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Authors: Gao, Aiting
Title: Activated Liquid Transport by Thermal Marangoni Effect and by Avoiding the Coffee Ring Effect
Online publication date: 24-Feb-2022
Language: english
Abstract: Liquid transport, particularly on the micrometer scale, is crucial for numerous applications, such as microfluidics for chemical or biochemical analysis, and inkjet printing. For flows on the micrometer scale, there is a significant influence of the interfaces. This is because the smaller the size of the system, the larger the area of the interfaces becomes compared to the enclosed volume. Hence, in order to create, control and manipulate flows on the microscale, interfaces play a significant role. My study focuses on how liquid flows, subjected to variations of the conditions at the interfaces, i.e., by introducing surface tension gradients, locally suppressed evaporation or local heating, are affected. In this respect, I investigated two cases: a) optical manipulation of liquid flows by the thermocapillary effect on superhydrophobic surfaces b) controlling evaporation to manipulate flows in an evaporating droplet on surfaces prewetted with oil A feasible way to trigger liquid motion is to introduce surface tension gradients on free liquid interfaces, which leads to the Marangoni effect. However, this approach has been rarely reported to trigger continuous flows in confined liquid systems, e.g., microfluidic devices, due to the lack of free liquid interfaces. In the first case of the two mentioned projects, I show experimentally that non-wetted superhydrophobic surfaces can provide the necessary free liquid interface to allow for Marangoni flows. I use laser light to heat this surface asymmetrically. Thereby fluid flows are created near the liquid interface via the gradients produced in the surface tension. With confocal microscopy experiments and computational fluid dynamics simulations, the flow velocity distribution in the liquid phase was analyzed. My results show that it is possible to introduce Marangoni flows on superhydrophobic surfaces with an optical approach. I find that besides the Marangoni flows, a notable buoyancy-driven flow may occur, which has not yet been considered in previous works on Marangoni flows at superhydrophobic surfaces. The contribution of these two mechanisms is investigated to provide guidelines for an efficient design of Marangoni pumping systems. In the second case of the investigated interface modifications, I address drying patterns of a droplet. In general, the evaporation of a droplet is non-uniform along its free liquid interface. This induces flows within the droplet, which ultimately affect its drying features. For instance, VIII a wide-spread phenomenon is the “coffee ring” effect, named after the ring-like structures of residues left after a drop of coffee has dried on a surface. However, in printing, patterning and coating processes, where uniform patterns are highly required, this effect presents a major challenge. It is hence highly desired to methods to suppress the coffee-ring effect. In this project, I introduce an approach that can directly control the evaporation profile at the droplet surface to suppress the coffee-ring effect. It is realized by depositing water droplets on a substrate pre-wetted by a with water immiscible oil. The low-surface-tension oil spontaneously climbs up and forms an apparent “wetting ridge” around the droplet edge. This oil “wetting ridge” helps to hinder liquid from evaporating at the droplet edge. This suppresses the outward capillary flows, which normally lead to the coffee ring. Consequently, upward flows are generated and drive non-volatile components, such as particles dispersed in the droplet, toward the droplet surface. In consequence, I have produced uniform depositions on the surface. This mechanism is verified by monitoring the dynamic behavior of contact lines, contact angles, and the migration process of dispersed particles in the evaporating droplet. As a possible application, I have produced a range of supraparticles (assembled from smaller particles) with disk-like shapes. Tests with chemically different types of particles have been performed by this straightforward approach to show its wide range of applicability. In summary, controlling liquid flows through manipulating liquid interfaces is applicable and leads to fascinating liquid behavior. This provides new possibilities in applications such as fluid pumping, transporting, and surface patterning.
DDC: 000 Allgemeines
000 Generalities
540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-a865694a-837c-434d-b1ef-2e4ec01ca9956
Version: Original work
Publication type: Dissertation
License: In Copyright
Information on rights of use:
Extent: XI, 100 Seiten, Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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