驅動訊號波形、噴孔幾何參數與噴孔流道親疏水性對噴墨頭液滴噴射行為影響之研究

Abstract

本研究利用數值模擬的方法，探討驅動訊號波形對ㄧ壓電式噴墨頭液滴噴射行為之影響。為了對驅動訊號波形與液滴形成之關係有一完整的了解，研究中將一雙極方波訊號區分成供液段、補液段、噴出段、間歇段、回拉段與靜止段，並找出每一波形區段的獨立參數，分析不同區段不同獨立參數組合，對噴出之主液滴體積、主液滴飛行速度、液滴斷裂時間與衛星液滴形成之影響。數值模擬部分，採用商業化計算流體力學軟體CFD-ACE+來進行；透過邊界條件給予相對於電壓訊號之壓電隔板形變位移量以模擬不同訊號波形之壓電致動行為；利用流體體積－片段線性界面重建法來做液氣界面的時變追蹤，並以連續表面力法來考慮流體表面張力的效應。研究結果顯示，主液滴體積主要與訊號噴出段所造成最大腔體形變量有關；主液滴飛行速度主要與訊號噴出段之斜率有關。此外，縮短訊號間歇段能有效地抑制衛星液滴的形成。為了探討噴孔流道親疏水性與噴孔流道曲率對液滴噴射行為的影響，我們簡化液滴生成與噴射機構，採用一噴孔片透過壓電材料的形變振動，將液體噴射而出的設計。研究中首先利用CFD-ACE+建立一數值模型，再設計一液滴噴射實驗，利用實驗觀測結果來驗證數值模型。研究結果顯示，當噴孔直徑越小，主液滴體積越小，主液滴飛行速度越快。當噴孔片振動振幅越大或振動頻率越快，會使主液滴體積變小，主液滴飛行速度變快。此外，在所考慮不同流道曲率中，線性流道曲率會產生較小主液滴體積和較快的主液滴飛行速度。當噴孔流道表面由親水變化到疏水時，主液滴體積會變小，主液滴飛行速度會變快。

Numerical calculations were performed to investigate the effect of the component of a single transducer pulse on the ejection of a drop for a drop-on- demand ink-jet printhead with a piezoelectric actuator. The flow field is governed by continuity and Navier-Stokes equations. A volume-of-fluid method with a piecewise-linear interface construction is used to track the complicated topological variation of the liquid-gas interface. The computer code was validated with experimental results present in the literature. The volume of the primary drop is closely related to the maximum displacement of chamber wall induced by piezoelectric actuator in the forward stroke; the velocity of the primary drop depends on the ratio of to the time period of the forward stroke, . Moreover, the formation of the primary drop depends weakly on the conditions of backward stroke considered. A decreased interval between forward and backward strokes might serve to suppress the formation of satellite drops owing to reducing the liquid thread length at pinching off to a value less than the upper limit . The breaking up of freely flying liquid thread from nozzle outlet has two modes – multiple breaking up and end pinching, and depends on the thread length at pinching off. In an investigation of the influence of liquid hydrophobicity and nozzle geometry on the drop formation process, a system of a nozzle plate connected to a flat-plate piezoelectric material was designed. The numerical models of the designed system were constructed and validated by comparing simulation results with experimental observations. The numerical results show that a decrease in nozzle exit diameter causes a decrease in drop volume and an increase in drop velocity. When the vibrational amplitude or frequency of the nozzle plate increases, which raises the input energy, drop volume appears to decrease and drop velocity to increase. Besides, the linear-type curvature seems to produce smaller drop volume and larger drop velocity. When the contact angle varies from 7.10 to 1700, drop volume seems to decrease and drop velocity to increase.