Surface forces and wetting features in drops and capillaries

Using the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory, which accounts for quantum mechanics and electrostatics at the macroscopic level, the thermodynamic expressions for (thermodynamic) equilibrium contact angles of drops on solid substrates and menisci in solid wall capillaries are, operationally and unambiguously, expressed in terms of the corresponding Derjaguin pressure. The latter’s S-shape is responsible for microdrops and other phenomena appearing on flat solid substrates

Kinetics of wetting and spreading of droplets over various substrates

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and spreading of non-Newtonian liquid (blood) over porous substrates. We showed that in spite of the enormous complexity of blood, the spreading over porous substrate can be described using a relatively simple model: a power low-shear-thinning non-Newtonian liquid. (iv) The kinetics of spreading of surfactant solutions. In this part, new results related to various surfactant solution mixtures (synergy and crystallization) are discussed, which shows some possible direction for the future revealing of superspreading phenomena. (v) The kinetics of spreading of surfactant solutions over hair. Fundamental problems to be solved are identified

Kinetics of Wetting and Spreading of Droplets over Various Substrates

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and spreading of non-Newtonian liquid (blood) over porous substrates. We showed that in spite of the enormous complexity of blood, the spreading over porous substrate can be described using a relatively simple model: a power low-shear-thinning non-Newtonian liquid. (iv) The kinetics of spreading of surfactant solutions. In this part, new results related to various surfactant solution mixtures (synergy and crystallization) are discussed, which shows some possible direction for the future revealing of superspreading phenomena. (v) The kinetics of spreading of surfactant solutions over hair. Fundamental problems to be solved are identified

Pore-scale modelling of wettability alteration in microporous carbonates

While carbonate reservoirs are recognized to be weakly- to moderately oil-wet at the core-scale, wettability distributions at the pore-scale remain poorly understood. In particular, the wetting state of micropores (pores <5 μm in radius) is crucial for assessing multi-phase flow processes, as microporosity can determine overall pore-space connectivity. Nonetheless, micropores are usually assumed to be water-wet and their role in multi-phase flow has often been neglected. However, oil-wet conditions in micropores are plausible, since oil has been detected within micropores in carbonate rocks. Modelling the wettability of carbonates using pore network models is challenging, because of our inability to attribute appropriate chemical characteristics to the pore surfaces in the presence of the oil phase and over-simplification of the pore shapes. First, we carry out an investigation of the prevalent wettability alteration scenario due to heavy polar compounds (e.g. asphaltenes) adsorption from the oil phase onto the surface, which occurs strictly after oil invasion. We develop a physically-plausible wettability distribution that we incorporate in a quasi-static two-phase flow network model which involves a diversity of pore shapes. The model qualitatively reproduces patterns of wettability alteration recently observed in microporous carbonates via high-resolution imaging. To assess the combined importance of pore-space structure and wettability on petrophysical properties, we consider a homogeneous Berea sandstone network and a heterogeneous microporous carbonate network, whose disconnected coarse-scale pores are connected through a sub-network of fine-scale pores. Results demonstrate that wettability effects are significantly more profound in the carbonate network, as the wettability state of the micropores controls the oil recovery. Second, we develop a novel mechanistic wettability alteration scenario that evolves during primary drainage, involving small polar non-hydrocarbon compounds present in the oil (e.g. alkylphenols, carbazoles, etc.). We implement a diffusion and adsorption model for these compounds that triggers a mild wettability alteration from initially water-wet to more intermediate-wet conditions. This mechanism is incorporated in the quasi-static pore-network model to which we add a notional time-dependency of the invasion percolation mechanism. The model qualitatively reproduces experimental observations where an early rapid wettability alteration occurred during primary drainage. Additionally, we are able to predict clear differences in the primary drainage patterns by varying both the strength of wettability alteration and the balance between the processes of oil invasion and wetting change, which control the initial water saturation for waterflooding. In fact, under certain conditions, the model results in higher oil saturations at predefined capillary pressures compared to the conventional primary drainage. In particular, it leads to the invasion of micropores even at moderate capillary pressures in the microporous carbonate network. Additionally, the model results in significant changes in the residual oil saturations after waterflooding, especially when the wetting state is altered from intermediate-wet to more oil-wet conditions during ageing

