Printing surface charge as a new paradigm to program droplet transport

Directed, long-range and self-propelled transport of droplets on solid surfaces, especially on water repellent surfaces, is crucial for many applications from water harvesting to bio-analytical devices. One appealing strategy to achieve the preferential transport is to passively control the surface wetting gradients, topological or chemical, to break the asymmetric contact line and overcome the resistance force. Despite extensive progress, the directional droplet transport is limited to small transport velocity and short transport distance due to the fundamental trade-off: rapid transport of droplet demands a large wetting gradient, whereas long-range transport necessitates a relatively small wetting gradient. Here, we report a radically new strategy that resolves the bottleneck through the creation of an unexplored gradient in surface charge density (SCD). By leveraging on a facile droplet printing on superamphiphobic surfaces as well as the fundamental understanding of the mechanisms underpinning the creation of the preferential SCD, we demonstrate the self-propulsion of droplets with a record-high velocity over an ultra-long distance without the need for additional energy input. Such a Leidenfrost-like droplet transport, manifested at ambient condition, is also genetic, which can occur on a variety of substrates such as flexible and vertically placed surfaces. Moreover, distinct from conventional physical and chemical gradients, the new dimension of gradient in SCD can be programmed in a rewritable fashion. We envision that our work enriches and extends our capability in the manipulation of droplet transport and would find numerous potential applications otherwise impossible.Comment: 11 pages, 4 figure

Creating robust superamphiphobic coatings for both hard and soft materials

Most superhydrophobic surfaces lose their water-repellency when either contaminated by oily liquids or by being mechanically damaged. Superamphiphobic surfaces are ones that repel both oil and water. However, to date such surfaces are hampered by being mechanically weak. Robust superamphiphobic surfaces with highly water and oil repellent properties are desired for a wide range of environments. Reported herein is a superamphiphobic coatings fabricated by a facile deposition method and followed by a low surface energy materials modification. These coatings can be applied on both hard and soft materials to repel water, glycerol, peanut-oil droplets and some organic solvents. Falling sand abrasion, UV irradiation and aqueous media immersion were used to test the mechanical robustness and durability of the superamphiphobic coatings. A multi-cycle stretch/release test was developed to characterize the robustness of the self-cleaning soft materials. A coated rubber-bond retained both water and oil repellency even after 50 stretch/release cycles. These tests show that the superamphiphobic coatings have remarkable mechanical robustness and UV/aqueous media resistance and can be readily applied to a wide variety of materials to form self-cleaning surfaces that are extremely robust and durable even under intense strains

Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface

Evaporation of multi-component drops is crucial to various technologies and has numerous potential applications because of its ubiquity in nature. Superamphiphobic surfaces, which are both superhydrophobic and superoleophobic, can give a low wettability not only for water drops but also for oil drops. In this paper, we experimentally, numerically and theoretically investigate the evaporation process of millimetric sessile ouzo drops (a transparent mixture of water, ethanol, and trans-anethole) with low wettability on a superamphiphobic surface. The evaporation-triggered ouzo effect, i.e. the spontaneous emulsification of oil microdroplets below a specific ethanol concentration, preferentially occurs at the apex of the drop due to the evaporation flux distribution and volatility difference between water and ethanol. This observation is also reproduced by numerical simulations. The volume decrease of the ouzo drop is characterized by two distinct slopes. The initial steep slope is dominantly caused by the evaporation of ethanol, followed by the slower evaporation of water. At later stages, thanks to Marangoni forces the oil wraps around the drop and an oil shell forms. We propose an approximate diffusion model for the drying characteristics, which predicts the evaporation of the drops in agreement with experiment and numerical simulation results. This work provides an advanced understanding of the evaporation process of ouzo (multi-component) drops.Comment: 41 pages, 8 figure

