Investigation of interfacial phenomena on bio-inspired micro/nano-engineered superhydrophobic surfaces
具有微/納米結構仿生超疏水表面的界面現象研究
Student thesis: Doctoral Thesis
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Award date | 2 Oct 2013 |
Link(s)
Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(87fb6106-d375-4398-9489-fc43cdae9f65).html |
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Other link(s) | Links |
Abstract
Nature is an important source of inspiration for developing advanced materials that
function from macroscale to nanoscale levels. The rapid advancement of
nanotechnology allows people to mimic nature and synthesize structures with desired
size and function that are not available in macroscopic systems. Hierarchical micro/nano
structures are often found in nature, such as certain plant leaves, insect wings, and bird
feathers. Surfaces with hierarchical structure are water-repellent or superhydrophobic
because water droplets can form nearly spherical shapes on such surfaces and roll off
rapidly when the surfaces are slightly inclined. This water-repellent or
superhydrophobic phenomenon has stimulated extensive research in producing artificial
superhydrophobic surfaces for a variety of applications. Generally, water has three
states, namely, liquid, solid, and vapor, and these states can change from one phase to
another by changing the temperature, pressure, or energy. Condensation
(vapor-to-liquid), evaporation (liquid-to-vapor), and frosting
(vapor-to-solid/vapor-to-liquid-to-solid) are the three common phase change phenomena.
This thesis aims to understand and control the interfacial phase change phenomena
(condensation, evaporation, and frosting) on bio-inspired micro-/nano-engineered
superhydrophobic surfaces, and translate the findings from these non-equilibrium
phenomena for various industrial applications, such as thermal management, biosensing,
and anti-frosting.
In the first part of this dissertation, we studied how to tailor surface roughness and
wettability to enhance the dropwise condensation heat transfer application. By studying
the roles of surface roughness (micro- or nano-scale) on condensation dynamics, we
have developed a novel strategy for engineering a hierarchical superhydrophobic
surface that can promote fast droplet nucleation and growth, as well as efficiently shed
condensate droplets. Moreover, the synergistic cooperation between the hierarchical
structures contributes directly to a continuous process of nucleation, coalescence, departure, and re-nucleation, enabling sustained dropwise condensation over prolonged
periods, which significantly dissipate heat by phase change. The insights learned from
this study could stimulate new theoretical and experimental efforts to probe the complex
condensation phenomenon.
Aside from the condensation phase change phenomenon, we also investigated
droplet evaporation on superhydrophobic surfaces. We systematically examined the
effects of surface roughness and droplet size on the whole evaporation process on
superhydrophobic surfaces. We found that during the droplet evaporation on the
micro-structured surfaces, the solid-liquid-vapor triple phase contact line pinning at the
first stage of evaporation and the Cassie-Wenzel transition at the late stage of
evaporation exhibited remarkable dependence on the surface roughness. We
demonstrated that when the droplet shrinks to a size that is comparable to the feature
size of the surface roughness, the line tension at the triple line becomes important in the
prediction of the critical droplet base size at Cassie-Wenzel transition. Moreover, we
showed that the hierarchical micro/nano-structured superhydrophobic surfaces not only
inhibits the onset of the contact line pinning phenomenon, but also prevents the
Cassie-Wenzel transition as opposed to that on micro-structured surfaces. Thus, the
unique evaporation phenomenon on such hierarchical surfaces can open up a novel
avenue in bio-sensing application.
Finally, we systematically performed a full spectrum phase change regime
including condensation, freezing, frosting, and defrosting at the whole surface level on a
series of samples with different wettability and surface roughness. We demonstrated that,
through the rational design of a novel hierarchical superhydrophobic surface with
nanograssed micro-truncated cone architecture, the subcooled condensate droplets can
constantly depart from the surface, significantly preventing heterogeneous ice
nucleation, and consequently retarding frost formation. Moreover, we found that this
kind of hierarchical surface also enabled efficient frost removal during the defrosting
process. By exploiting the synergistic effects of micro/nanoscale roughness to achieve
superior performances in these two opposite phase transition processes
(frosting/defrosting), we believe this study could shed new light on the development of
efficient materials for dropwise condensation and anti-frosting.
- Nanotechnology, Biomimicry, Surface chemistry, Hydrophobic surfaces