Stability, transport and applications of polyaphrons in porous media

Polyaphrons are a kind of macroemulsion. The most distinctive feature of polyaphrons is their high stability. On the other hand, certain polyaphrons can be effectively destabilized by multivalent ions. In this dissertation, we devise a novel procedure that utilizes the fact that polyaphrons can be destabilized by certain ions to deliver light chemicals to lower the density of dense non-aqueous phase liquid (DNAPL) contaminants in situ. We first investigate how the stability of diluted polyaphrons is affected by the properties of the continuous phase. We find that polyaphrons can be destabilized by a low concentration of Al3+ or Ca2+ in the continuous phase while the cations have a stabilizing effect at high concentrations. Our results suggest that upon dilution polyaphrons may experience a structure transition, i.e. from bilayer structure to mixed monolayer structure. We examine the efficiency of our novel procedure on removing 1,2-dichlorobenzene (DCB) from a sand column. The results show that our procedure effectively prevents downward migration of DCB during surfactant flooding. Depending on the injection strategy and initial distribution of DCB, as mush as 97% of the DCB entrapped in the sand column is removed, most of which is in a bulk organic phase lighter than water. Since the coalescence between aphrons is a crucial step in the polyaphron treatment, we develop a boundary element method (BEM) model to study the coalescence behavior of a pair of drops of equal size in a constricted tube. Our simulations show that the capillary number Ca plays an important role in determining whether the drops coalesce. At low Ca, drops hardly deform and coalescence occurs at the entrance of the pore throat, whereas significant deformation enables the drops move through the pore without coalescence at high Ca. Coalescence is favored at intermediate values of the viscosity ratio. The destabilizing effect of added electrolytes is found to be insignificant for the drop interaction for 10 micron-size drops, but significant for micron-size drops. Among the geometric-related parameters, the drop/pore size ratio appears to be the most significant: coalescence does not occur when this ratio is equal to or below unity