Easily Achievable Methods for Making Superamphiphobic Surfaces

Superamphiphobic materials have surfaces that display a contact angle above 150° for both low and high-tension liquids [1]. Superamphiphobic surfaces present exciting and innovative properties for commercial and industrial applications. Fabrication of superamphiphobic surfaces often require advanced techniques and chemicals. Easier and cheaper methods for making these surfaces are desirable to produce them sustainably, durably and on a big-scale. In this thesis, we explore whether surfaces of different materials can be rendered superamphiphobic in an environmentally sound way and with easily accessible chemicals and instruments found in most labs. Three different techniques described in the literature were taken as starting points for the pursuit of easily accommodable methods. For the first technique, glass and stainless steel substrates were dip-coated in a waterborne coating system consisting of a fluorinated compound and silica particles, to give the proper structured surfaces for superamphiphobicity [2]. In the second technique, thermal treatment was used on silicon oil to produce a layer of soot which deposited directly on to glass and steel substrates to give the proper surface structure and composition for superamphiphobicity [3]. Thirdly, aluminum was etched in hydrochloric acid in order to give a micro structured surface. The aluminum was then immersed in a solution of HNO_3 and CuSO_4 in order to provide a hierarchical structure by the addition of copper particles, and dip-coated in a fluorinated solution to minimize the surface energy [4]. The unmodified and modified substrates were characterized by electron microscopy imaging and elemental analysis to elucidate the structure and composition of the surfaces. The wetting properties and apparent surface energy of the substrates were determined from optical contact angle measurements. Superhydrophobic surfaces were readily achieved for all the tested materials. Oleophobicity was not achieved without the addition of fluorinated compounds, and none of the methods chosen yielded superamphiphobic surfaces. The easily achievable methods have not provided surface structures of the necessary quality to uphold superamphiphobicity.Masteroppgave i nanovitenskapMAMN-NANONANO39

Fabrication and manipulation of non-wetting surfaces and drops : from super liquid-repellency and the Leidenfrost effect to liquid marbles

