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Doctoral Thesis
DOI
10.11606/T.43.2017.tde-10072017-094009
Document
Author
Full name
Alexandre Barros de Almeida
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2017
Supervisor
Committee
Alencar, Adriano Mesquita (President)
Coutinho, Kaline Rabelo
Galvão, Douglas Soares
Henriques, Vera Bohomoletz
Scott, Luis Paulo Barbour
Title in Portuguese
Gotas e pontes capilares na escala nanométrica
Keywords in Portuguese
dinâmica molecular
gás de rede
gotas
nanoescala
pontes capilares
teoria capilar
Abstract in Portuguese
O fenômeno da capilaridade na escala macroscópica é descrito pela teoria capilar (TC) que se utiliza de superfícies contínuas para modelar as interfaces formadas entre dois meios, sendo um líquido e o outro líquido, gasoso, sólido. A TC é empregada em diversas áreas da biologia, ambientes de microgravidade e em aplicações na escala nanométrica, como no microscópio de força atômica. Essa aproximação por superfícies contínuas pode não ser adequada para sistemas na escala nanométrica, em que são reportados comportamentos anômalos como no preenchimento de líquidos em nanocanais e nanotubos de carbono, oscilações nas medidas de força de adesão capilar e grandes valores de pressões de Laplace negativas. Esses fatos motivam o estudo do fenômeno da capilaridade na escala nanométrica por meio de simulações computacionais. Aqui, utilizamos a dinâmica molecular para estudar a interface de gotas e pontes capilares constituídas de água do modelo SPC/E com volumes da ordem de 100 nanômetros cúbicos e aderidas a placas de cristobalita hidrofóbicas/hidrofílicas. Comparamos as propriedades dessas gotas e pontes capilares com as previsões da TC macroscópica, que são baseadas nos ajustes dos perfis e em cálculos analíticos. Especificamente, confrontamos os perfis das interfaces, os ângulos de contato, as forças de adesão capilar, as pressões de Laplace e o valor da tensão superficial da água. Essas análises foram divididas em três etapas. Na primeira etapa, estudamos as gotas e pontes capilares com simetrias axial e translacional, em que a altura da ponte capilar permaneceu constante. Na segunda etapa, focamos nossos estudos nas pontes capilares com simetria axial (ponte SA) e estudamos o processo de ruptura dessa. Finalmente, na terceira etapa, estudamos as flutuações, que não são previstas pela TC, em sistemas mais simples, como no caso de gotas livres, que não estão aderidas a placas, e em gotas com simetria axial. Mostramos que a TC macroscópica é capaz de explicar satisfatoriamente sistemas com volumes da ordem de 100 nanômetros cúbicos, em que submetemos nossos resultados a comparações rigorosas das soluções analíticas da TC, sendo essa capaz de prever a dependência do ângulo de contato nas alturas das rupturas das ponte SA e os volumes das gotas formadas após a ruptura.
Title in English
Droplets and capillary bridges at the nanoscale
Keywords in English
capillary bridges
capillary theory
Droplets
lattice gas
molecular dynamics
nanoscale
Abstract in English
The capillarity phenomenon at macroscopic scale are described by the capillarity theory (CT), which uses continuous surfaces to model the interfaces formed between two media, wherever one medium is liquid and the other can be liquid, gas or solid. The CT is employed in several areas ranging from biology, microgravity environments and applications on the nanoscale, such as in the atomic force microscope. However, the continuous approach may not be adequate for systems at nanoscale, where anomalous behaviors have been reported, such as the filling of liquids in nanochannels and carbon nanotubes, oscillations in measurements of capillary adhesion force and large negative values of Laplace pressures. These facts motivate the study of capillarity phenomenon at the nanometric scale by computational simulations. Here, we use the molecular dynamics to study the droplets and capillary bridges interfaces composed of SPC/E water model and volumes in the order of 100 cubic nanometer, and attached to hydrophobic/hydrophilic cristobalite walls. We have compared the droplets and capillary bridges properties with the macroscopic CT predictions, which are based on profile fitting and analytic calculations. Specifically, we have compared the interface profiles, the contact angles, the capillary adhesion forces, the Laplace pressures and the water surface tension. These analyzes were divided into three steps. In the first step, we have studied droplets and capillary bridges with axial and translational symmetries, where the capillary bridge height remained constant. In the second step, we have focused our studies on capillary bridges with axial symmetry (AS bridge), and we have studied the bridges rupture process. Finally, in the third step, we have studied the fluctuations, which are not predicted by the CT, in simpler systems, such as free droplets, which are not attached to walls, and droplets with axial symmetry. We have shown that the macroscopic CT is able to satisfactorily predict systems with volumes in the order of 100 cubic nanometer, in which we have been submitted our results to rigorous comparisons to the analytic CT solutions, which is able to predict the dependence of contact angle on the AS bridge rupture heights, and the volumes of droplets formed after rupture.
 
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Publishing Date
2017-07-31
 
WARNING: The material described below relates to works resulting from this thesis or dissertation. The contents of these works are the author's responsibility.
  • ALMEIDA, A. B., BULDYREV, S. V., and Alencar, A. M. Crackling Sound Generation During the Formation of Liquid Bridges: A Lattice Gas Model [doi:10.1016/j.physa.2013.03.038]. Physica. A [online], 2013, vol. 392, p. 3409-3416.
  • ALMEIDA, A. B., e Alencar, A. M. Study of surface free energy on a lattice-gas model applied to Liquid bridge. In Encontro Nacional de Fisica da Materia Condensada, Aguas de Lindoia/SP, 2013. Anais do ENFMC., 2013. Resumo. Dispon?vel em: http://www.sbf1.sbfisica.org.br/eventos/enfmc/xxxvi/sys/resumos/R0423-1.pdf.
  • LLOVERA-GONZÁ, et al. Speckle patterns during the spreading of lung surfactant [doi:10.1117/12.2026326]. In VIII Iberoamerican Conference on Optics (RIAO/OPTILAS 2013), Porto, Portugal, 2013. RIAO Proceedings., 2013.
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