Elsevier

Applied Surface Science

Volume 426, 31 December 2017, Pages 253-261
Applied Surface Science

Full Length Article
Effect of droplet shrinking on surface acoustic wave response in microfluidic applications

https://doi.org/10.1016/j.apsusc.2017.07.140Get rights and content

Highlights

  • Both contact angle and radius of a micro-size droplet can influence the experiment of the surface acoustic wave-based microfluidic applications.

  • The change in contact radius effects the attenuation more than that in contact angle, especially on hydrophilic and super-hydrophilic surfaces.

  • For larger liquid volume, the mostly stable duration of the SAW response is due to the only longer shrinking duration of the contact angle.

  • For the long-time actuation and manipulation in experiments, the effect of the evaporation phenomenon should be considerable.

Abstract

The effect of the contact angle and radius of a microsize droplet on the surface acoustic wave (SAW) response for microfluidic applications is reported. It is studied through the dynamic change of the droplet shape during the evaporation process. An aluminium nitride SAW device, operating at 125.7 MHz, is utilized to investigate the deformation of the droplet shape (contact angle and contact radius) caused by shrinking. The large cavity placed on the propagation path distorts the in-band SAW response one time at the centre frequency. The fractional coefficient of the SAW insertion loss, before and after dropping the liquid on the propagation path, is continuously recorded. The change in the fractional coefficient shows that the radiated acoustic kinetic energy depends on the contact area between the sessile micro-size droplet and the SAW device more than the contact angle of the droplet. Three droplet volumes have been considered, namely 0.05, 0.1 and 0.13 μl, and the electrical results show a better agreement with the theoretical data than the optical image data. The average duration of the fractional coefficient change for these cases is 420, 573 and 760 s, respectively. The effect of the hydrophobicity versus hydrophilicity of the contact surface on the duration of the fractional coefficient change is studied by coating the SAW with a silicon oxide or hexamethyldisilazane (HMDS) thin layer. For the same 0.05 μl sessile droplet on the hydrophobic surface, this duration is on average 110 s longer than that on the hydrophilic surface.

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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