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Title: An experimental investigation on droplet generation in microfluidic T-junctions and characterisation of the resulting nanoparticle stabilsed emulsions
Author: Loizou, Katerina
ISNI:       0000 0004 6351 3815
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2016
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The utilisation of microfluidic devices and methods at a submillimeter scale to manipulate and control fluid flows has been spreading fast over fields such as chemistry, physics or biology. Microfluidics have been used as a platform in an attempt to miniaturise processes and techniques in handheld Lab-on-Chip devices. Specifically, droplet microfluidics have found several applications in a wide range of industries such as pharmaceutical, food or cosmetic industry. The generation of monodispersed, nanoliter droplets using such networks is beneficial in terms of the intrinsic control over different parameters that these devices offer. These emulsions can be considered as building blocks for particles, encapsulating carriers for chemicals or even microreactors. The stabilisation of such droplets to generate Pickering emulsions is another promising aspect, as these have found applications in the industries mentioned above or as precursors for shell or particle generation. Incorporating particles in emulsion systems to stabilise the system against coalescence is not a new concept. As emulsions are inherently unstable due to the increased surface to volume ratios, formation of stabilised emulsions using particles or surfactants has received significant attention over the years. Particle stabilised emulsions are also used as drug delivery precursors. The backbone of this study is the investigation of droplet generation in microfluidic T-junctions, in an attempt to revisit fundamental mechanisms governing droplet generation. Experimental tests examining the effect of flow and fluid properties as well as geometrical characteristics on the droplet generation mechanism are presented. A scaling law to predict droplet volume is constructed from conclusions drawn for the parametric space tested. The proposed scaling law is evaluated using several different fluids and it is compared with existing scaling laws. In another context, the transition between droplet generation regimes is investigated. Transition between regimes at constant Capillary numbers is reported and an intermediate regime observed between squeezing and dripping 1 has been identified. The intermediate or transitional regime identified is reproducible, stable over time and reoccurring. It is customary that the transition between regimes to be characterised using Capillary number. However in this work it is demonstrated that regime transition occurs at constant Capillary numbers when the flowrate ratio is the parameter being altered. The second part of this study focuses on the utilisation of T-shaped microfluidic junctions to generate Pickering emulsions. Nanoparticles of different geometries and contact angles are employed to generate nanofluids of different physical properties in order to assess their ability to stabilise nanoliter droplets generated in microfluidic T-junctions. The effect of nanofluids on the droplet generation mechanism is described, as it is suggested that two different modes of droplet generation are observed. The effect of nanoparticle concentration on droplet size and stability is also assessed. Nanoparticle stabilised droplets are observed to collapse after 7 days in storage. It is postulated that as the encapsulated liquid diffuses through the particle laden interface, the decrease of the encapsulated volume together with the transition of the liquid film to a solid film are causing the collapse. The behaviour of droplets that are stable against coalescence in different environments is also summarised. It was found that nanoparticle stabilised droplets collapse when heated. Additionally, the effect of varying pH of the encapsulated liquid is also investigated. It is concluded that weak acidic solutions can be encapsulated. In this thesis, it is demonstrated that nearly monodisperse microdroplets generated using a microfluidic T-Junction are successfully stabilised using a type of bioactive and biodegradable nanoparticles.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available