Adsorption and wetting : experiments, thermodynamics and molecular aspects

Adsorption and wetting are related phenomena. In order to improve knowledge of both and their relations, experiments, thermodynamics and a theoretical interpretation have been connected, starring n-alkanes.Starting from the Gibbs adsorption equation thermodynamic relations between vapour adsorption and wetting are derived. The surface pressure of a film, formed by vapour adsorption on a solid surface, is calculated by integrating the vapour adsorption isotherm. The surface pressure at the saturated vapour pressure determines, together with the interfacial tension of the liquid, the difference between the interfacial tension of a clean solid and a solid- liquid interface. Moreover, the surface pressure is related to the spreading tension and contact angle in a solid-liquid-vapour system. The thermodynamic equations derived are generally valid and the approach covers wetting on both flat and powdered solids. From the individual surface pressure values of two immiscible liquids, wetting and displacement in a solid-liquid-liquid system can be assessed. The procedure is illustrated for a silica-water-octane system. Silica is one of the most abundant minerals on earth and the oil/water wettability of silica can be considered a model for oil displacement in reservoir rocks.By combining the Young equation with the Gibbs adsorption equation, the contact angle and the work of adhesion of an aqueous electrolyte solution on a charged solid is investigated as a function of the solid surface charge density or the electrolyte concentration. In the case of partial wetting, the solid-solution-vapour contact angle is a maximum at the point of zero charge of the solid. The contact angle decreases and the work of adhesion increases with increasing absolute value of the surface charge. The derived equations are used to study the wettability of silica under changing electrolyte conditions. The surface charge density of silica Aerosil OX-50 at a number of indifferent KCl concentrations, ranging from 0.01 M to 1M, is determined as a function of pH by potentiometric titrations. The silica surface charge increases with increasing ionic strength and increasing pH. At its point of zero charge ( p H 0 ≈ 3) silica is already completely wetted by water. Charging of the surface results in an even better water wettability although this can not be observed experimentally. At pH = 9 and 1 M KCl the silica surface charge equals -0.25 C/m 2. Compared to the uncharged silica, this surface charge decreases the silica-water interfacial tension by 22 mJ/m 2. Under usual conditions the electrolyte adsorption at the solid-vapour interface will be less than at the solid-water interface. With respect to an uncharged silica ( pH =3), the silica surface charge of -0.25 C/m 2decreases the silica-(water) vapour interfacial tension by maximally 22 mJ/m 2whereas it increases the work of adhesion by maximally 22 mJ/m 2. By combining the present approach with theoretical equations describing the adsorption of charge determining ions on solids with different kinds and amounts of chargable groups the wettabililty of such solids as a function of their charging behaviour can be described theoretically. This remains a task for the future.The vapour adsorption of different n-alkanes, cyclohexane, toluene and water on bare and methylated pyrogenic silica (Aerosil OX-50) has been studied gravimetrically. Linear adsorption isotherms of the n-alkanes and of cyclohexane on both substrates are found until high relative vapour pressures. The same holds for toluene on methylated silica. The linearity of the isotherms indicates relatively weak lateral interactions between adsorbed molecules. On bare silica, the adsorption of the n-alkanes studied (C7-C9) is, expressed in moles/m 2, independent of the chain length. The adsorption strongly increases after the coverage corresponding to a monolayer of alkanes, oriented perpendicular to the surface, has been reached (at p/p 0 ≈0.8). Methylation of the silica decreases the adsorption of all adsorptives studied. Until just before saturation the octane adsorption on methylated silica is below that of a monolayer parallel to the surface. The shape of these adsorption isotherms indicates that on bare silica n-alkanes predominantly adsorb end-on, perpendicular to the surface, whereas on methylated silica, the adsorption is rather parallel. From the adsorption data surface pressure isotherms are constructed and the work of adhesion is obtained. The work of adhesion reveals that the Lifshits-van der Waals part of the silica surface tension is reduced from 44 mJ/m 2for bare (pyrogenic) silica to 30 mJ/m 2for methylated silica. The adsorption data are also converted to disjoining pressure isotherms. At low film thicknesses, these can be described by an exponential short-range interaction. The classical macroscopic models are not very suited for the description of such thin films for which the molecular organization and the discrete character of the adsorbed layer are extremely important. However, thin adsorbed layers can be described on the basis of microscopic models for adsorption. Also surface pressures of simple systems can be obtained from classical adsorption equations (e.g., Langmuir, Volmer, BET, Polyani). However, for chain molecules like n-alkanes these models are inadequate as they are unable to describe the structure of the molecules and their adsorbed layers. This problem can be overcome by using, for instance, a more recent self-consistent-field (SCF) theory, orginially developed by Scheutjens and Fleer and extended by Leermakers; and others.The SCF theory is applied for the description of chain molecular fluids and their interfaces. Hereto a fluid is considered as a mixture of chains and monomeric vacancies. The latter account for the free volume in the system. Intermolecular interactions are described in terms of Flory- Huggins (FH) parameters. For the homologous series of linear alkanes, these parameters are generalized and assessed from a fit to vapour pressure data. In the SCF lattice-fluid theory, each alkane is described as a chain of segments with a volume of 0.027 nm 3each. The segment- segment interactions (for which the FH parameter is zero by definition) are reflected in a non zero FH interaction parameter for a chain segment-vacancy contact χAO .Under the conditions mentioned χ AO equals 580/ T ( T in K) for all n-alkanes. With these parameters n-alkane bulk properties such as the vapour pressure, density, critical point and heat of vaporization can be obtained together with structural