Título:	Numerical study of complex fluid-interface systems : from ionic surfactant-covered droplets to electrohydrodynamic phenomena
Outros títulos:	Estudo numérico de interfaces líquidas complexas : de gotas recobertas por surfactantes iônicos a fenômenos eletro-hidrodinâmicos
Autor(es):	Barbosa, Juan Linhares
Orientador(es):	Oliveira, Taygoara Felamingo de
Assunto:	Emulsões Surfactantes iônicos Eletro-hidrodinâmica Level set Ghost fluid
Data de publicação:	24-Nov-2025
Data de defesa:	4-Abr-2025
Referência:	BARBOSA, Juan Linhares. Numerical study of complex fluid-interface systems: from ionic surfactant-covered droplets to electrohydrodynamic phenomena. 2025. 132 f., il. Dissertação (Mestrado em Ciências Mecânicas) — Universidade de Brasília, Brasília, 2025.
Abstract:	The stability and control of emulsion flows are key aspects in designing products across industries such as environmental engineering, oil recovery, food processing, and pharmaceuticals. Emulsion stability hinges on interfacial phenomena, where ionic surfactants—amphiphilic molecules with charged head and tail groups—play a critical role by adsorbing at droplet interfaces, reducing surface tension, and imparting electrostatic repulsion to hinder coalescence. Surfactants redistribute over the droplet surface under imposed flow, coupling interfacial mechanics with charge transport mechanisms. This study investigates two interconnected aspects of droplet electrohydrodynamics: (1) the interplay among the effect of surfactant charge, induced electric field, and bulk flow stresses on the interfacial force balance and droplet dynamics, and (2) the electrohydrodynamic response of droplets under applied electric fields, focusing on charge convection and Quincke rotation. In the first part, we numerically study surfactant-laden droplets in shear flow, employing a unified framework that integrates electric, interfacial and hydrodynamic effects. A coupled numerical methodology combines the projection method to solve the Navier-Stokes equations, the level-set technique to capture the interface, and the closestpoint method to resolve surfactant transport. This approach captures the balance among shear-driven advection, surfactant diffusion, and electromigration of ionic charges on the droplet on droplet interface. We demonstrate that droplet deformation is governed by the Mason number (ratio of viscous to electric forces), the mobility ratio (charge transport efficiency), the capillary number (ratio of viscous and capillary forces), the Peclet number (ratio of convective and diffusive transport of surfactant), the surface surfactant coverage, and surfactant elasticity. Surface charge density depend linearly on surfactant concentration, the flow field advects charges to high-curvature regions, while diffusion redistributes charges along the interface and electromigration shifts peak concentrations away from droplet tips. The second part examines electrohydrodynamics of clean droplets (without surfactants or imposed shear flow) under uniform electric fields. Surface charge convection and Quincke rotation—a spontaneous rotation due to charge-induced torque—are analyzed. The numerical framework is extended with the ghost fluid method to handle interfacial discontinuities (e.g., permittivity, conductivity) and a diffusive interface model that smoothens transitions across the interface. By comparing sharp (ghost fluid) and diffuse interface treatments, we identify regimes where charge convection dominates droplet deformation and triggers Quincke rotation. The tilt angle during rotation agrees with rigid sphere theory at low electric capillary numbers but shows a slight mismatch under strong electric fields, highlighting the role of droplet deformation and interfacial mobility. Quantitative comparisons with experimental data and theoretical models validate our methodology, underscoring its capability to bridge the gap between sharp and diffuse interface paradigms. This work advances the current understanding of droplet dynamics in multifield environments, offering insights for optimizing emulsion stability and electrohydrodynamic manipulation in industrial and biophysical applications.
Unidade Acadêmica:	Faculdade de Tecnologia (FT) Departamento de Engenharia Mecânica (FT ENM)
Informações adicionais:	Dissertação (mestrado) — Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, Programa de Pós-Graduação em Ciências Mecânicas, 2025.
Programa de pós-graduação:	Programa de Pós-Graduação em Ciências Mecânicas
Licença:	A concessão da licença deste item refere-se ao termo de autorização impresso assinado pelo autor com as seguintes condições: Na qualidade de titular dos direitos de autor da publicação, autorizo a Universidade de Brasília e o IBICT a disponibilizar por meio dos sites www.unb.br, www.ibict.br, www.ndltd.org sem ressarcimento dos direitos autorais, de acordo com a Lei nº 9610/98, o texto integral da obra supracitada, conforme permissões assinaladas, para fins de leitura, impressão e/ou download, a título de divulgação da produção científica brasileira, a partir desta data.
Agência financiadora:	Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) e Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
Aparece nas coleções:	Teses, dissertações e produtos pós-doutorado

Os itens no repositório estão protegidos por copyright, com todos os direitos reservados, salvo quando é indicado o contrário.