Full Length ArticleEffect of droplet shrinking on surface acoustic wave response in microfluidic applications
Introduction
The leakage phenomenon of surface acoustic waves (SAWs) into a liquid forms a significant limitation for the employment of SAW devices in liquid applications. If we consider the unlimited ambient liquid medium on the propagation path of a SAW device, most of the SAW energy is emitted into the liquid medium [1], [2]. This emitted energy is related to a longitudinal component of the SAWs which is referred to as compressional waves, Rayleigh surface acoustic waves, or longitudinal waves [3], [4], [5]. In practice, when the liquid medium has an insignificant volume (less than a microliter) like a small droplet, the emitted kinetic energy can transport and manipulate fluids such as separating, trapping, driving, mixing, jetting and atomizing [6], [7], [8]. Diverse mechanisms, such as microfluidic sensor, actuation and manipulation of the micro-objects have been studied [7], [8], [9], [10]. Depending on the envisioned SAW microfluidic applications, the applied power and operating frequency need to be determined. For example, the centre frequency of the SAW fluid actuation and manipulation at microscale can be in the range of 0.01–1000 MHz. Low input power (in the order of mWatts) generates a preliminary acoustic streaming on the free surface of the sessile droplet for vibration, mixture, driving applications while higher input power (from 1 W) leads to breakup of the stabilizing interface of the sessile droplet [11], [12].
For a micro-size sessile droplet, typical phenomena in the sensor, actuation and manipulation are acoustic streaming flow, acoustic radiation force, jetting phenomenon which are influenced by the formation of the droplet on the surface. Besides, an evaporation phenomenon plays a crucial role in microfluidic SAW applications owing to the presence of the changeable contact angle and area between the liquid and the piezoelectric substrate in the short transient time [8], [10], [13]. One obvious aspect of the droplet evaporation is an unstable and turning-back SAW response during and after the short duration of the evaporation duration. Therefore, the shrinking sessile droplet placed on the propagation path can have a relevant effect on the microfluidic-actuation and −manipulation applications of the SAW device. The presence of adjacent energy from the liquid-solid interface is influenced by the behaviour of the droplet (surface tension, density, contact area and shrinking droplet shape) [14] and the properties of the piezoelectric device (surface wettability, surface roughness, input power and centre frequency) [7], [8].
This paper reports on the effect of the change of the droplet volume, through the contact angle and radius, on the SAW device response. The theoretical analysis shows that the shrinking behaviour of both the contact angle and radius during the droplet evaporation process influences the variation duration of the SAW response. This is expressed by the fractional coefficient of the SAW response before and after dropping liquid on the propagation path of the SAW device. The slow or fast attenuation duration of the fractional coefficient depends on the variation of the contact angle, radius or both at the same time. It is also caused by the fluid-structural solid interface due to the presence of the pinning force relative to the change in contact angle. Besides, as the surface property affects the droplet evaporation process and the contact area region between the liquid and the piezoelectric medium, it also influences the duration of the variation of the SAW response.
Section snippets
Fundamentals of the Rayleigh SAW in liquid domain
In Snell’s law on the refraction phenomenon, the incidence angle of the denser medium must be smaller than the critical angle at the contact to avoid the probable total internal reflection. For a piezoelectric medium, the Rayleigh angle in the forward medium is considered. For example, the longitudinal component of the SAW is radiated into the less dense medium at the Rayleigh angle [15]. In order to have the refraction phenomenon at the contact surface between piezoelectric and less dense
Emission of the SAW energy in the evolving micro-droplet
During the evaporation process of the sessile droplet, the domain of the liquid medium varies continuously because of the shrinking contact angle, radius and height. Therefore, the measured insertion loss changes during the evaporation process. To analyse this variation, the evolution of the droplet in the time domain and the variable fractional coefficient of the lost energy are considered.
SAW device with the hydrophilic surface
Although SAWs may be generated upon the surface of various piezoelectric materials such as quartz, lithium niobate, zinc oxide, PZT, here a CMOS compatible material, aluminium nitride thin film, is chosen. The fabrication procedure for the SAW device includes two main parts, namely patterning the interdigital transducer (IDT) fingers on a 1 μm aluminium nitride (AlN) thin film and two top-surfaces (one hydrophilic and one hydrophobic surface).
For the 125.7 MHz SAW device, a 50-Ω IDT impedance
Results and discussions
The data from the top-view optical image and the corresponding SAW response are shown in Fig. 9a. In the first 450 s, the contact area of the sessile droplet (1.45% evaporated contact area) is mostly constant, thus the insertion loss changes insignificantly. We refer to this situation as the CR mode which has a constant radius and changeable contact angle (from 65.5° down to 20.0°). This contact angle change can also be observed by the bright spot on the optical images, caused by the reflected
Conclusions
The energy emitted into the liquid medium is different for the dissimilar contact area between the sessile droplet and the piezoelectric material. In additions, the evaporation phenomenon of the sessile droplet contributes as well. The dynamic change caused by the contact angle and radius of the sessile droplet occurs during the evaporation process. The change in contact radius influences the insertion loss more than the change in contact angle, especially on hydrophilic and super-hydrophilic
Acknowledgements
The authors would like to acknowledge the support of Dr. Rene Poelma and Tom Scholtes at EKL lab, Dr. Atef Akhnoukh at Electronics Research lab of the Delft University of Technology, Dr. An Tran at Advanced Institute for Science and Technology, Hanoi University of Science and Technology and Dr. Catalin V. Rusu from the Computer Science Department at Babes-Bolyai University.
This work is partly supported by a Vietnamese Government scholarship.
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