and thermodynamic properties of the liquidvapour ( LV ) interface. The calculations reveal that chain ends are the major constituents on the vapour side of the (alkane) LV interface. For longer chains and lower temperatures the (relative) preference of the chain ends to protrude into the vapour phase is more pronounced. The calculated variation of the n-alkane LV interfacial tension (γ) with temperature and chain length agrees quantitatively with experimental data. If the theory is applied for temperatures below the (experimental) n-alkane freezing points, positive dγ/d T values occur and a maximum in the LV interfacial tension is found at T/T C ≈0.12, irrespective of the chain length of the molecule. In experimental studies close to the n-alkane freezing points similar observations have been made. However, a comparion of these experimental observations with our theoretical predictions should be performed with some reservation as the theory describes a frozen phase as an isotropic supercooled fluid.The lattice fluid theory description of the n-alkane interfacial properties may be improved by considering chain-flexibility constraints, such as trans-gauche conformations and/or by distinghuishing (the parameters of the) CH 3 and CH 2 segments. It is rather straightforward to extend the present theory to more complex systems such as fluid mixtures, or fluids (vapour and liquid) at solid surfaces. The interfacial tensions of such interfaces can be inferred from the theory so that the work of adhesion and contact angles on these interfaces can be investigated as a function of temperature and chain length of the (wetting) liquid. Some of these aspects are elaborated in the last Chapter.The adsorption, structure and thermodynamics of (aliphatic) chain molecular fluids at rigid surfaces and at solids with thermally grafted (aliphatic) chains is also investigated. Vapour adsorption isotherms, inclusive the meta- and unstable regions, of an octameric fluid on various substrates are calculated. The octameric molecules are modelled as B-A 6 -B chains where A represents a CH 2 segment and B a CH 3 segment. On a bare solid, the influence of adsorption energy differences between the A and B segments of the chain molecule is investigated together with the influence of the chain flexibility. For semi-flexible chains with high chain end-adsorption energies the shape of the calculated isotherm qualitatively agrees with the linear vapour adsorption isotherms measured for n-alkanes on bare silica. With the theory adsorption isotherms resembling the ones measured for n-alkanes on methylated silica can be obtained. This requires semi-flexible chains with interaction parameters that favour a rather parallel adsorption of the chain-molecules with respect to the surface. A reduction of the chain flexibility, for instance by applying the RIS scheme, increases the tendency of the adsorbed molecules to line up. In general, this increases the adsorbed amounts when the interaction parameters favour end-on adsorption whereas this reduces the adsorbed amounts when the interaction parameters are in favour of parallel adsorption. On a poorly wetted rigid solid, a decreasing contact angle was calculated for increasing chain length of the (aliphatic) wetting liquid An . The contact angles and the (Zisman) critical surface tension for wetting decrease with increasing temperature. When the temperature approaches the critical temperature of the wetting fluid, complete wetting occurs. Furthermore, it is established that the temperature dependence of the contact angle mainly results from the influence of the temperature on the liquid-vapour and solid-liquid interfacial tensions.On solids with grafted chains, octamer ( A8 ) adsorption isotherms and contact angles are calculated for different grafting densities and grafted chain lengths. Grafting aliphatic chains on a very poorly wetted (bare) solid decreases the contact angle of the octameric liquid. On such a solid, the contact angle as a function of the grafting density passes through a minimum. The rise beyond the minimum has an entropic origin. Grafting of chains on a completely wetting (bare) solid eventually results in a finite contact angle; higher grafting densities give rise to higher contact angles. When longer chains are grafted, lower contact angles result for both substrates. The calculations provide insight into the wettability of a substrate by chain-molecular fluids on a molecular level. Partial wetting of chain molecules can be explained from an autophobic effect: due to the ordering (anisotropy) of the molecules present in the thinnest adsorbed layer that can form at saturation, molecules of the isotropic liquid are repelled. The liquid does not spread on the thin film and droplet formation results. The calculations reveal that the partial wetting of chain molecular liquids on grafted solids is largely due to the enrichment of the grafted layer by middle segments of the liquid molecules.In this thesis the thermodynamics and molecular aspects of adsorption and wetting have been investigated and coupled by means of vapour adsorption isotherms and a lattice fluid theory. At present a reasonable (semi) quantitative agreement between theory and experiments has been achieved. For the (near) future, some other investigations based on the present theory are challenging. Firstly, a better quantitative agreement with experimental data is feasible by optimizing the description of the aliphatic molecules, for instance by incorporating differences between end and middle segments and their mutual interactions, or, in our case, their interactions with a vacancy. Secondly, the theory is able to describe random, heterogeneous surfaces and rough substrates so that contact angles of chain molecular liquids on such substrates can be inferred and compared to theories such as developed by Cassie and Wenzel. Moreover, the theory can be extended to a twodimensional SCF approximation instead of the one dimension we used in this work. This renders calculation of the (density) contour plot of a droplet feasible. By comparing the contact angle of this contour plot with the equilibrium contact angle, calculated in onedimensional SCF, the effect of the drop size, its curvature and line tension on the contact angle can be studied. Finally, a "dynamic" version of the SCF theory is currently being developed in the department of Physical and Colloid Chemistry. In the future, this theory will be suited to investigate the dynamics of evaporation of chain molecular liquids as well as the dynamics of their adsorption, spreading and contact angles on solid substrates

