enabled acoustic trapping of bacteria and nanoparticles in continuous flow systems

Acoustic trapping of sub-micron particles can allow enrichment and purification of small-sized and low-abundance microorganisms. In this paper, we investigate the dependency of the ability to capture sub-micron particles on the particle concentration. Based on the findings, it is demonstrated that seed particles can be introduced to acoustic trapping, to enable capture of low-abundance sub-micron particles. Without using seed particles, continuous enrichment of 490 nm polystyrene particles is demonstrated in a rectangular capillary with a locally generated acoustic field at high particle concentrations, i.e. above 1% wt. Trapping of sub-micron particles at significantly lower concentrations was subsequently accomplished by seeding 10-12 micrometer-sized particles in the acoustic trap prior to the sub-micron particle capture. Furthermore, the new seeded-particle-aided acoustic trapping technique was employed for the continuous enrichment of bacteria (E. coli) with a capture efficiency of 95%. Finally, seed particle assisted acoustic trapping and enrichment is demonstrated for polymer-based particles down to 110 nm in diameter.

Acoustic radiation- and streaming-induced microparticle velocities determined by microparticle image velocimetry in an ultrasound symmetry plane

We present microparticle image velocimetry measurements of suspended microparticles of diameters from 0.6 to 10 μm undergoing acoustophoresis in an ultrasound symmetry plane in a microchannel. The motion of the smallest particles is dominated by the Stokes drag from the induced acoustic streaming flow, while the motion of the largest particles is dominated by the acoustic radiation force. For all particle sizes we predict theoretically how much of the particle velocity is due to radiation and streaming, respectively. These predictions include corrections for particle-wall interactions and ultrasonic thermoviscous effects and match our measurements within the experimental uncertainty. Finally, we predict theoretically and confirm experimentally that the ratio between the acoustic radiation- and streaming-induced particle velocities is proportional to the actuation frequency, the acoustic contrast factor, and the square of the particle size, while it is inversely proportional to the kinematic viscosity.

Acoustofluidics 14: Applications of acoustic streaming in microfluidic devices

In part 14 of the tutorial series "Acoustofluidics--exploiting ultrasonic standing wave forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we provide a qualitative description of acoustic streaming and review its applications in lab-on-a-chip devices. The paper covers boundary layer driven streaming, including Schlichting and Rayleigh streaming, Eckart streaming in the bulk fluid, cavitation microstreaming and surface-acoustic-wave-driven streaming.

Ultrasonic enhancement of bead-based bioaffinity assays

Ultrasonic radiation forces can be used for non-intrusive manipulation and concentration of suspended micrometer-sized particles. For bioanalytical purposes, standing-wave ultrasound has long been used for rapid immuno-agglutination of functionalized latex beads. More recently, detection methods based on laser-scanning fluorometry and single-step homogeneous bead-based assays show promise for fast, easy and sensitive biochemical analysis. If such methods are combined with ultrasonic enhancement, detection limits in the femtomolar region are feasible. In this paper, we review the development of standing-wave ultrasonic manipulation for bioanalysis, with special emphasis on miniaturization and ultrasensitive bead-based immunoassays.