Merging, spreading and jumping nanodroplets

Droplet-based systems appear in various aspects of our daily lives: in understanding the process of atmospheric storm cloud formation -involving very large length and time scales; in determining the shelf life of emulsion-based products such as mayonnaise - involving intermediate scales; and in design and optimization of next-generation micro/nano-fluidic devices such as nanopipe cooling materials - operating at much smaller scales. There are clear differences in the dominant physics that underpins their functioning, when these systems scale from macro to nano. As a result, many of the experimental observations at micro/nano-scales are often counter-intuitive and fascinating. Some such examples relevant to future nano-engineered technologies include: order of magnitudes higher water flow rate through nanotubes than predicted by traditional theories, passive water droplet transport to hotter regions on a heated surface and faster evaporation rates from nanoscale menisci. In this thesis, unconventionally large and computationally expensive molecular dynamics simulations are used to study problems involving nanodroplets, which have a wide range of engineering applications. The novelty in this work includes: (a) the investigation of previously unexplored realms of nanoscale interfacial fluid flows using high-fidelity molecular simulations and (b) uncovering the theoretical and fundamental explanation of how molecular motion affects the nanodroplet dynamics of three problems: merging, spreading and jumping nanodroplets. In the first problem, coalescence of two water droplets is studied, focusing on the first contact and growth of the bridge that connects both droplets. Many mathematical models in the literature host a `singularity' in the beginning of coalescence, where calculated quantities like velocity and pressure diverge at this point. Such singularities are unphysical, and what happens in reality is investigated in more detail in this thesis. The thermal motion of constituent molecules is found to have substantial impact not only in initiating coalescence, but also in developing the liquid bridge in the initial stages. For large droplets, a hydrodynamic instability develops owing to the attraction between confronting interfaces of the droplets as they approach each other. However, no evidence of such instability is observed at the nanoscale. Instead, the first contact happens because of the interfacial thermal fluctuations on droplets' surfaces meeting from opposite sides. Thereafter, coalescence proceeds in an observed `thermal regime', where, as molecular simulations show, the bridge grows as a result of gradual cohesion of the confronting interfaces of the droplets due to collective molecular jumps. This continues until a `thermal length scale' is achieved, which is found to scale as square-root of the size of the coalescing droplets. Only after these molecular-driven processes finish does the bridge evolve in the manner that we had previously understood. The relevance of the observed molecular thermal motion on droplet-droplet interactions is tested on droplet-surface interactions and found to extend also to these problems with small variations in the observed physics. When a liquid wets a solid surface, which is essential for applications in coating technologies, agriculture and printing, to name a few, a regime of contact line motion, which is very similar to the thermal regime in coalescence, is found to precede the contact line motion that we have traditional understood. The extent of this regime not only scales as square-root of the droplet size, but also depends on the attraction from the underlying wall. The dependence of this length scale on the equilibrium contact angle is explained based on the local profile of the droplet near the wall when the first contact happens. In this `thermal-vdW regime', the interfacial molecules of the droplet get deposited directly on to the surface, before it gives way to the traditional picture of contact line motion, where the molecules at the three-phase-zone hop over the potential energy landscape above the wall atoms. The existence of this new regime of droplet wetting on atomically smooth surfaces is further validated by comparison of the contact line motion with what is described by the molecular kinetic theory, with which the late stage dynamics closely match. The third problem combines the droplet-droplet and droplet-surface interactions and investigates the molecular physics of coalescence-induced jumping of nanodroplets from non-wetting surfaces, which is relevant for heat transfer and self-cleaning applications. Here, the effect of molecular thermal motion and ambient gas rarefaction on the jumping speed of a droplet is investigated. While the presence of an outer gas reduces the jumping speed by introducing an additional dissipation mechanism into the system, the interfacial thermal fluctuations