Surface acoustic wave-based generation and transfer of droplets onto wettable substrates

Fluid manipulation using surface acoustic waves (SAW) has been utilized as a promising technique in the field of microfluidics due to its numerous advantages, over other active techniques, such as low power requirement, facile fabrication methods, and non-invasive nature. Even though SAW-based generation of micron-sized droplets through atomization has been studied, the role of substrate wettability on the characteristics of the transferred droplets has not been explored to date. Here, we study the generation and effective transfer of micron-sized droplets using SAW onto wettable substrates whose water contact angles vary from 5 degrees to 145 degrees. The characteristics of transferred droplets after impacting the wettable substrates are characterized in terms of the contact line diameter and polydispersity index. A theoretical model is formulated to predict the initial average size of the transferred droplets on the wettable substrates of different contact angles. The variation of polydispersity and number density with contact angle is explained by considering droplet coalescence and bouncing. The relevance of the technique in biological assays is demonstrated by transferring droplets of streptavidin protein samples onto a substrate.