Ultrasonic forces enhance optical detection of micron-size particles in water

Water utilities employ the latest technologies to remove or lower levels of solutes, particulates, and microorganisms. Cultivation techniques, turbidity measurements, particle counters, and electrochemistry are widely used to determine and monitor the quality and safety of treated water. Growing indicator microorganisms—such as Escherichia coli—on a specific culture medium can take 24–36 hours, and the processes of medium preparation, inoculation of plates, colony counting, and biochemical characterization are labor-intensive. Turbidity measurements are routinely used to determine cell populations above 10,000 cells per milliliter, even though lower levels of microbial contamination have been reported to cause serious waterborne health outbreaks.¹

Optical signals gathered from low particle concentrations are insufficient for detection, characterization, or enumeration of contaminants in most cases. However, one way of optically detecting micron-size particles and bacterial cells in drinking water is by focusing particles using ultrasonic waves. Ultrasonic forces that arise from the particle-fluid interactions in a standing ultrasonic wave field can be used to concentrate trace quantities of particles into the small optical field-of-view within a few seconds, thus enhancing real-time water quality monitoring.

One way to improve optical detection limits is to reduce the liquid volume in a dilute suspension until the particle concentration increases to a detectable level. Ultrasonic concentration (UC) is an attractive method for this purpose for several reasons. First, it offers separation without a physical barrier, such as a filter that can get plugged over time. Second, it potentially requires little maintenance, since there are no moving parts. Third, it provides rapid concentration of particles (within seconds). Fourth, it is controllable via ultrasonic frequency and amplitude modulation. And, finally, it may be adapted to be used in a continuous system.

In UC, forces arising from the particle-fluid interactions due to differences in density and compressibility are used to separate the suspended particles into equally-spaced bands in a standing ultrasonic wave field (SUWF).^2,3 Particles much smaller than the ultrasonic wavelength and more dense than the fluid are driven to a nearby pressure node by the primary acoustic force. The speed of a single particle's response to this force is proportional to its volume. Secondary and transverse acoustic forces further retain and aggregate the particles. UC has also been used to separate biological cells without affecting cell viability^4,5

Before incorporating UC into an optical sensor design, the degree of concentration achievable with ultrasonic forces needs to be evaluated over a range of process parameters. Two 2mm-thick piezoceramic disks were driven by an amplified 975kHz sinusoidal waveform to generate an SUWF in a chamber. ⁶ A collimated beam of 540nm light was used to illuminate the contents of the chamber and an analog linear array detector was used to measure the amount of transmitted light over the length of the chamber and time. The amount of transmitted light was calibrated for different particle concentrations. The ratio of particle concentration at any given time to the initial concentration (C₀) was defined as the concentration factor (CF).

Particle suspensions were pumped through the chamber at different flow rates (Q = 0, 50, 100, and 200μl/min) and sonicated at different transducer driving voltages (V_T = 13.0, 16.5, and 20 V_rms). Banding was observed within seconds of exposure to the field (See Figure 1), and each band was approximately 100 – 200μ m wide. CF was negatively correlated to Q, V_T, and C₀. At high flow rates, drag forces exceeded the ultrasonic forces that retained the particles. At high transducer voltages, the temperature of the suspending fluid increased and resulted in an unstable SUWF. CF values of 10 or greater were achieved with dilute suspensions of 0.025% w/w (see Figure 2). This implies that when UC is coupled with an optical technique, the detection limit can be improved by one order of magnitude.

Effective UC of smaller particles and biological cells will require higher ultrasonic frequencies (2–5MHz), resulting in much closer particle bands. A more-sensitive optical technique with a small optical field of view, such as fluorescence correlation spectroscopy,⁷ would be appropriate to couple with UC for enhanced detection and differentiation of particles and cells in water samples. The concentration of particles using UC systems based on piezoceramic tubes,⁶ which are more adaptable for in-line sensing applications, needs to be further evaluated

This material was based on work supported under National Science Foundation and American Association of University Women fellowships and a United Kingdom Internal Research Support Grant. Any opinions, findings, conclusions, or recommendations expressed here are those of the authors and do not necessarily reflect the views of these organizations. The authors also thank Jim Ash and Lee Rechtin for their assistance.