Sub-wavelength phononic crystal liquid sensor

Manzhu Ke, Mikhail Zubtsov, Ralf Lucklum
2011 Journal of Applied Physics  
We introduce an acoustic liquid sensor based on phononic crystals consisting of steel plate with an array of holes filled with liquid. We both theoretically and experimentally demonstrate sensor properties considering the mechanism of the extraordinary acoustic transmission as underlying phenomenon. The frequency of this resonant transmission peak is shown to rely on the speed of sound of the liquid, and the resonant frequency can be used as a measure of speed of sound and related properties,
more » ... ke concentration of a component in the liquid mixture. The finite-difference time domain method has been applied for sensor design. Ultrasonic transmission experiments are performed. Good consistency of the resonant frequency shift has been found between theoretical results and experiments. The proposed scheme offers a platform for an acoustic liquid sensor. An ultrasonic broadband measurement of absorption and velocity in liquids and liquid systems has been proven to be an almost universal tool for many applications in basic research as well as in characterization of materials, manufacturing processes, and control routines. 1 Sound waves in liquids induce perturbations, affecting equilibrium and resulting in characteristic sound absorption and velocity dispersion. Precise attenuation or velocity measurements yield valuable information about thermodynamic and kinetic parameters of the liquid that is often difficult to obtain by other methods. Though these amplitude-based or bulk-resonant measurement techniques are relatively easy in the use at macro-scale, they are rather challenging in applications at micro-scale, in particular, in combination with micro-fluidic devices, because of a short propagation path of ultrasonic waves. During the last three decades, phononic crystal structures (PnC) have gained much attention due to their rich physics and great utility in the control of sound propagation. The reported advances in PnCs implementation and fabrication promise both their efficient and beneficial integration into existing technologies and far greater experimentation on a wide variety of potential applications. 2, 3 Recently, an extraordinary acoustic transmission (EAT) through a structure with regularly perforated apertures immersed in a liquid has been demonstrated when acoustic waves are incident either on one-dimensional gratings with narrow apertures 4 or on 2D perforated metal plates with arrays of holes with sub-wavelength size. [5] [6] [7] [8] [9] There are no generally recognized theory or conceptual framework concerning EAT, though the behavior of acoustic waves propagating through these structures has been a subject of intensive research. Lu et al. 4 have attributed the resonant enhancement in EAT to the coupling between the compositions of diffractive waves excited on the surfaces of the gra-ting and the Fabry-Perot (FP) resonant modes inside the apertures. Hou et al. 5, 6 have demonstrated that evanescent waves may alter FP resonance conditions and FP resonance can occur in aperture with the length, which is even less than the half wavelength of incident wave. They have also found that both the periodicity and area fraction of apertures may affect the FP resonance condition and that this resonance is smoothly linked to the surface-wave-like modes induced by the periodic structure factor. Estrada et al. 7 point out that there is a number of several transmission mechanisms involved in EAT and producing complex interplay phenomena: transmission assisted by cutoff-free waveguide modes of individual apertures, interaction among holes arranged in array, and direct transmission through the bulk material. Wang 9 has introduced an acoustic view of this phenomenon, specifically focusing on the properties of the liquid. In this paper, we demonstrate how extraordinary acoustic transmission in phononic crystals can be used to determine mechanical material properties of liquids. We follow the route of Huang's research paper, 10 which succeeded in realizing the sub-wavelength nanofluidics in photonic crystal sensors by using the similar resonance transmission properties. They applied this method to detect refractive index changes in aqueous solutions and showed enhancements in the sensor performance and the sensitivity. We have studied a specific arrangement of PnC which is typical for EAT experiments, specifically, a thin plate with the thickness comparative to the half wavelength of the source of sound, regularly perforated with circular holes and immersed in liquid. We have chosen the size of the structure in millimeter scale range. The structure (shown in Fig. 1(a) ) we used here consists of a steel plate with liquid holes in square arrays. The lattice distance (denoting as a) is 1.5 mm, the thickness (t) of the plate is 0.5 mm and the diameter (d) of the hole is 0.5 mm. Different liquid material properties are obtained by gradually changing the liquid in the holes and surroundings from pure Distilled water (DI-water) through a series of liquid mixtures with different molar concentration to pure 1-propanol. a)
doi:10.1063/1.3610391 fatcat:wll4h6sa2nhnvmmhpetzq4bm7u