Marinaro Giovanni, Riekel Christian, Gentile Francesco
Faculty of Mechanical Science and Engineering, Institute of Process Engineering, Technische Universität Dresden, 01062 Dresden, Germany.
Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 01328 Dresden, Germany.
Micromachines (Basel). 2021 Feb 12;12(2):185. doi: 10.3390/mi12020185.
Aqueous solution droplets are supported quasi contact-free by superhydrophobic surfaces. The convective flow in evaporating droplets allows the manipulation and control of biological molecules in solution. In previous works, super-hydrophobic drops on nano-patterned substrates have been used to analyze otherwise undetectable species in extremely low concentration ranges. Here, we used particle image velocimetry (PIV) for studying the flow field in water droplets containing polystyrene particles on a pillared silicon super-hydrophobic chip. The particles describe vortex-like motions around the droplet center as long as the evaporating droplet maintains a spherical shape. Simulations by a Finite Element Method (FEM) suggest that the recirculating flow is due to the temperature gradient along the droplet rim, generating a shear stress. Notably, the characteristics of the internal flow can be modulated by varying the intensity of the temperature gradient along the drop. We then used the flow-field determined by experiments and an approximate form of the Langevin equation to examine how particles are transported in the drop as a function of particle size. We found that larger particles with an average size of μ36 μm are preferentially transported toward the center of the substrate, differently from smaller particles with a 10-fold lower size that are distributed more uniformly in the drop. Results suggest that solutions of spherical particles on a super-hydrophobic chip can be used to separate soft matter and biological molecules based on their size, similarly to the working principle of a time-of-flight (ToF) mass analyzer, except that the separation takes place in a micro-sphere, with less space, less time, and less solution required for the separation compared to conventional ToF systems.
水溶液微滴被超疏水表面以准无接触的方式支撑。蒸发微滴中的对流流动能够对溶液中的生物分子进行操控和控制。在之前的研究中，纳米图案化基底上的超疏水液滴已被用于分析极低浓度范围内原本无法检测到的物质。在此，我们使用粒子图像测速技术（PIV）来研究在带柱状结构的硅基超疏水芯片上含有聚苯乙烯颗粒的水滴内的流场。只要蒸发的水滴保持球形，颗粒就会围绕液滴中心做类似涡旋的运动。有限元方法（FEM）模拟表明，回流是由沿液滴边缘的温度梯度产生剪应力所致。值得注意的是，内部流动的特性可通过改变沿液滴的温度梯度强度来调节。然后，我们利用实验确定的流场和朗之万方程的近似形式来研究颗粒如何作为粒径的函数在液滴中传输。我们发现，平均粒径为36μm的较大颗粒优先向基底中心传输，这与粒径小10倍的较小颗粒在液滴中分布更均匀的情况不同。结果表明，超疏水芯片上的球形颗粒溶液可用于根据软物质和生物分子的大小进行分离，这类似于飞行时间（ToF）质量分析仪的工作原理，只是分离发生在微球中，与传统ToF系统相比，所需空间更小、时间更短、分离所需溶液更少。