make the jumping of nanodroplets a stochastic process. An analytical model of drag from outer gas is developed explaining the reduction of the jumping speed with respect to that in near-vacuum conditions. The thermal-capillary waves on the droplet surface renders the jumping speed to be statistically distributed with smaller droplets having wider and skewed distributions. It is shown that the jumping dynamics of nanodroplets is governed not just by Ohnesorge number as previously thought, but also by Knudsen number and thermal fluctuation number. Despite their increased importance at the nanoscale, this is the first time that the effect of thermal capillary waves is properly quantified in studies concerning the dynamics of nanodroplets. Moreover, this thesis is intended to inspire the reader to look at many other traditional problems with singularities from a fundamental molecular perspective. It may be the case that the thermal regime of droplet coalescence and the thermal-vdW regime of droplet spreading are two special classes of a larger set of interface evolution dynamics and this requires further systematic molecular investigations and quantifications. Furthermore, the models developed in this thesis can be integrated in CFD simulations in the future as better initial/boundary conditions. Coupled with insights from the theoretical analyses presented throughout this thesis, the results can be used to study many natural systems and to predict performance characteristics of futuristic micro/nano-fluidic devices, which employ nanodroplets for heat-transfer and various other emerging technologies such as self-cleaning and anti-icing surfaces

Bibliography of Lewis Research Center technical publications announced in 1984

This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1984. All the publications were announced in the 1984 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses

Nanoengineering approaches for guiding cellular behavior

Tissue engineering aims to replace restore or help regeneration of injured tissue or organ with scaffolds that mimic the natural extracellular matrix (ECM). The design of such scaffolds requires deeper understanding of the factors that determine the cellular behavior. This thesis is focused on the cell-biomaterials interaction, but it strives to go beyond the classical material science, looking for new options to obtain control over the cell behavior. Cellular interaction with artificial substrata is a well-described paradigm usually attributed to the adsorption of adhesive proteins from the surrounding medium. The recognition of these proteins triggers an order of specific signaling events, reminiscent of the natural interaction of cells with ECM, affecting strongly their behavior. One important aspect of such interaction, however, is the organization of the matrix proteins - a hallmark for the ordinary ECM. Recent studies in our group showed that also in-vitro the cells tend to create organization. They remodel the adsorbed matrix proteins (in a fibril-like pattern) as an attempt to make their own provisional ECM. This phenomenon, described basically for fibronectin (FN), appears to involve also other matrix proteins, such as fibrinogen (FBG), and even collagen IV and vitronectin, which, being non fibrillar proteins by their nature, also undergo linear reorganization. Thus, cells somehow “prefer” fibrillar assemblies trying to imprint such patterns in their microenvironment. Other types of protein arrangements, however, for example network-like assemblies, which are also typical for the ECM, are insufficiently studied and this comprises an essential part of this work. In the first part of the thesis, particular attention is devoted on the peculiar behavior of adsorbed FN and FBG in the nanoscale observed by atomic force microscopy (AFM). Joint work with the group of Prof Salmeron-Sanches from the Polytechnic University in Valencia revealed that apart from the classical view for rather stochastic adsorption of matrix proteins, the lateral protein-protein interactions prevail on some surfaces giving rise to self-assembly in a network-like structures with significant consequences on the cell behavior. The thesis focuses particularly on the biological activity of these networks. The performed studies clearly suggested that the modulation of the network formation (using model surfaces with varying density of -OH groups) has evident impact on cell adhesion and functionality ¿ a fact, confirmed with two different cell systems: fibroblasts and endothelial cells. Another line of performed research lie on the fact that matrix proteins can sequestrate from the surrounding liquid phase to form structures of various shapes, including fibres with a diameter of only few nanometres and lengths up to centimetres; thus resembling the natural ECM components. A fascinating possibility to mimic similar structures is to engineer nanofibers, based on matrix proteins, via electrospinning