Investigation of Emitting Liquid with Various Sizes to the Same Height Using Acoustic Droplet Ejection

Recently, several contactless liquid handling methods were developed to reduce residues and disposals. Among them, acoustic droplet ejection (ADE) is the most convincing method as it could generate a large quantity of liquid in mid-air without physically touching it. However, the height and size of droplets produced by the conventional ADE setup typically exhibit a proportional relationship, which contradicted the requirements of the application. Therefore, this study demonstrated a novel ADE setup that could create different sizes of droplets at the same height by merely alternating the electrical driving signal. The acoustic pressure distribution along the junction of liquid and air would be changed by introducing a low voltage driving period and a pinhole to form a water mount before ejection. As a result, the size of the droplet would still increase with the duty cycles where the height remains the same. In this study, a simulation was conducted to verify the feasibility of the proposed method and evaluate the parameters for the practical ADE setup. Next, a driving circuit comprising a high-voltage pulser and a Field-programmable gate array is developed. Besides, a resin model made by a 3D printer is used to focus the acoustic wave and fix the acoustic source. The droplet ejection process was recorded using a high-speed camera and analyzed by a custom droplet detection algorithm. First, the conventional ADE experiment is conducted to verify the proposed ADE system. After that, a plate with a pinhole is placed on the top of the resin model, and a low voltage driving period is added to the driving signal. Three different driving voltages (65V, 70V, and 75V) and pinhole diameters of 3.7 mm were used to verify the proposed ADE setup. Successfully demonstrating the approach that different sizes of droplets with the same ejecting height could be achieved in certain potential operation regions by tuning V_High. This finding should initiate more applications of ADE and play an essential role in environmental sustainability.