Das Augenmerk dieser Dissertation liegt auf Tropfen, die den Kontakt mit Oberflächen minimieren. Die Haftkraft der Tropfen an die Oberfläche wird dadurch deutlich reduziert, was in einigen Fällen kann dazu führen kann, dass die Tropfen leicht über sie hinweg gleiten können. Das geringe Benetzungsvermögen der Tropfen beruht auf Luftpolstern, die sich zwischen Tropfen und Oberfläche befinden. Die Luftpolster resultieren entweder aus oberflächen- oder aus tropfenspezifischen Eigenschaften. Ersteres ist der Fall für super-flüssigkeits-abweisende Oberflächen und den Leidenfrost-Effekt, Zweiteres für flüssige Murmeln. Die vorgelegte Dissertation erläutert die grundlegenden Konzepte dieser drei Herangehensweisen, geht auf aktuelle Forschungsentwicklungen ein und präsentiert eigene Forschungsbeiträge auf allen drei Gebieten. Ein Schlüsselelement super-flüssigkeitsabweisender Oberflächen sind Oberflächen-strukturen im Nano- bis Mikrometerbereich. Diese Strukturen sind allerdings empfindlich gegenüber mechanischer Beanspruchung. Es gibt zahlreiche Bestrebungen, mechanisch verstärkte Oberflächen herzustellen, allerdings ist die Charakterisierung ihrer mechanischen Eigenschaften nicht vereinheitlicht. Kraft-sensitive Messungen ermöglichen den Vergleich von Materialeigenschaften. Es werden die mechanischen Eigenschaften eines Kerzenruß-basierten Testsystems für super-flüssigkeitsabweisende Oberflächen untersucht. Der Einfluss der Berußungsposition innerhalb der Kerzenflamme auf die Benetzungs-eigenschaften wird zunächst evaluiert. Anschließend wird die Rolle der Reaktionsparameter auf die mechanische Stabilität beleuchtet. Dazu wird Rasterkraftmikroskopie mit einem Silikamikropartikel als Eindringkörper gewählt. Die Ergebnisse der Messungen werden mit dem Benetzungsverhalten der jeweiligen Oberflächen in Beziehung gesetzt, wodurch die mechanische Stabilität gegen das Benetzungsverhalten abgewogen werden kann. Beim Leidenfrost-Effekt gleitet ein Tropfen kontaktlos auf seinem Dampfkissen über eine Oberfläche. Dazu muss die Oberfläche ausreichend heiß sein, bevor der Tropfen mit ihr in Kontakt kommt. Kürzlich konnte gezeigt werden, dass zunächst ruhende Tropfen auf strukturierten Oberflächen anfangen können zu springen, wenn der Umgebungsdruck sukzessiv verminder wird. Dabei wird ein Leidenfrost-ähnlicher Zustand erreicht. Hier wird gezeigt, dass dieser Effekt nicht nur durch eine Druckänderung, sondern auch durch eine Temperaturänderung der Oberfläche zustande kommen kann. Unter anderem werden tropfenähnliche Hydrogele verwendet, die nahezu nur aus Wasser bestehen (>90 Gew%). Es wird gezeigt, dass Hydrogeltropfen durch einen Temperaturgradienten von flachen Oberflächen gelöst werden und danach kontinuierlich springen können. Diese Beobachtung setzt die kontrollierte Wärmeübertragung in Zusammenhang mit dem Springen von Tropfen und beleuchtet dabei die Rolle der Tropfenform und Elastizität. Flüssigen Murmeln sind Tropfen, die mit Pulverteilchen ummantelt sind. Die Teilchenschale verhindert, dass die innere Flüssigkeit mit der Oberfläche in Berührung kommt. Flüssige murmeln können sowohl über feste als auch flüssige Oberflächen bewegt werden. Die nichtinvasive, ferngesteuerte Bewegung solcher flüssigen Murmeln ist dabei von großem Interesse. Um dies zu erreichen werden flüssige Murmeln hergestellt, deren Schalenmaterial Licht in Wärme umwandelt. Schwimmt die flüssige Murmel auf einer Wasseroberfläche, kann durch Lichteinstrahlung ein Hitze-basierter Marangoni Fluss auf der Grenzfläche erzeugt werden, der die flüssige Murmel antreibt. Es wird ebenfalls gezeigt, wie die innere Flüssigkeit durch äußere Einflüsse gezielt freigesetzt werden kann, um eine Reaktion auszulösen. Zusammenfassend konnten mittels Oberflächenindentation mechanisch ausbalancierte, super-flüssigkeitsabweisende Oberflächen erhalten werden. Außerdem wurden der Leidenfrost-Effekt und flüssige Murmeln genutzt, um gezielt die Bewegung von Tropfen zu manipulieren.Controlling wetting, i.e. how drops interact and spread on surfaces, is of interest from a fundamental, physicochemical point of view and has relevance in many industrial processes like printing and spray coating. A great deal of attention is dedicated to the situation where drops ball up on a surface to minimize any contact between both. The reduced contact area results in a comparatively low adhesion force between the surface and the drop. Under certain conditions the lateral adhesion forces become so small, that drops easily move over the surface. A high drop mobility on a given surface is appealing for numerous applications, e.g. self-cleaning surfaces or the use of drops as microreactors. The non-wetting behavior results from entrapment of air pockets between drops and the surface. The entrapment of air pockets between both is either due to surface properties of the substrate, like in the case of super liquid-repellent surfaces and the Leidenfrost effect, or due to surface properties of the drop, like in the case of liquid marbles. In the presented thesis, I introduce the fundamentals, discuss state of the art research and challenges, and finally present contributions in all three fields. Super liquid-repellent surfaces rely on