Compact SAW aerosol generator

Abstract

In this work, we discuss and demonstrate the principle features of surface acoustic wave (SAW) aerosol generation, based on the properties of the fluid supply, the acoustic wave field and the acoustowetting phenomena. Furthermore, we demonstrate a compact SAW-based aerosol generator amenable to mass production fabricated using simple techniques including photolithography, computerized numerical control (CNC) milling and printed circuit board (PCB) manufacturing. Using this device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of fluid supply position and geometry, SAW power and fluid flow rate on the device functionality. These factors in turn influence the droplet size distribution, measured here, that is important for applications including liquid chromatography, pulmonary therapies, thin film deposition and olfactory displays.

Keywords: Aerosol source; Atomization; Fluid supply; Mass scale production; Microchannel; Miniaturization; SAW; Surface acoustic wave.

Fig. 1

Tilted photomicrograph of a SAW chip with two IDTs and SU-8 microchannel structure (IDT aperture indicated by dotted white lines); inset shows magnified microchannel outlet

Fig. 2

Assembly sketch of the compact SAW aerosol generator with its individual components: 1 = chip holder with heat conductive foil (white plane), 2 = SAW chip, 3 = O-ring, 4 = fluid block, 5 = PCB, 6 = screw; Insets magnify: I) PCB with 7 = strip line and 8 = spring pins; II) SAW chip with 9 = IDT and 10 = SU-8 structure (SAW propagation directions indicated); III) partial SU-8 structure with 11 = channel outlet and indicated, measured sSAW amplitude distribution (u_3,max) at the acoustic boundary positioned in respect to the channel outlet

Fig. 3

Typical sSAW amplitude distribution (out of plane amplitude û₃) between two IDTs (λ = 90 μm, w = 500 μm, w/λ = P_Load = 200 mW, 533 × 76 measurement points); IDT positions, important regions of the wave field (1, 2, 3) and their arbitrarily chosen boundaries (black dotted lines), the IDT aperture boundaries (white dashed lines) and employed channel outlet positions (A, B, C) indicated

Fig 4

Aerosol generation using the compact sSAW aerosol generator: a view on tilted setup during Ethanol atomization (140 μl/min), b Comparison of the aerosol beam for three different fluids at the maximum possible flow rate for a given setup and constant SAW parameters: DI-Water (1 ml/min), Ethanol (0.55 ml/min) and 50%v aqueous Glycerol solution (0.03 ml/min)

Fig 5

Comparison of three different fluid supply positions; distance measured from the boundary of the IDT aperture: Position a 100 μm (region 1), b 400 μm (region 2–3) and c 650 μm (region 3); Positions indicated in measured SAW field (Fig. 3); boundary of the IDT aperture close to the channel outlet (dotted yellow lines) indicated

Fig 6

Micrographs of the SAW chip for continuous fluid supply (100 μl/min) with different sSAW power (2 × 1.5…3.5 W) at improved fluid supply position (i.e. Pos. A in Figs. 3 and 5); boundaries of the IDT aperture (dotted yellow lines) and SAW propagation directions indicated

Fig. 7

Side view on the aerosol beam for different fluid flow rates (DI-water); differences in the aerosol beam angle caused by convection of the surrounding air

Fig. 8

Atomization regimes observed for a 90 μm sSAW chip with improved fluid (DI water) supply position (i.e. Pos. A (Figs. 3 and 5)); Power-flowrate combinations for droplet size measurements indicated (I-III)

Fig. 9

Measured droplet size distributions (volume fraction q₃, averages of 3x15s measurements) for three chosen combinations of flow rate and power (Fig. 8); logarithmic normal distribution functions fitted to individual peaks indicated by dashed lines (the addition of individual fitted functions results in almost ideal reproduction of the measured curve)