Ice formation on micropillar patterned surfaces and freezing of nanofluid droplets

• Main Author: Zhao, Yugang
• Other Authors: Yang Chun, Charles
• Format: Theses and Dissertations
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/68835
• Institution: Nanyang Technological University

Ice is ubiquitous in nature, whilst its accretions may cause severe negative consequences. Owning to the progress in micro/nano fabrication technology, bioinspired roughness-induced superhydrophobicity has shown great potential in retarding ice nucleation, reducing interfacial ice adhesion and mitigating ice accumulation. Despite extensive progress, most of the existing studies on superhydrophobic surfaces were concerned of the freezing of individual deposited or impacting droplets. Thus, it is apparent that relatively little is known about the anti-icing performance of these surfaces in condensation frosting. Recent observations revealed the fact that in condensation frosting, frost spreading arises from a two-dimensional interdrop freezing wave propagation which initiates at geometry/chemical defects. As the heterogeneous nucleation of ice is inevitable, it is therefore of more significance to study the freezing wave propagation among condensed droplets. Surface interdrop freezing wave propagation among condensed droplets is of essential importance that determines ice invasion and following frost growth in condensation frosting. Whist forgoing literature considered only superhydrophobic surfaces with robust nanoscale or hierarchical roughness are capable of retarding interdrop freezing wave propagation, we report that interdrop freezing wave propagation on microscale pillar pattern engineered surfaces can also be profoundly suppressed with modulating condensate droplet using micropillar engineered surfaces. In particular, we examined the effects of pillar size and pillar density on freezing propagation velocity and ice coverage by exploiting condensation frosting on a series of silicon based micropillar pattern engineered surfaces. Our results show that with properly chosen pattern design the freezing propagation velocity is suppressed by one order of magnitude compared to that of smooth surfaces. On the other hand, the ratio of ice coverage which causes heat transfer breakdown is also significantly reduced. Additionally, an analytical model is then constructed to describe the freezing propagation with microstructured surface effects and to reveal the dynamics of ice bridging. We believe the concept of modulating condensate droplets distribution to retard interdrop freezing wave propagation might shed new light on the design of durable icephobic surfaces. Besides of studying the freezing droplets interaction, we also investigated the freezing of an individual nanofluid droplet. In contrast with a conical singular tip formed when freezing a pure water droplet, a flat plateau emerges on top of icecrystalline as the result of freezing a nanofluid droplet. This intriguing observation stems from a non-uniform distribution of nano particles which alters the phase propagation profile and eventually causes a macroscopic geometrical change of the ice crystalline. An outward Marangoni flow which drives nanoparticle from central to wedge vertexes, and a radical osmotic flow arising from particle concentration gradient play important roles during this process. Also, a numerical model is developed to represent the physical roots and agrees reasonable with experimental results. Finally, to further explore its potential for industrial applications we investigated the solidification of nanofluid droplets on energetically patterned substrates. For a droplet confined with a triangle three phase lines, we observed the top plateau geometry of an ice crystalline switches from a triangle, to hexagon, counter triangle, and hexagon again while increasing droplet height. A similar scenario happens for show that with properly chosen pattern design the freezing propagation velocity is suppressed by one order of magnitude compared to that of smooth surfaces. On the other hand, the ratio of ice coverage which causes heat transfer breakdown is also significantly reduced. Additionally, an analytical model is then constructed to describe the freezing propagation with microstructured surface effects and to reveal the dynamics of ice bridging. We believe the concept of modulating condensate droplets distribution to retard interdrop freezing wave propagation might shed new light on the design of durable icephobic surfaces. Besides of studying the freezing droplets interaction, we also investigated the freezing of an individual nanofluid droplet. In contrast with a conical singular tip formed when freezing a pure water droplet, a flat plateau emerges on top of ice crystalline as the result of freezing a nanofluid droplet. This intriguing observation stems from a non-uniform distribution of nano particles which alters the phase propagation profile and eventually causes a macroscopic geometrical change of the ice crystalline. An outward Marangoni flow which drives nanoparticle from central to wedge vertexes, and a radical osmotic flow arising from particle concentration gradient play important roles during this process. Also, a numerical model is developed to represent the physical roots and agrees reasonable with experimental results. Finally, to further explore its potential for industrial applications we investigated the solidification of nanofluid droplets on energetically patterned substrates. For a droplet confined with a triangle three phase lines, we observed the top plateau geometry of an ice crystalline switches from a triangle, to hexagon, counter triangle, and hexagon again while increasing droplet height. A similar scenario happens for that of square pattern. A numerical model is developed to represent the mechanism that this geometry change of top plateaus arises from horizontal osmotic flow as the result of particle accumulations. Reasonable agreement was obtained between the numerical results and experimentally observations. We believe this observation will shed new light to 3D print and micro casting.