Microfluidic investigation of gynamical surfactant rearrangement at the water/oil interface during single droplet formation

复杂流体和多相流的输送，如胶体悬浮液、聚合物或乳液，是自然和工业过程在所有长度尺度上的共同特征。通过利用典型的微流体层流条件，可以实现对一系列流动特性的精确控制。微流体的一个主要优点是可以通过研究单个液滴的形成动力学来研究乳剂的形成。特别是在液滴形成过程中，新的界面迅速形成。表面活性剂的存在保证了乳液的稳定性。表面活性剂分子向新界面迁移的速度可能比界面本身形成的速度更快或更慢，这取决于其相对大小和与体系的亲和力[Kovalchuk， Nowak, Simmons, Langmuir, 32(20):5069-77, 2016]，尽管目前还没有进行微观研究。对这一现象的研究是令人着迷的，其含义对于工业和基础科学来说都是非常有趣和重要的。该博士项目的预期产出之一将是通过先进的光学技术，在不同表面活性剂存在的情况下，对两相体系（油中水或油中水）中单个液滴形成的实验研究。特别是，我们将设计和实现不同的微流体装置，我们将广泛使用光学显微镜和快速相机记录（高达每秒50,000帧），以评估分散相和连续相之间界面行为的差异。速度场将用一种名为“幽灵粒子测速”的创新技术进行分析[Buzzaccaro等人，物理学]。启。， 111, 2013；Pirbodaghi， Vigolo等，实验室芯片，2015 [j]。还需要对微流体中乳剂的生成进行基础研究，并对微流体装置内单液滴形成的流体动力学进行计算建模。

The transport of complex fluids and multiphase flows such as colloidal suspensions, polymers or emulsions is a common feature of natural as well as industrial processes at all length scales. By exploiting laminar flow conditions typical of microfluidics, it is possible to achieve precise control over a range of flow characteristics. One major benefit of microfluidic is the possibility to investigate the formation of emulsions by studying the dynamics of formation of each single droplet. In particular, during droplet formation, a new interface is formed rapidly. The stability of the emulsion is guaranteed by the presence of surfactants. Depending on the relative size and affinity with the system, the surfactant molecules can migrate towards the new interface faster or slower than the speed at which the interface itself is created [Kovalchuk, Nowak, Simmons, Langmuir, 32(20):5069-77, 2016], although no microscopic study has been performed so far. The investigation of this phenomenon is fascinating and the implications are extremely interesting and important for both industrial and fundamental science reasons.One of the expected outputs of this PhD project will be the experimental study of the formation of a single droplet in a two-phase system (either water in oil or oil in water) in the presence of different surfactants by means of advanced optical techniques. In particular, we will design and realize different microfluidic devices, and we will extensively use optical microscopy and fast camera recordings (up to 50,000 frames per second) in order to evaluate differences in the behaviour of the interface between dispersed and continuous phase. The velocity field will be analysed with an innovative technique called Ghost Particle Velocimetry [Buzzaccaro et al., Phys. Rev. Lett., 111, 2013; Pirbodaghi, Vigolo et al., Lab Chip, 15, 2015].It will also be required to perform a fundamental study on the generation of emulsions in microfluidic and the computational modelling of the fluid dynamics of the single droplet formation inside the microfluidic device.