On the mechanisms of shrinkage reducing admixtures in self con-solidating mortars and concretes

'Zur Wirkungsweise schwindreduzierender Zusatzmittel in selbstverdichtenden Mörteln und Betonen' Problemstellung und Zielsetzung 1. Der Einsatz selbstverdichtender Mörtel und Betone im Bauwesen erbringt klare Vorteile. Dies sind im Wesentlichen eine erhöhte Betonierleistung, verbesserte Betonierqualität für bewehrten Beton im Allgemeinen und für filigrane, eng bewehrte Bauteile im Besonderen. Die mit den traditionellen Methoden des Betonbaus verbundenen Lärmemissionen werden erheblich reduziert. Der Wegfall der für herkömmlichen Beton notwendigen Verdichtungsarbeit reduziert den manuellen Aufwand und die damit verbundenen Gesundheitsrisiken. Das im selbstverdichtenden Beton benötigte hohe Bindemittelleimvolumen ist der Betondauerhaftigkeit abträglich. Es bewirkt, dass selbstverdichtende Betone ein erhöhtes Schwindmaß sowie eine höhere Rissneigung aufweisen. Ersteres kann für Betonbauteile zu erheblichen Verformungen oder Zwangsspannungen führen, während Letzteres die Dauerhaftigkeit des Baustoffes Beton aufgrund einer Begünstigung rissinduzierter Schädigungsmechanismen stark beeinträchtigt. 2. Herkömmliche Methoden zur Schwindreduktion und Rissvermeidung verfolgen hauptsächlich das Ziel, die im Beton einzusetzenden Bindemittelmengen zu reduzieren. Für selbstverdichtende Betone ist dieses Konzept nur sehr begrenzt anwendbar, da die Selbstverdichtung dieser Betone relativ hohe Bindemittelleimvolumen erfordert. Eine Möglichkeit das Ausmaß des Schwindens und damit die Rissanfälligkeit selbstverdichtender Betone zu senken, besteht in der Anwendung schwindreduzierender Betonzusatzmittel. Eingeführt in den achtziger Jahren des 20ten Jahrhunderts in Japan, erweisen sich diese Zusatzmittel als effiziente Methode zur Verbesserung der Qualität bindemittelreicher Hochleistungsbetone im Allgemeinen und selbstverdichtender Betone im Besonderen. 3. Während die Wirksamkeit schwindreduzierender Betonzusatzmittel in zahlreichen anwendungsorientierten Studien nachgewiesen werden konnte, ist das Wirkprinzip nur unzureichend erforscht. Eines der Hauptziele dieser Arbeit ist deshalb die gründliche Erforschung des Wirkmechanismus schwindreduzierender Betonzusatzmittel. 4. Weiterhin besteht Unklarheit, wie diese Zusätze in den Chemismus der Zementhydratation eingreifen und ob dies der allgemeinen Dauerhaftigkeit des Baustoffes Beton abträglich ist. Ein wichtiges Ziel dieser Arbeit ist deshalb die gründliche Erforschung der Zementhydratation in Gegenwart einer repräsentativen Auswahl verschiedener Typen schwindreduzierender Betonzusatzmittel. 5. Die Nachhaltigkeit der Anwendung schwindreduzierender Zusatzmittel ist bedeutend für die Betondauerhaftigkeit. Ob schwindreduzierende Zusatzmittel auslaugbar sind und ob eine Auslaugung die Schwindreduktion langfristig beeinträchtigt, sind weitere Fragen, denen im Rahmen dieser Arbeit nachgegangen wird. Stand der Wissenschaft 6. Schwinden und Quellen von zementären Baustoffen wird im Allgemeinen mittels makroskopisch-thermodynamischer Ansätze beschrieben. Der stark vereinfachte Ansatz kapillaren Unterdrucks bzw. hydrostatischen Drucks als treibende Kraft für hygrische Verformungen wird weitgehend abgelehnt. Vielmehr wird im Bereich moderater Luftfeuchten der Spaltdruck und im Bereich niedriger Luftfeuchten die Oberflächenenergie zur Beschreibung der hygrischen Volumenstabilität herangezogen. 7. Schwindreduzierende Betonzusatzmittel bestehen überwiegend aus synergistischen Abmischungen nicht-ionischer Tenside mit Glykolen. Die amphiphilen Eigenschaften der nicht-ionischen Tenside führen zu einer Senkung der Oberflächenspannung des Zementporenwassers. In Abhängigkeit ihrer Konzentration in wässrigen Elektrolyten bilden nicht-ionische Tenside Mizellen und/oder Flüssigkristalle. Beobachtet wurden Mischungslücken und Aussalzungen dieser organischen, oberflächenaktiven Substanzen. Durch die Zugabe von Glykolen wird die Mischbarkeit nicht-ionischer Tenside mit wässrigen Elektrolyten stark erhöht und führt zu einer Absenkung der Bildung von Flüssigkristallen und organischen Aussalzungen sowie zu einer verminderten Adsorption des Tensides an Feststoffoberflächen. Der ausschließlich in der Patentliteratur erwähnte Synergieeffekt bei der Abmischung nicht-ionischer Tenside mit Glykolen zu Schwindreduzierern bezieht sich auf eine erhöhte Schwindreduktionskapazität des Zusatzmittels und beruht auf der Abmilderung aller Effekte, die zu einer Abscheidung des Tensides aus der wässrigen Lösung führen. 8. Eine Implementierung der spezifischen chemisch-physikalischen Eigenschaften schwind-reduzierender Zusatzmittel in bestehende Modelle zur Beschreibung des Trocknungsschwindens ist der Fachliteratur nicht zu entnehmen. Mit Ausnahme des Kapillardruckmodells zur Vorhersage des Trocknungsschwindens lassen sich Charakteristika schwindreduzierender Betonzusatzmittel, im Speziellen ihrer Oberflächenaktivität, nicht bzw. nur unzureichend in bestehende Modelle zum Trocknungsschwinden implementieren. Methodik 9. Die Oberflächenaktivität einer repräsentativen Auswahl an Schwindreduzierern wurde in makroskopischen Versuchen an synthetischen als auch an extrahierten Zementporenwässern quantifiziert. Dies umfasste auch die Quantifizierung von Mischungslücken und organischen Aussalzungen. 10. Ein in dieser Arbeit entwickelter theoretischer Ansatz zur Auswertung herkömmlicher Messungen der Oberflächenspannung erlaubt eine Abschätzung der Oberflächenspannung der Porenlösung im trocknenden, zementären Porensystem. 