The Drying of Inkjet Printed Drops on Patterned Substrates

Inkjet printing provides a promising method for the fabrication of OLED displays but currently, inkjet printed displays are not commercially viable. This thesis focuses on understanding the drying processes that occur once drops have been delivered to the patterned substrates necessary for OLED devices. To this end, internal flows in drops evaporating within wells were investigated and the changing drop profiles during drying were imaged. A method was suggested for successful fabrication of OLED devices. Particle tracking was carried out on both pure solvents and binary solvent mixtures within square wells. Due to the large particle size in comparison to the depth of fluid these experiments were not very informative, though they did confirm evaporation was faster at the contact line than in the centre of the drops. Evaporation was also slightly faster in the corners of the wells relative to the straight edges. Studies on pure solvents identified the influence of evaporation rate on profile development in drying drops. Two main drying regimes were identified and the main influence on drop profile development was found to be the evaporation rate of the solvent. Slow drying drops gave U-shaped profiles and fast drying drops gave W-shaped profiles. The influence of thermal effects on drop profiles was also considered. Thermal Marangoni flows were found to have a profound influence on profile development, with drops giving M-shaped profiles. Thermal effects could not always be reliably reproduced and it was concluded that further experimentation in this area was necessary. The lack of repeatability in the results was assumed to be due to the sensitivity of the drop profile to its initial behaviour. Binary solvent mixtures were also found to have an impact on profile progression during drying. Solutal Marangoni flows gave M-shaped profiles in the case where the more volatile solvent had a lower surface tension and enhanced drainage from the corners of the wells towards the centre in the case where the more volatile solvent had a higher surface tension. The thesis then moved on to investigate the effect of active materials on drop profiles. The active materials used were found to increase the surface tension of the solvents, giving M-shaped profiles when dissolved in single solvents. In some slow drying solvents, diffusion of the material evened out concentration gradients during drying and U-shaped profiles were seen. When solvent mixtures which had shown flows in opposition to those caused by active materials were used to print the actives, the profile development showed enhanced drainage from the corners of the wells suggesting solvent driven Marangoni flows were dominant over active material driven Marangoni flows. Crystallisation of the active material in this case showed re-circulatory flows were present with the active materials following the flows. This suggested particle tracking should be possible in these systems. A proposed method for obtaining flat deposits from printed drops was then presented, along with some initial results towards that goal. The initial results were promising but more investigation is needed in this area

Levitation of non-magnetizable droplet inside ferrofluid

The central theme of this work is that a stable levitation of a denser non-magnetizable liquid droplet, against gravity, inside a relatively lighter ferrofluid -- a system barely considered in ferrohydrodynamics -- is possible, and exhibits unique interfacial features; the stability of the levitation trajectory, however, is subject to an appropriate magnetic field modulation. We explore the shapes and the temporal dynamics of a plane non-magnetizable droplet levitating inside ferrofluid against gravity due to a spatially complex, but systematically generated, magnetic field in two dimensions. The effect of the viscosity ratio, the stability of the levitation path and the possibility of existence of multiple-stable equilibrium states is investigated. We find, for certain conditions on the viscosity ratio, that there can be developments of cusps and singularities at the droplet surface; this phenomenon we also observe experimentally and compared with the simulations. Our simulations closely replicate the singular projection on the surface of the levitating droplet. Finally, we present an dynamical model for the vertical trajectory of the droplet. This model reveals a condition for the onset of levitation and the relation for the equilibrium levitation height. The linearization of the model around the steady state captures that the nature of the equilibrium point goes under a transition from being a spiral to a node depending upon the control parameters, which essentially means that the temporal route to the equilibrium can be either monotonic or undulating. The analytical model for the droplet trajectory is in close agreement with the detailed simulations. (See draft for full abstract).Comment: This article has been published in a revised form in Journal of Fluid Mechanics http://dx.doi.org/10.1017/jfm.2018.733. Copyright: copyright holde

Transport and deposition patterns in drying sessile droplets

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106826/1/aic14338.pd

Axisymmetric critical points of a nonlocal isoperimetric problem on the two-sphere

On the two dimensional sphere, we consider axisymmetric critical points of an isoperimetric problem perturbed by a long-range interaction term. When the parameter controlling the nonlocal term is sufficiently large, we prove the existence of a local minimizer with arbitrary many interfaces in the axisymmetric class of admissible functions. These local minimizers in this restricted class are shown to be critical points in the broader sense (i.e., with respect to all perturbations). We then explore the rigidity, due to curvature effects, in the criticality condition via several quantitative results regarding the axisymmetric critical points.Comment: 26 pages, 6 figures. This version is to appear in ESAIM: Control, Optimisation and Calculus of Variation

