PhD Defence Duarte Ferreira Gonçalves Da Rocha | Marangoni flows in evaporating droplets and gradient fields

Marangoni flows in evaporating droplets and gradient fields

Duarte Ferreira Gonçalves Da Rocha is a PhD student in the department Physics of Fluids. (Co)Promotors are prof.dr. D. Lohse & dr. C. Diddens from the faculty of Science & Technology (TNW), University of Twente.

Pick up a glass of wine and let it rest. Within minutes, thin rivulets creep up the glass wall before falling back in slow, irregular drops: the "tears of wine". This is a Marangoni flow: fluid motion driven by gradients in surface tension along an interface. Far from a curiosity, such flows shape deposition patterns in applications such as inkjet printing, pesticide sprays and spray-dried pharmaceuticals. Because the flows are small, fast, and hard to measure directly, numerical simulation is often the only way to understand what happens inside a droplet a fraction of a millimetre across.

This thesis is a computational and theoretical study of Marangoni flows in two settings: inside evaporating droplets, and in liquid systems subjected to externally imposed gradients of temperature or composition.

Part I develops an efficient numerical method for azimuthal stability analysis (chapter 1), predicting whether a symmetric flow will spontaneously break into an asymmetric pattern, including in systems with evolving, moving geometries.

Part II applies this to evaporating droplets. It shows that a trace surfactant contamination suppresses the thermal Marangoni flow expected in evaporating water droplets (chapter 2); it examines how droplet geometry controls the onset of chaotic Marangoni instabilities in binary mixtures (chapter 3); and how neighbouring droplets and rivulets influence each other's evaporation and internal flow through shared vapour fields (chapter 4).

Part III turns to externally imposed gradients. It studies how bubbles grow near an advancing ice front during freezing (chapter 5); and how an oil droplet in a co-axial microfluidic jet can be propelled upstream, against the imposed flow, by a self-generated Marangoni force (chapter 6).

Together, these results show how sensitive Marangoni flows are to seemingly minor factors and how far-reaching their consequences can be.