11. Der Einfluss schwindreduzierender Zusatzmittel auf den Hydratationsmechanismus, d.h. Hydratphasenbestand und Hydratationskinetik, wurde mittels Thermogravimetrie, Röntgenphasenanalyse bzw. isothermer Wärmeleitungskalorimetrie erfasst. Zusätzlich wurde Elektronenmikroskopie zur Beschreibung der Mikrostrukturen und energiedispersive Röntgenspektroskopie zur qualitativen Bestimmung von niedrig konzentrierten Hydratphasen eingesetzt. Die Veränderungen der Komposition des Zementporenwassers in Gegenwart schwindreduzierender Zusatzmittel wurden analysiert. Die spezifische Adsorption schwindreduzierender Zusatzmittel an Zementhydraten wurde an hydratisierendem Zement als auch an synthetischen Hydratphasen untersucht. 12. Der Mechanismus der Auslaugung schwindreduzierender Zusatzmittel wurde in Standtests untersucht, während praxisnahe Konditionen mittels zyklischer Auslaugung und Trocknung in Langzeittests simuliert wurden. 13. Die Beschreibung der hygrischen Eigenschaften von Zementstein und Mörteln erfolgte anhand von Schwind- und Desorptionsisothermen. Basierend auf thermodynamischen Ansätzen wurden unter Verwendung dieser Schwind- und Desorptionsisothermen Energiebilanzen erstellt, die eine Unterscheidung zwischen Verformungsenergie und Energie zur Erzeugung von Oberfläche im Trocknungsprozess zementärer Baustoffe zulassen und somit eine Abgrenzung der Einflussnahme von Schwindreduzierern auf diese spezifische Energieverteilung ermöglichen. Im Wesentlichen erzielte Ergebnisse 14. Schwindreduzierende Betonzusatzmittel nehmen aufgrund ihrer amphiphilen Eigenschaften Einfluss auf den Hydratationsmechanismus von Portlandzementen. In Gegenwart dieser Zusatzmittel ist die Löslichkeit für anorganische Salze verringert. Die Konzentration von Calcium-, Kalium- und Sulfationen sinkt mit zunehmender Konzentration des Zusatzmittels. Während der Induktionsperiode der Portlandzementhydratation führt dies zur temporären Ausfällung von Calcium-Kalium-Sulfathydrat. Eine Veränderung des Hydratphasenbestandes in Gegenwart von schwindreduzierenden Zusatzmitteln kann nicht signifikant unterschieden werden. Somit sind nachteilige Auswirkungen auf die Dauerhaftigkeit derartig modifizierter Betone aufgrund eines veränderten Hydratphasenbestandes nicht zu erwarten. 15. Die stark verzögernde Wirkung von Schwindreduzierern in Kombination mit polycarboxylat-basierten Fließmitteln beruht nicht auf der Adsorption des Schwindreduzierers am hydratisierenden Klinker. Vielmehr kann davon ausgegangen werden, dass die verminderte Löslichkeit für Salze in der Porenlösung den Reaktionsumsatz absenkt und/oder eine spezifische Adsorption des nicht-ionischen Tensides an Portlanditkeimen deren Wachstum hemmt und damit die Auflösung von silikatischen Klinkerphasen. 16. Schwindreduzierende Zusatzmittel weisen eine spezifische Adsorption an Portlandit auf, einem Nebenprodukt der Hydratationsreaktionen eines Hauptbestandteils von Portlandzement. Ein verstärktes Kristallwachstum von Portlandit in lateraler Dimension führt zu einer Zunahme der spezifischen Oberfläche des hydratisierten Zementsteines. Für nass nachbehandelte Zementsteine bedeutet dies eine Zunahme der Gelporosität auf Kosten der Kapillarporosität. Eine Einflussnahme auf die Gesamtporosität lässt sich nicht feststellen. 17. Die Zunahme der spezifischen Oberfläche von Zementstein in Gegenwart von Schwindreduzierern bewirkt eine verstärkte physikalische Adsorption von Zementporenwasser am Feststoff. Für Betone mit niedrigem w/z-Wert oder unzureichender Nachbehandlung kann dieser Prozess zu einer Reduktion des für die Hydratation verfügbaren Wassers führen und in einem vermindertem Hydratationsgrad resultieren. Dies könnte eine Ursache für die in der Literatur beschriebenen Einbußen bezüglich mechanischer Eigenschaften beim Einsatz von Schwindreduzieren sein. 18. Schwindreduzierer sind im hohen Maße auslaugbar. Jedoch zeigen zyklische Langzeittests, dass ein signifikanter Austrag des Zusatzmittels in vorwiegend trockener Exposition nicht zu erwarten ist. Die Nachhaltigkeit des Einsatzes dieser Zusatzmittel ist gegeben, wenn die Anwendung im Beton das Ziel der Reduktion des Trocknungsschwindens verfolgt. 19. Die schwindreduzierende Wirkung der nicht-ionischen Tenside beruht vorwiegend auf der Reduktion der Oberflächenspannung der Grenzfläche „flüssig/gasförmig“ des trocknenden Zementsteines. Inwieweit diese Oberflächenspannung durch das nicht-ionischeTensid herabgesetzt wird, ist von der Gesamtkonzentration im Allgemeinen und im Speziellen von der Konzentration des Tensides in der Oberfläche abhängig. Da im Zuge des Trocknens diese Grenzfläche wächst, kann bei gegebener Gesamtkonzentration des Zusatzmittels im Beton dessen Konzentration in der Grenzfläche sinken, woraufhin die Oberflächenspannung ansteigt und die Schwindreduktion sinkt. 20. Im Ergebnis dieser Arbeit ist es möglich, die Entwicklung sowohl der Oberfläche als auch ihrer Oberflächenspannung im Trocknungsprozess zu quantifizieren und diese Ergebnisse in einen einfachen konzeptionellen, thermodynamischen Ansatz zur Minimierung der freien Energie des trocknenden, zementären Porensystems zu überführen. Die Verwendung dieses konzeptionellen Ansatzes erlaubt es, den Wirkmechanismus schwindreduzierender Betonzusatzmittel zu beschreiben.Self Consolidating Concrete – a dream has come true!