technology - an approach extensively explored in the second part of the thesis. It was evidently shown that the cells readily recognize such fibrils and attach to them much faster than on planar substrata. Thus, one can anticipate that mimicking the organization of ECM with nanofibers will help to understand how cells respond to such an environment, an issue that is fundamental for biology. Besides, this approach represents an additional tool for controlling the cell behavior as proposed in this thesis. Therefore nanofibers based on natural matrix proteins (e.g., fibrinogen, fibronectin) and synthetic polymers (e.g. polylactic acid, (PLA); poly(ethylacrilate), (PEA)) were systemically elaborated. Their implication as a model system revealed that by varying with the composition, the organization and the mechanical properties of these fibres a tight control over the cellular response may be obtained.La ingeniería de tejidos tiene el objetivo de reemplazar, restaurar o regenerar tejidos dañados con matrices que mimetizan la matriz extracelular natural (MEC). El diseño de dichas matrices requiere un profundo conocimiento de los factores que gobiernan el comportamiento celular. Esta tesis se centra en las interacciones entre células y biomateriales, pero pretende ir más allá de los límites de la ciencia de materiales con el fin de contribuir al desarrollo de nuevos métodos para controlar el comportamiento celular. Las interacciones de las células con sustratos artificiales son un paradigma bien caracterizado, atribuido a la adsorción de proteínas adhesivas del medio circundante. El reconocimiento de dichas proteínas desencadena eventos de señalización celular reminiscentes de las interacciones naturales célula-MEC que afectan al comportamiento de las células adherentes. Un aspecto importante de dicha interacción es la organización de las proteínas de la matriz, que caracteriza la MEC. Estudios recientes han demostrado que in vitro las células tienden a crear organización, remodelando las proteínas de matriz adsorbidas (en un patrón fibrilar) a fin de generar su propia MEC provisional. Este fenómeno descrito básicamente para la fibronectina (FN), parece involucrar otras proteínas de la matriz como el fibrinógeno (FBG), el colágeno IV y la vitronectina, las cuales, no siendo proteínas fibrilares por naturaleza, experimentan una reorganización linear. Así, las células, de alguna manera, “prefieren” una disposición “fibrilar” intentando grabar dichos patrones en su microambiente. Otros tipos de disposición típicos en la MEC, p.ej. en forma de red, han sido poco estudiados, y constituirán una parte esencial de esta tesis. En la primera parte del trabajo, se presta atención especial al comportamiento peculiar de la FN y la FBG adsorbidas, observado mediante microscopía de fuerza atómica (MFA). Un trabajo conjunto con el Prof. Salmerón-Sánchez de la Universidad Politécnica de Valencia reveló que aparte de adsorción estocástica, en algunas superficies las interacciones laterales proteína-proteína prevalecen, dando lugar al auto-ensamblaje en estructuras de red con efectos significativos sobre el comportamiento celular. Esta tesis se centra especialmente en la actividad biológica de estas redes. Los estudios realizados claramente sugieren que la modulación de la reticulación (usando superficies modelo con densidad variable de grupos hidroxilo) tiene un evidente impacto en la adhesión y funcionalidad celular- confirmado mediante dos sistemas celular: fibroblastos y células endoteliales. Otra línea de investigación se basa en el hecho de que las proteínas de la matriz en solución se pueden secuestrar de la fase líquida circundante, formando estructuras diversas, incluyendo fibras con un diámetro de unos nanómetros, similares a los componentes de la MEC. La ingeniería de nanofibras basada en las proteínas de la matriz, mediante tecnología de “electrospinning”, ofrece posibilidades fascinantes para mimetizar estas estructuras. Esta aproximación se explora en la segunda parte de esta tesis, en la que se demuestra que las células se anclan más rápido y reconocen más fácilmente estas fibras. Por lo tanto, puede anticiparse que mimetizar la organización fibrilar de la MEC mediante nanofibras puede contribuir a comprender como las células responden a su entorno, un fenómeno esencial para la biología. Además, este método representa una herramienta adicional para controlar el comportamiento celular como se propone en esta tesis. Por lo tanto, nanofibras basadas en las proteínas naturales de la matriz (p.ej. FBG o FN) así como en polímeros sintéticos (p.ej. ácido poliláctico (PLA), o el polietacrilato (PEA)) fueron sistemáticamente elaboradas. Su implicación como sistemas modelo reveló que variando la composición, organización y propiedades mecánicas de estas fibras puede obtenerse un control preciso sobre las respuestas celulares.