surface chemistry and, most importantly, on a nano- to micrometer-sized surface texture which stabilizes the air pockets between drop and substrate. However, surface textures on this size scale have low wear resistance what hampers the practical breakthrough of super liquid-repellent surfaces so far. Much attention is thus dedicated improving the mechanical strength of super liquid-repellent surfaces, but consistent approaches to quantify the mechanical durability of such surfaces are missing. Ultimately, a consistent test protocol requires force-sensitive indentation measurements to obtain quantitative results. Here, I investigate the mechanical properties of candle soot-templated super liquid-repellent surfaces. First off, the influence of the soot collection height on the wetting properties of the surface is explored. Then, I investigate the role of the reaction parameters on the mechanical properties of the candle soot-based test system. Therefore, force-sensitive measurements using colloidal indenters mounted to a scanning probe microscope are conducted. Comparison of these results to wetting experiments allows the careful balance of mechanical strength against repellency. In the Leidenfrost state, a drop hovers on a hot plate due to steady evaporation of the liquid. This creates an air cushion between both, drop and surface, and prevents any contact between them. The Leidenfrost effect is only observed if the surface temperature of the substrate exceeds the boiling point of the liquid by a lower, critical value before the drop comes close to it. Otherwise, surface and drop contact and the drop quickly evaporates. Whereas this is known, it was shown recently that drops on a superhydrophobic surface can enter a Leidenfrost-like state, starting from ambient conditions by a continuous decrease of the surrounding pressure. The surface texture restricts the vapor flow of the evaporating water drop which leads to an upward force. This results in the drop to jump from the substrate and continuously bounce at increasingly higher heights. In my thesis, I show that not only a pressure-time gradient but also a temperature-time gradient can lead to a similar effect. In particular, jumping and bouncing is observed even on smooth hot substrates for elastic hydrogel balls containing more than 90 wt% water. This study connects controlled heat transfer to drop bouncing depending on the drop elasticity. Liquid marbles are powder encapsulated drops. In contrast to the two aforementioned approaches, the non-wettability of liquid marbles is a drop, not a substrate property. The particle powder shell of liquid marbles is porous and entraps air pockets. The shell prevents the inner liquid from wetting the substrate and affords high mobility both, on solid and liquid substrates. In terms of applications, they serve as flexible, microliter sized reservoirs to carry analytes and reactants. The movement of these reservoirs is under investigation to prepare drop delivery systems which are controllable in space and time. If necessary, the inner liquid can be released to initiate a reaction or analysis. In this thesis, I introduce photo-thermally responsive liquid marbles. Such liquid marbles can be propelled over the air-water interface to a desired place at a given timing. Propulsion of the liquid marble is generated by shining light on the shell material. The light is converted into heat and the heat dissipates into the water leading to a heat gradient on the water surface close to the liquid marble. This in turn causes a Marangoni flow on the water surface, pushing the liquid marble forward. The inner liquid can be on-demand released by an external stimulus. In conclusion, contributions in all three fields are presented leading to mechanically optimized super liquid-repellent surfaces and two strategies were exploited to move and manipulate drops using the Leidenfrost effect and liquid marbles.175 Seite

Super liquid repellent surfaces for anti-foaming and froth management

Wet and dry foams are prevalent in many industries, ranging from the food processing and commercial cosmetic sectors to industries such as chemical and oil-refining. Uncontrolled foaming results in product losses, equipment downtime or damage and cleanup costs. To speed up defoaming or enable anti-foaming, liquid oil or hydrophobic particles are usually added. However, such additives may need to be later separated and removed for environmental reasons and product quality. Here, we show that passive defoaming or active anti-foaming is possible simply by the interaction of foam with chemically or morphologically modified surfaces, of which the superamphiphobic variant exhibits superior performance. They significantly improve retraction of highly stable wet foams and prevention of growing dry foams, as quantified for beer and aqueous soap solution as model systems. Microscopic imaging reveals that amphiphobic nano-protrusions directly destabilize contacting foam bubbles, which can favorably vent through air gaps warranted by a Cassie wetting state. This mode of interfacial destabilization offers untapped potential for developing efficient, low-power and sustainable foam and froth management