Baw-induced Streaming, Faraday Waves, and atomization on a paper substrate

Acoustofluidics offers contactless manipulation, high versatility, and good biocompatibility. Microfluidic paper-based analytical devices (μPAD) have key advantages like multiplexing, rapid response, and portability. The present study is a first step that combines acoustics with μPADs to develop Acoustic-μPADs (A-μPADs). We focus on the fundamental aspects of acoustic field-driven flow in a sessile drop placed on a hydrophobic paper substrate. Young's modulus and acoustic impedance were used to sort and select suitable paper. A simple, two-step fabrication process was used to impart hydrophobicity to paper, which eliminates liquid imbibition. A temporal variation of contact angle (with and without acoustics) was used to set the experiment duration. We investigate the effects of excitation frequency and amplitude on the drop. We have observed and recorded streaming, Faraday waves and atomization in this study. The velocity field in the drop was obtained from Micro-Particle Image Velocimetry (μPIV). Faraday waves and atomization were captured using high-speed imaging (8000 fps). A key finding of this study is that the system exhibits three different phenomena: internal streaming, Faraday waves, and atomization on a single device. These are captured in a flow regime map in the parameter space of dimensionless frequency and amplitude. These maps can be used as a guide for applications such as mixing (streaming), patterning (Faraday waves), and mist generation (atomization).