(?) Self Consolidating Concrete (SCC) is mainly characterised by its special rheological properties. With-out any vibration this concrete can be placed and compacted under its own weight, without segrega-tion or bleeding. The use of such concrete can increase the productivity on construction sites and en-able the use of a higher degree of well distributed reinforcement for thin walled structural members. This new technology also reduces health risks since in contrast to the traditional handling of concrete, the emission of noise and vibration are substantially decreased. The specific mix design for self consolidating concretes was introduced around the 1980s in Japan. In comparison to normal vibrated concrete an increased paste volume enables a good distribution of aggregates within the paste matrix, minimising the influence of aggregates friction on the concrete flow property. The introduction of inert and/or pozzolanic additives as part of the paste provides the required excess paste volume without using disproportionally high amounts of plain cement. Due to further developments of concrete admixtures such as superplasticizers, the cement paste can gain self levelling properties without causing segregation of aggregates. Whereas SCC differs from normal vibrated concrete in its fresh attributes, it should reach similar properties in the hardened state. Due to the increased paste volume it usually shows higher shrinkage. Furthermore, owing to strength requirements, SCC is often produced at low water to cement ratios and hence may additionally suffer from autogenous shrinkage. This means that cracking caused by drying or autogenous shrinkage is a real risk for SCC and can compromise its durability as cracks may serve as ingression paths for gases and salts or might permit leaching. For the time being SCC still exhibits increased shrinkage and cracking probability and hence may be discarded in many practical applications. This can be overcome by a better understanding of those mechanisms and the ways to mitigate them. It is a target of this thesis to contribute to this. How to cope with increased shrinkage of SCC? In general, engineers are facing severe problems related to shrinkage and cracking. Even for normal and high performance concrete, containing moderate amounts of binder, a lot of effort was put on counteracting shrinkage and avoiding cracking. For the time being these efforts resulted in the knowledge of how to distribute cracks rather to avoid them. The most efficient way to decrease shrinkage turned out to be to decrease the cement content of concrete down to a minimum but still sufficient amount. For SCC this obviously seems to be contradictory with the requirement of a high paste volume. Indeed, the potential for shrinkage reduction is limited to some small range modifications in the mix design following two major concepts. The first one is the reduction of the required paste volume by optimising the aggregate grading curve. The second one involves high volume substitution of cement, preferentially using inert mineral additives. The optimization of grading curves is limited by several severe practical issues. Problems start with the availability of sufficiently fractionated aggregates. Usually attempts fail because of the enormous effort in composing application-optimized grading curves or mix designs. Due to durability reasons, the substitution rate for cement is limited depending on the application purpose and on environmental exposure of the hardened concrete. In the early 1980s Shrinkage Reducing Admixtures (SRA) were introduced to counteract drying shrinkage of concrete. The first publications explicitly dealing with SRA go back to Goto and Sato (Japan). They were published in 1983, which is also the time when the SCC concept was introduced. SRA modified concretes showed a substantial reduction of free drying shrinkage contributing to crack prevention or at least a significant decrease of crack width in situations of restrained drying shrinkage. Will shrinkage reducing admixtures contribute to a broader application of SCC? Within the last three decades performance tests on several types of concrete proved the efficiency of shrinkage reducing admixtures. So, at least in terms of shrinkage and cracking, concretes in general and SCC in particular can benefit from SRA application. But "One man's meat is another man's poison" and with respect to long term performance of SRA modified concretes there are still several issues to be clarified. One of these concerns the impact of SRAs on cement hydration. It is therefore an issue to know if changes in the hydrated phase composition, induced by SRA, result in undesired properties or decreased durability. Another issue is that the long term shrinkage reduction has to be evaluated. For example, one can wonder if SRA leaching may diminish or even eliminate long term shrinkage reduction and if the release of admixtures could be a severe environmental issue. It should also be noted that the basic mechanism or physical impact of SRA as well as its implementation in recent models for shrinkage of concrete is still being discussed. The present thesis tries to shed light on the role of SRA in self consolidating concrete focusing on the three questions outlined above: basic mechanisms of cement hydration, physical impact on shrinkage and the sustainability of SRA-application. Which contributions result from this study? Based on an extensive patent search, commercial SRAs could be identified to be synergistic mixtures of non-ionic surfactants and glycols. This turns out to be most important information for more than one reason and is the subject of chapter 4. An abundant literature focuses on properties of these non-ionic surfactants. Moreover, from this rich pool of information, the behaviour of SRAs and their interactions in cementitious systems were better understood through this thesis. For example, it could be anticipated how SRAs behave in strong electrolytes and how surface activity, i.e. surface tension, and interparticle forces might be affected. The synergy effect regarding enhanced performance induced by the presence of additional glycol in SRAs could be derived from the literature on the co-surfactant nature of glycols. Generally it now can be said that glycols ensure that the non-ionic surfactant is properly distributed onto the paste interfaces to efficiently reduce surface tension. In literature, the impact of organic matter on cement hydration was extensively studied for other admixtures like superplasticizer. From there, main impact factors related to the nature of these molecules could be identified. In