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Microparticles and of Mixtures Of Experimental Investigations of Drying Droplets Containing Microparticles and of Mixtures of Poly(Ethylene Oxide)/Microparticles Yohana Msambwa A thesis submitted in partial fulfillment of the requirements of Nottingham Trent University for the degree of Doctor of Philosophy May 2015 This work is the intellectual property of the author, and may also be owned by the research sponsor(s) and/or Nottingham Trent University. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re- use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantial copy is required, should be directed in the first instance to the author. ii Abstract Ring-stains are seen when droplets of liquid containing particles are left to dry on a surface: a pinned contact line leads to outward radial flow, which is enhanced by the diverging evaporative flux at the contact line. As a result, suspended particles in the drops are transported to the edge of the droplet, and deposited in a circular stain. In the first section of this study, we investigated how the width and height of ring in water droplets containing suspensions of polystyrene microparticles with diameters ≤0.5µm vary with experimentally controlled parameters, including particle size, contact angle, concentration, evaporation rate and orientation of the droplets. Our studies found, for the first time, that the drying rate plays an important role in determining the shape of the final deposit which may contribute to a better understanding of a coffee ring effect. At low drying rates, nearly all the particles are deposited in the ring and the width and height of the ring follow a power law. However there is a significant deposition of particles in the center of the droplets at the fastest drying rates. In the second section, we investigated the drying dynamics of the droplets using optical microscopy and optical coherence tomography both at low and high drying rates. The results indicate that, as expected, the outward capillary flow to the edge dominates for 0.5m and 2m particles at low drying rates. However, at the highest drying rates, particle flow is reversed towards the center for 0.5m and 2m particles and is attributed to Marangoni flow driven by a high temperature gradient between the edge and apex of the drop. For 5m at low drying rates, sedimentation is found to be more significant than outward flow and at higher drying rates the outward flow dominates over sedimentation. The final section involved mixing polystyrene microparticles with Poly(ethylene oxide) (PEO) polymer. Drying droplets of aqueous solutions of PEO is known to form either the familiar “coffee-ring” or a tall “central pillar” depending on the iii concentration of the polymer, its molecular weight, humidity and drying rate. The objective of this study was to investigate the effect of particle sizes and concentration on drying of water droplets containing a water/PEO solution mixture. Results show that the four stages in the pillar forming of PEO drying process is disrupted, and even completely destroyed, with the additional of polystyrene microparticles. The extent at which the pillars are destroyed depends on both the concentration and sizes of the polystyrene microparticles as well as on PEO concentration in the mixture. A range of deposit was observed, as a result, including the standard particle ring stain, polymer pillars and puddles. We present a novel and versatile technique, using the skewness of the height profile, to distinguish quantitatively between the various deposits. We show that this simple parameter seems to illustrate the observed deposit types, with a positive skewness for pillars, close to zero for flat puddle deposits and negative skewness for ring stains. The skewness values have the potential to be useful for characterizing deposits in a wide variety of system iv Declaration The experiments described in this thesis were carried out by myself and, where indicated, in collaboration with colleagues. The data analysis and interpretation is my own work. This thesis has been written entirely by myself. Yohana Msambwa v Acknowledgements I would like thank my supervisors Dr. Fouzia Ouali, Dr. David Fairhurst and Prof. Carl Brown for their unrelenting support and guidance throughout this work. I would like thank Dr. Haida Liang for helping me with appropriate suggestions regarding optical coherence tomography. I would also like to thank my independent assessor Dr. Michael Newton for his valuable inputs and comments on my research work. I wish to thank the technicians, Dave Parker and Stephen Elliot for their help in several occasions and Gordon Arnott for his expertise in scanning electron microscope. I would like to thank all my fellow laboratory mates in particular Naresh Sampara, Kyle Baldwin and Anthony Turner for their support. I greatly appreciate the unconditional love and support from my mother Frida Merere and other family members. I appreciate the financial support of Dar Es Salaam University College of Education, a constituent college of the University of Dar Es Salaam. vi Table of Contents Abstract ....................................................................................................... iii Declaration ................................................................................................... v Acknowledgements ........................................................................................ vi Table of Contents ......................................................................................... vii List of Figures ............................................................................................. xiii List of Tables ............................................................................................. xxvi Notation ...................................................................................................xxvii 1. Introduction ............................................................................................ 1 1.1 Motivation ............................................................................................ 2 1.2 Liquid-solid interface ............................................................................. 3 1.2.1 Surface Tension .............................................................................. 3 1.2.2 Wetting .......................................................................................... 4 1.2.3 Young’s Equation ............................................................................. 5 1.3 Droplet Evaporation of Pure Liquids ......................................................... 8 1.3.1 Theory of Droplet Evaporation .......................................................... 8 1.3.2 Modes of Droplet Evaporation. ........................................................ 12 1.3.3 Temperature ................................................................................. 13 1.3.4 Marangoni Flow ............................................................................. 16 1.3.5 Thermal Conductivity of the Substrate. ............................................ 18 1.3.6 Air Pressure .................................................................................. 20 vii 1.4 Evaporation of Droplets Containing Particles ........................................... 20 1.4.1 Coffee Ring Effect .......................................................................... 21 1.4.2 Particle Ring Growth ...................................................................... 23 1.4.3 Relative Humidity and Ring Stain Formation ..................................... 28 1.4.4 Forces between the particles in solution and the substrate ................. 30 1.5 Evaporation of Droplets Containing Polymers .......................................... 34 1.5.1 What is a polymer? ........................................................................ 34 1.5.2 Behaviour of PEO solutions ............................................................. 37 1.5.3 PEO droplets ................................................................................. 39 1.5.4 PEO and particle droplets ............................................................... 42 1.6 The Present Work ................................................................................ 43 2. Experimental Techniques and Methods ..................................................... 46 2.1 Introduction ....................................................................................... 47 2.2 Sample Preparation ............................................................................. 48 2.2.1 PS Microparticle Suspensions .......................................................... 48 2.2.2 PS-PEO Solution ............................................................................ 48 2.2.3 Substrates .................................................................................... 49 2.2.4 pH Measurement ........................................................................... 51 2.2.5 Zeta and Surface Potential Measurement.......................................... 52 2.3 Evaporation of Sessile Droplets ............................................................. 52 2.3.1 Experimental Set-up ...................................................................... 52 2.3.2 Experimental Procedures ...............................................................
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