Department of Chemistry, and ‡Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Anal Chem. 2014 Feb 18;86(4):2229-37. doi: 10.1021/ac403968d. Epub 2014 Feb 4.
Lab-in-a-drop, using ultrasonic levitation, has been actively investigated for the last two decades. Benefits include lack of contact between solutions and an apparatus and a lack of sample cross-contamination. Understanding and controlling mixing in the levitated drop is necessary for using an acoustically levitated drop as a microreactor, particularly for studying kinetics. A pulsed electrostatic delivery system enables addition and mixing of a desired-volume droplet with the levitated drop. Measurement of mixing kinetics is obtained by high-speed video monitoring of a titration reaction. Drop heterogeneity is visualized as 370 nl of 0.25 M KOH (pH: 13.4) was added to 3.7 μL of 0.058 M HCl (pH: 1.24). Spontaneous mixing time is about 2 s. Following droplet impact, the mixed drop orbits the levitator axis at about 5 Hz during homogenization. The video's green channel (maximum response near 540 nm) shows the color change due to phenolphthalein absorption. While mixing is at least an order of magnitude faster in the levitated drop compared with three-dimensional diffusion, modulation of the acoustic waveform near the surface acoustic wave resonance frequency of the levitated drop does not substantially reduce mixing time.
利用超声悬浮技术实现的微滴实验室在过去二十年中得到了广泛的研究。这种技术的优点包括溶液和仪器之间无接触，以及避免了样本交叉污染。为了将悬浮微滴用作微反应器，需要深入了解并控制悬浮微滴中的混合过程，尤其是在研究动力学时。采用脉冲静电滴液系统，可以将所需体积的液滴滴加到悬浮微滴中并实现混合。通过高速视频监测滴定反应，可以测量混合动力学。通过向 3.7 μL 的 0.058 M HCl（pH：1.24）中加入 370 nl 的 0.25 M KOH（pH：13.4）可视化液滴的不均匀性。自发混合时间约为 2 秒。在液滴撞击后，混合液滴在均化过程中以约 5 Hz 的频率围绕着悬浮体的对称轴旋转。视频的绿色通道（最大响应接近 540nm）显示了由于酚酞吸收而导致的颜色变化。尽管与三维扩散相比，悬浮微滴中的混合速度至少快了一个数量级，但在悬浮微滴的表面声波共振频率附近调制声波波形并不能显著缩短混合时间。