addition, here again, the literature on non-ionic surfactants provides sufficient information to anticipate possible interactions of SRA with cement hydration based on the nature of non-ionic surfactants. All in all, the extensive study on the nature of non-ionic surfactants, presented in chapter 4, provides fundamental understanding of the behaviour of SRAs in cement paste. Taking a step further to relate this to the impact on drying and shrinkage required to review recent models for drying and shrinkage of cement paste as presented in chapter 3. There, it is shown that macroscopic thermodynamics of the open pore systems can be successfully applied to predict drying induced deformation, but that surface activity of SRA still has to be implemented to explain the shrinkage reduction it causes. Because of severe issues concerning the importance of capillary pressure on shrinkage, a new macroscopic thermodynamic model was derived in a way that meets requirements to properly incorporate surface activity of SRA. This is the subject of chapter 5. Based on theoretical considerations, in chapter 5 the broader impact of SRA on drying cementitious matter could be outlined. In a next step, cement paste was treated as a deformable, open drying pore system. Thereby, the drying phenomena of SRA modified mortars and concrete observed by other authors could be retrieved. This phenomenological consistency of the model constitutes an important contribution towards the understanding of SRA mechanisms. Another main contribution of this work came from introducing an artificial pore system, denominated the normcube. Using this model system, it could be shown how the evolution of interfacial area and its properties interact in presence of SRAs and how this impacts drying characteristics. In chapter 7, the surface activity of commercial SRAs in aqueous solution and synthetic pore solution was investigated. This shows how the electrolyte concentration of synthetic pore solution impacts the phase behaviour of SRA and conversely, how the presence of SRA impacts the aqueous electrolyte solution. Whilst electrolytes enhance self-aggregation of SRAs into micelles and liquid crystals, the presence of SRAs leads to precipitation of minerals as syngenite and mirabilite. Moreover, electrolyte solutions containing SRAs comprise limited miscibility or rather show miscibility gaps, where the liquid separates into isotropic micellar solutions and surfactant rich reverse micellar solutions. The investigation of surface activity and phase behaviour of SRA unravelled another important contribution. From macroscopic surface tension measurements, a relationship between excess surface concentration of SRA, bulk concentration of SRA and exposed interfacial area could be derived. Based on this, it is now possible to predict the actual surface tension of the pore fluid in the course of drying once the evolution of internal interfacial area is known. This is used later in this thesis to describe the specific drying and shrinkage behaviour of SRA modified pastes and mortars. Calorimetric studies on normal Portland cement and composite binders revealed that SRA alone show only minor impact on hydration kinetics. In presence of superplasticizer however the cement hydration can be significantly decelerated. The delaying impact of SRA could be related to a selective deceleration of silicate phase hydration. Moreover, it could be shown that portlandite precipitation in presence of SRA is changed, turning the compact habitus into more or less layered structures. Thereby, the specific surface increases, causing the amount of physically bound water to increase, which in turn reduces the maximum degree of hydration achievable for sealed systems. Extensive phase analysis shows that the hydrated phase composition of SRA modified binders re-mains almost unaffected. The appearance of a temporary mineral phase could be detected by environmental scanning electron microscopy. As could be shown for synthetic pore solutions, syngenite precipitates during early hydration stages and is later consumed in the course of aluminate hydration, i.e. when sulphates are depleted. Moreover, for some SRAs, the salting out phenomena supposed to be enhanced in strong electrolytes could also be s

Monte Carlo Methods for Estimating Interfacial Free Energies and Line Tensions

Excess contributions to the free energy due to interfaces occur for many problems encountered in the statistical physics of condensed matter when coexistence between different phases is possible (e.g. wetting phenomena, nucleation, crystal growth, etc.). This article reviews two methods to estimate both interfacial free energies and line tensions by Monte Carlo simulations of simple models, (e.g. the Ising model, a symmetrical binary Lennard-Jones fluid exhibiting a miscibility gap, and a simple Lennard-Jones fluid). One method is based on thermodynamic integration. This method is useful to study flat and inclined interfaces for Ising lattices, allowing also the estimation of line tensions of three-phase contact lines, when the interfaces meet walls (where "surface fields" may act). A generalization to off-lattice systems is described as well. The second method is based on the sampling of the order parameter distribution of the system throughout the two-phase coexistence region of the model. Both the interface free energies of flat interfaces and of (spherical or cylindrical) droplets (or bubbles) can be estimated, including also systems with walls, where sphere-cap shaped wall-attached droplets occur. The curvature-dependence of the interfacial free energy is discussed, and estimates for the line tensions are compared to results from the thermodynamic integration method. Basic limitations of all these methods are critically discussed, and an outlook on other approaches is given