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Title: Charge of water droplets in non-polar oils
Author: Schoeler, Andreas Martin
ISNI:       0000 0004 5368 328X
Awarding Body: King's College London
Current Institution: King's College London (University of London)
Date of Award: 2015
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Water-in-oil microdroplets are an attractive “tool” in lab-on-a-chip devices, as they offer simple compartmentalisation, constitute tiny reaction chambers and can be used to perform “digital” operations. One of the many benefits they offer is the ability to manipulate droplets by electric fields, which can be implemented on-chip, using electrodes and suitable wiring. Water droplets dispersed in a non-polar oil are manipulated by exploiting the fundamental phenomenon of electrophoretic motion, i.e. motion in response to an external, electric field. There are surprisingly little data regarding the electrophoretic mobility of water droplets dispersed in a non-polar oil and this work aims to elucidate some of the properties of droplet charge from measurements of the electrophoretic mobility of individual water droplets in two different, non-polar oils of similar, physical fluid properties: silicone and paraffin oil. Single droplets of varying pH and ion concentrations were investigated and it was found that the effective initial droplet charge (i.e. the charge a water droplet has before making contact with a biased electrode) is always positive and independent of pH and ion concentration. When the anionic surfactant SDS (Sodium Dodecyl Sulfate) was dispersed in the water phase, the initial droplet charge could be altered from positive to negative at concentrations greater than 1 g/l. However, using cationic surfactant CTAB (Hexadecyltrimethyl Ammonium Bromide) had no impact on droplet surface charge. Once the droplet touches a biased electrode, the droplet charge in increased by a factor of 10 and any surfactant charge effects are overridden. Lastly, complex oil-in-water-in-oil and water-in-oil-in-water-in-oil droplets were created and their electrophoretic mobility was studied. It was found that the inner droplet does not affect electrophoretic motion of the core shell drop, regardless of size and composition, nor does it experience the same (if any) electric field strength the outer water shell is subjected to. This is advantageous in a variety of applications. For example, oil droplets of varying types and sizes could accurately be transported and manipulated at the same speed using monodisperse water shells, which can be either thick or ultrathin. This could also be used for the manipulation of materials that would otherwise be damaged by an electrical field.
Supervisor: Sajjadi-Emami, Shahriar ; Mesquida, Patrick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available