Тъканното инженерство цели възстановяване или регениране на увредени тъкани или органи посредством матрици (“scaffolds”), които имитират естествения екстрацелуларен матрикс (ЕЦМ). Разработването на такива подложки обаче изисква дълбоко познаване на факторите, определящи клетъчното поведение. Тази дисертация се фокусира върху взаимодействието клетка-биоматериал като се стреми да надникне отвъд класическото материалознание, тъсейки нови подходи за осъществяване на контрол върху поведението на клетката. Клетъчното взаимодействие с изкуствени субстрати е добре описан парадигъм, който обикновено се свързва с адсорбцията на адхезивни протеини от заобикалящата среда. Разпознаването на тези протеини запуска редица сигнални мехамизми, наподобяващи естественото взаимодействие на клетките с екстрацелуларния матрикс, и повлиява поведението на адхериралите клетки. Важен аспект на едно такова взаимодействие е организацията на адхезивните матриксни протеини – знакова характеристика за нативния екстрацелуларен матрикс. Предишни изследвания в нашата група показаха, че дори in-vitro клетките са склонни да създават организация. Те ремоделират адсорбираните матриксни протеини (най- често във фибриларна форма) в опит да създадат собствен, кратковременен екстрацелуларен матрикс. Въпреки, че този феномен е установен и описан предимно за фибронектина (ФН), се оказва, че той включва и други матриксни протеини, например фибриноген (ФБГ), и дори колаген тип IV и витронектин (ВН), които, макар и нефибриларни по своята природа, също претърпяват линейна реорганизация. Следователно, клетките по някакъв начин „предпочитат“ фибриларната организация на протеините и се опитват да „отпечатат“ подобни структури в най-непосредствено си обкръжение. Други типове организация, обаче, като например мрежовиднaта, също типичнa за ЕЦМ, са недосатъчно изследвани и това съставлява съществена част от настоящата дисертация. Първата част на този тезис обръща особено внимание на специфичното поведение на адсорбирани ФН и ФБГ в наноразмерната област, което може да бъде наблюдавано посредством атомно-силова микроскопия (АФМ). Съвместната работа с групата на проф. Салмерон-Санчес от Политехническия университет на Валенсия (в момента в Университета на Глазгоу (Шотландия)), показа, че отделно от класическото разбиране за стохастична адсорбция на матриксните протеини, върху някои повърности може да пробладава латералното взаимодействие между техните молекули, водещо до самоорганизирането им в мрежоподобни структури, имащи значителни последствия както върху поведението на протеина, така и върху неговата биоактивност. Настоящият труд е фокусиран именно върху биологичната активност на тези супрамолекулни структури, докато феноменът на образуването им сам по себе си, както и неговите наноинженерни аспекти, са обект на отделни изследвания. Проучванията, проведени в обхвата на тази дисертация ясно доказаха, че чрез промяна в повърхностната плътност на –ОН групи може силно да се повлияе способността за мрежовидна самоорганизация на някои белтъци, а от там и клетъчното поведение – факт, потвърден при две отделни клетъчни системи: фибробласти и ендотелни клетки. Друга линия на изследвания се основава на факта, че, в разтвор, белтъците могат да секвестират от заобикалящата ги течна фаза и да формират различни структурни форми, включително фибри с диаметър от само няколко нанометра и дължина до сантиметри, наподобяващи естественната организация на ЕЦМ. Интригуваща възможност за имитиране на подобни структури е разработването на нановлакна, базиращи се на матриксни протеини, посредством технологията на електроовлакняване (електроспининг) – подход, който е широко застъпен във втората част на дисертацията. Там безспорно е показано, че клетките се прикрепят по- добре към такива нановлакна и явно ги разпознават – феномен, който макар и не напълно разбран, бе потвърден в редица изследвания. Следователно, имитирането на фибриларната организация на ЕЦМ би спомогнала да разберем как клетките реагират на подобна среда – въпрос, който е фундаментален за клетъчната биология. Наред с това, подобен подход предствавлява и допълнителен инструмент за контрол на клетъчното поведение, въпрос които подробно е застъпен в настоящия тезис. И така, базирайки се на естествени матриксни протеини, какъвто е фибриногенът, и на синтетични полимери, каквито са полимлечната киселина (PLA) и полиетилакрилатът (PEA), бяха разработени нов тип хибридни нановлакна и тяхното използване като модална система разкри, че варирайки с тяхната композиция, организация и механични свойства би могло да се осъществи прецизен контрол върху клетъчния отговор. Настоящата дисертация се базира на пет оригинални публикации, организирани в пет отделни глави (Глава 2 до Глава 6), подредени в логична и йереархична последователност, и носещи едноименни със съответната публикация заглавия. Подробното Въведение (Глава 1) и кратките уводни бележки към всяка глава отразяват същественото в публикуваната работа и негйното положение спрямо цялостната тематика на дисертацията, формулирана ясно в раздела “Цел и задачи”