Sharp interface limit of an energy modelling nanoparticle-polymer blends

We identify the $\Gamma$-limit of a nanoparticle-polymer model as the number of particles goes to infinity and as the size of the particles and the phase transition thickness of the polymer phases approach zero. The limiting energy consists of two terms: the perimeter of the interface separating the phases and a penalization term related to the density distribution of the infinitely many small nanoparticles. We prove that local minimizers of the limiting energy admit regular phase boundaries and derive necessary conditions of local minimality via the first variation. Finally we discuss possible critical and minimizing patterns in two dimensions and how these patterns vary from global minimizers of the purely local isoperimetric problem.Comment: Minor changes. Rephrased introduction. This version is to appear in Interfaces and Free Boundarie

Doctor of Philosophy

dissertationDNA microarrays have been plagued with analytical problems with quantitation, metrics, figures of merit, and reliability and reproducibility issues, hindering their acceptance in clinical and diagnostic settings. The main deficiency in the printed DNA format is the microspot heterogeneity occurring during array fabrication and further amplified during target hybridization. Work described in this dissertation focuses on assessment of DNA microarray spots generated with conventional pin-type contact printing of fluorescently labeled DNA probes, on industry-standard commercial polymer-coated array slides and their hybridization with complementary oligomer DNA target. Printing of probe DNA microspots shares many features of commonly reported droplet evaporation dynamics that lead to different drying patterns and spot morphologies. This study directly identifies and analyzes different DNA probe chemical and spatial microenvironments within spots, analyzed with high-resolution time-of-flight secondary ion mass spectrometry (TOF-SIMS) chemical imaging, confocal epifluorescence, and probe microscopy force imaging methods. Drying of DNA probe spots shows Marangoni flow effects with high densities of probe DNA-Cy3 located in spot centers and nonhomogeneous DNA distributed radially within printed spots with both TOF-SIMS imaging and epifluorescence microscopy. Target hybridization kinetics and duplex formation were assessed using real-time in situ confocal imaging, and confirmed radial hemispherical diffusion-mediated distribution of target capture from spot edge to its interior. Kinetic modeling indicates pseudo-first order kinetics due to transport limitations and local density-dependent probe interactions with diffusing target. Fluorescence resonance energy transfer (FRET) and photobleaching results show that the high- density probe overcrowding in spots facilitates a broad range of target binding interactions regardless of dye orientations. Moreover, lateral probe density heterogeneity observed with high-resolution imaging techniques confirmed with confocal microscopy produces equally iv heterogeneous target capture under normal assay conditions, showing how spot drying produces signal variability. These methods are the first to interrogate single printed array spots providing new support that microspot signal heterogeneity is not purely a result of target hybridization but is initially sourced during immobilization of probes with droplet printing techniques. This will guide new thinking on immobilized density influence on assay performance and how to approach assay endpoints, either kinetically or at equilibrium binding, by modifying spot molecular environments to reliably capture their signal

Small Volume Fraction Limit of the Diblock Copolymer Problem: I. Sharp Interface Functional

We present the first of two articles on the small volume fraction limit of a nonlocal Cahn-Hilliard functional introduced to model microphase separation of diblock copolymers. Here we focus attention on the sharp-interface version of the functional and consider a limit in which the volume fraction tends to zero but the number of minority phases (called particles) remains O(1). Using the language of Gamma-convergence, we focus on two levels of this convergence, and derive first and second order effective energies, whose energy landscapes are simpler and more transparent. These limiting energies are only finite on weighted sums of delta functions, corresponding to the concentration of mass into `point particles'. At the highest level, the effective energy is entirely local and contains information about the structure of each particle but no information about their spatial distribution. At the next level we encounter a Coulomb-like interaction between the particles, which is responsible for the pattern formation. We present the results here in both three and two dimensions.Comment: 37 pages, 1 figur

Development and evaluation of nanocrystal formulations and their incorporation into topical semisolid vehicles

Nanosuspension technology is a strategy to increase the bioavailability of poorly soluble drugs and has been used for various routes of drug administration. Nanocrystals consist of pure drug, thus allowing the administration of a high drug dose in a small volume. This is advantageous for topical administration to increase the concentration gradient for the drug uptake into the skin. Moreover, the nm size of the drug (< 1μm) allows improved skin penetration and enhanced drug delivery to the target site. Nanosuspensions can be used as prepared or be loaded into semisolid vehicles. There are some challenges, which need to be solved. Stabilizers are unavoidable for aqueous nanosuspensions. However, stabilizers can also be a limiting factor for stability, e.g., during freeze-thaw cycling or sterilization. In addition, stabilizers might interact with the excipient in semisolid dosage forms, resulting in a different performance of the nanocrystals. The nanocrystal stability upon topical administration is also still an open question. Therefore, a systematic investigation of nanocrystal-loaded topical formulations was performed upon preparation, incorporation into semisolid vehicles and administration