An insight into the effects of mix-wettability micro/nano structured surface on droplet spreading and heat transfer: with molecular dynamic simulations
Publication Date
July 27, 2026
Creator
Zhu, Chenyue
Abstract
With the advancement of electronic manufacturing technology, electronic devices are rapidly evolving toward higher integration and miniaturization. Consequently, the heat flux generated within limited spatial environments has increased dramatically, creating an urgent need for efficient heat dissipation methods to ensure safe operation. Nucleate boiling heat transfer is characterized by a small temperature difference and a high heat transfer coefficient, making it an effective approach for managing high heat flux
Compared to macroscopic droplet spreading and capillary flow, the phenomena and mechanisms at the micro/nanoscale are significantly more complex. At this scale, the influence of intermolecular forces becomes non-negligible, and potential scale effects must be carefully considered. Furthermore, theories based on macroscopic behavior may no longer be applicable and require corresponding modifications or supplements. Therefore, an in-depth investigation into the micro-mechanical mechanisms of droplet spreading and capillary flow is urgently needed. Extensive research indicates that composite wettability surfaces (mixed hydrophilic/hydrophobic surfaces) can effectively enhance boiling heat transfer. However, due to constraints in fabrication and experimental costs, it is challenging to conduct microscale boiling heat transfer experiments on these composite surfaces. Moreover, the rules governing how the distribution of hydrophilic/hydrophobic regions affects boiling heat transfer, along with the underlying physical mechanisms, remain incompletely understood.
Therefore, this study focuses on the micro/nanoscale processes of droplet spreading and capillary flow. Addressing the micro-mechanical mechanisms as the core scientific problem, I combine molecular dynamics (MD) simulations with theoretical analysis to deeply investigate how solid-liquid interactions influence surface wettability and droplet spreading, as well as the capillary flow processes and energy dissipation mechanisms under various wettability conditions. Concurrently, using MD simulations, I alter the area fraction and spatial distribution of hydrophobic regions to explore the physical principles of pool boiling heat transfer on both smooth and structured mixedwettability surfaces. I analyze the effects of solid-liquid interaction strength (surface wettability), wall temperature, and composite wettability micro/nanostructures on the bubble growth process, the number of evaporated atoms, interfacial thermal resistance, and heat flux. This reveals the physical mechanisms behind boiling heat transfer enhancement and provides a reference for optimizing structured surfaces. The main contributions are as follows:
1. Mechanism of Solid-Liquid Interactions on Surface Wettability and Droplet Spreading Dynamics
I investigated the wetting and spreading processes of droplets on solid surfaces under varying solid-liquid interaction strengths. While solid-liquid interactions are crucial in modulating wettability and spreading, current research covers a limited range of interaction strengths and lacks a deep understanding of how liquid molecular layers form or how precursor films affect spreading. The MD simulations indicate that surface wettability strengthens as the solid-liquid interaction increases. However, the droplet spreading velocity does not increase monotonically; excessively strong interactions actually hinder spreading. Based on the simulation results, I identified the conditions for generating a liquid precursor film and revealed its impact and mechanisms during spreading. Utilizing a simplified solid-liquid interaction model, I elucidated how interaction strength governs wettability and the formation of molecular layered structures. These findings deepen the understanding of micro-mechanical spreading mechanisms and provide a theoretical basis for the targeted regulation of wettability, making them highly relevant for engineering applications like self-cleaning surfaces, anti-icing/fogging, and microfluidic control.
2. Energy Dissipation Mechanisms in Nanoscale Capillary Flow and a Modified Theoretical Model
I revealed the energy dissipation mechanisms during nanoscale capillary flow and proposed a theoretical model incorporating dynamic contact angle effects and asymmetric wall wettability. While wettability dictates micro/nanoscale capillary flow, most studies focus on uniform wettability channels, leaving asymmetric nanochannels and their energy dissipation mechanisms underexplored. Through theoretical analysis and MD simulations, I established the critical conditions for capillary flow based on energy conservation and conversion. I modified the classical Lucas-Washburn model using the Cox-Voinov law to account for dynamic contact angles and asymmetric wettability, verifying the modified model's accuracy via MD simulations. These outcomes provide effective tools for analyzing and controlling micro/nanoscale capillary flow, offering broad application prospects in novel materials, microfluidic chips, ion/water transport in cementitious materials, and spontaneous imbibition of fracturing fluids in shale reservoirs.
3. MD Simulation of Pool Boiling Heat Transfer on Smooth Mixed-Wettability Surfaces
I explored the variations in bubble growth, evaporation atom count, and interfacial thermal resistance on smooth hydrophilic/hydrophobic composite surfaces under different interaction strengths and wall temperatures. The results demonstrate that hydrophobic regions promote bubble nucleation, accelerate growth, and enhance evaporation. Compared to purely hydrophilic surfaces, the time Δt required from bubble nucleation to the coalescence of a vapor film on the wall is reduced by up to 61.3%. Higher wall temperatures also shorten both the initial bubble growth time and the vapor film formation time. However, as the hydrophobic area fraction β increases, solid-liquid heat transfer begins to deteriorate. At β = 0.25, a 4 × 4 distribution pattern achieves the minimum thermal resistance (only 2% higher than a purely hydrophilic surface) due to the "dense" layout of surrounding low-potential-energy zones. As the hydrophobic fraction increases further, interfacial thermal resistance rises sharply, reaching up to a 368.68% increase compared to purely hydrophilic walls.
4. MD Simulation of Pool Boiling Heat Transfer on Structured Mixed-Wettability Surfaces
I constructed structured mixed-wettability surfaces using a 4 × 4 hydrophobic distribution pattern to investigate the changes in bubble growth, evaporation, and heat flux under various wall temperatures and hydrophobic fractions. The results show that as the projected area fraction of the micro/nanostructures β increases, the area of the low-potential-energy zone on the hydrophobic structured surface decreases. Additionally, as the micro/nanostructure height increases, the absolute value of solidliquid potential energy drops, leading to poorer contact and hindered heat transfer. Consequently, the liquid phase temperature change ΔT during bubble growth and the Critical Heat Flux (CHF) gradually decrease with higher. Conversely, for hydrophilic structured surfaces, an increase in β enlarges the solid-liquid contact area. Driven by capillary forces, the structural voids remain filled with liquid atoms, ensuring stable heat transfer and preventing "dry-out." With increasing, the ΔT improved by 14.0%, 10.1%, and 9.4%, respectively. However, increasing β did not significantly impact CHF, showing only minor improvements of 3.0%, 1.2%, and 1.8% compared to the baseline of β = 0.25.
Item Type
ethesis
Thesis Type
PhD
Supervisors
University of Nottingham
Mark, Aston
University of Nottingham
Subjects (LC)
Associated Schools / Departments
UoN Repository URI
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