Abstract
With the development of ink jet printers or spotters for biological applications, the control size and homogeneity of small fluids as well as their successful deposition on various substrates has gained significant importance. We study the impingement process of such small droplets onto a solid substrate and investigate their evaporation and drying process by means of surface acoustic wave (SAW) transmission experiments. We observe a strong fluid-SAW interaction resulting in an attenuation which toward the end of the evaporation and drying process exhibits characteristic oscillations being related to temperature, SAW amplitude, and fluid properties such as viscosity and wetting angle. It is found that oscillations can only be seen in pinned, very flat droplets, while the pinning can be roughly controlled via the SAW amplitude. The frequency of those characteristic oscillations is superlinear with respect to temperature, rather proportional to the evaporation rate and can change drastically depending on the liquid used. In our experiments, isopropanol, ethanol, and water with various glycerol concentrations were loaded onto the SAW delay line. Based on the experimental results, simulations using the finite element method were performed. We find that a standing wave pattern within the droplet is responsible for the transmission oscillations. These findings might lead to new methods, e.g., sensory systems, being able to examine droplets according to their physical or chemical properties.
Highlights
Since the development of application involving combustion, spray cooling, ink jet printing, and others, the physics of evaporation has become of great interest
We study the impingement process of such small droplets onto a solid substrate and investigate their evaporation and drying process by means of surface acoustic wave (SAW) transmission experiments
The evaporation dynamics of small droplets on a solid substrate were investigated by employing SAW transmission experiments
Summary
Since the development of application involving combustion, spray cooling, ink jet printing, and others, the physics of evaporation has become of great interest. The evaporation process is investigated by sampling the droplet with surface acoustic waves. Surface acoustic waves have become an essential part of many applications as well and are widely employed in a variety of devices for telecommunication,[7,8] radio frequency signal processing,[9] biochemical sensory systems,[10] microfluidics,[11,12] and more. In the field of microfluidics, SAW is often employed on the so-called lab-on-a-chip, where they can effectively be used for various tasks like inducing an acoustic streaming in small amounts of fluids. This can already be done at low SAW amplitudes and allow an easy way for acoustic mixing. SAW can be used for droplet heating or, if its power is high enough, for jetting and atomization.[13]
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