Molecular dynamics simulation of the wetting characteristics of a nanofluid droplet on rough substrate

doi:10.1016/j.molliq.2020.114204

Journal of Molecular Liquids

Volume 319, 1 December 2020, 114204

https://doi.org/10.1016/j.molliq.2020.114204 Get rights and content

Highlights

•
Rough patterns will affect the wettability of nanofluids on it.
•
Increasing the fluid volume fraction will slow down its dynamic diffusion.
•
Droplets containing hydrophilic nanoparticles have worse wettability.
•
Droplet spreading is related to surface tension and viscosity.

Abstract

Wettability of a nanofluid droplet (ND) on rough substrate with fence pattern was researched by molecular dynamics (MD) method. The Embedded Atom Method (EAM) potential was used to model the interatomic interaction between nanoparticle atoms, solid rough substrate atoms and between these two. The effects of the surface morphology of substrate, the surface wettability of the nanoparticle, and the nanoparticle volume fraction (NVF) on the wettability of ND were studied. The vapor-liquid surface tension and viscosity of nanofluid were also discussed in details. The results show that the initial equilibrium contact angle (θ_e) of ND on a rough substrate is larger than that on a smooth substrate. When the interaction parameter between the substrate atoms and oxygen atoms increases, θ_e decreases. For hydrophilic rough substrates, the larger the NVF of hydrophilic nanoparticles in the ND, the greater the equilibrium contact angle of the ND, and the worse the wetting properties of the ND, which determines that the main driving force for the dynamic spreading process of the ND is the vapor-liquid surface tension and viscosity of the ND.

Introduction

Wettability of ND is very important for industrial applications, such as automatic DNA mapping [1], optofluidic control [2], nanoimprint [3], electronic chips [4] and nano/microfluidics [5]. Nowadays, the research on the wettability of nanodroplets mainly focuses on the wettability of sessile pure water nanodroplets on smooth or rough substrates [[11], [12], [13]], or the wettability of nanofluid nanodroplets on smooth substrates [8]. However, due to the fact that nanoparticles significantly affect the wetting properties of nanodroplets [[2], [3], [4], [5], [6], [7], [8], [9]], and for dynamic wetting on the rough substrate, the droplet easily penetrates into the fence pattern of the rough surface, forming a solid-liquid composite contact surface different from the droplet wetted on the smooth surface.This complex solid-liquid wetting surface may also have an effect on the wettability of droplets, but in previous studies, the influence of nanodroplets on wettability under these two factors was rarely discussed in detail.

Many researchers have studied the influencing factors in the dynamic diffusion process, such as the nanoparticle volume fraction [6], the nanoparticle surface wettability [7] and the substrate roughness [8]. Vafaei et al. [7] investigated the equilibrium contact angle of nanodroplet containing nanoparticles and found that the contact angle is related to the mass concentration and size of the nanoparticle. Qiang et.al [9] proposed that the equilibrium contact angle of Ar nanofluid soars with the increase of nanoparticle energy coefficient. Li et al. [8] comparatively studied the smooth and rough substrates, and found that as the interaction energy between solid and liquid Ar atoms increases, the equilibrium contact angle of the nanodroplets also increases. In addition, the surface wettability of the substrate and nanoparticles also have a significant role on the force balance at the triple line. It has been researched that the wetting behavior of ND on a smooth surface is mainly determined by the viscosity of the liquid and the surface tension of the gas-liquid [10,11], and follows the scaling law, R ∝ t^1/n, where the index n is determined by the competition mechanism of the surface tension, viscosity and disjoining pressure of the liquid [[12], [13], [14]].

Tanner proposed the scaling law of nanodroplets spreading radius with time during the dynamic spreading process, that is, R ∝ t^1/7 [15]. Yuan and Zhao [16] pointed out that if the influence of the precursor film is ignored, the dynamic spreading process of ND is mainly dominated by the viscosity and disjoining pressure of ND, and the equilibrium contact angle of ND approximately is zero, R varies with the 5th power of T, R ∝ t^1/5. Li et al. [17] proved that when viscosity and surface tension dominate the dynamic spreading process of ND, R varies with the 7th power of T R ∝ t^1/7.

Previous research shows that nanoparticles can regulate vapor-liquid surface tension [18,19] and viscosity [20,21] of nanofluids. Lu et al. [22] investigated that the variation of the vapor-liquid surface tension of the ND is related to the nanoparticle surface wettability. For hydrophobic nanoparticles, they reduce the vapor-liquid surface tension of nanodroplets, while hydrophilic nanoparticles increase the vapor-liquid surface tension of nanodroplets, and the viscosity remains constant.

Although the different factors affecting the wettability of the ND have mentioned above, there is no unified conclusion as to whether the wettability behaviors of the ND on the rough substrate and the smooth substrate are completely consistent. For example, although the wettability is mainly controlled by the surface tension and viscosity of the liquid for smooth surface [17], but it is still not clear whether this wetting mechanism is also applicable to rough substrates. Moreover, whether the nanoparticles affect the wettability of droplets on rough substrates remains to be discussed. Therefore, the synergistic effect of various factors affecting the wettability of ND on rough substrates is still worthy of further discussion.

In this work, it has been discussed in detail how factors such as surface wettability of fence pattern substrate or nanoparticles and nanoparticle volume fraction regulate the ND spreading process, focusing on both contact angle in equilibrium and the contact radius of dynamic spreading process.

Section snippets

Simulation details

In this study, the simulation system includes a nanodroplet and a rough substrate with fence pattern. The nanodroplet contains randomly distributed nanoparticles, and the base liquid is water molecules. Fig. 1 shows the initial configuration of ND (φ = 0.05%) spreading on the fence pattern substrate. The size of simulation box is 20 nm × 20 nm × 18 nm. In Fig. 1, the purple is water molecules that contains oxygen and hydrogen atoms, the orange spheres in water molecules represent nanoparticles

Effects of fence pattern on substrate wettability

Keeping the same NVF of nanoparticles and the interaction parameters of water molecules and nanoparticles kept constant, (ε_N−O=0.013 eV, φ = 0.2%), the wettability of nanoparticles on rough and smooth substrate was studied. Fig. 3, Fig. 4 show the two-dimensional density contours of droplets on rough and smooth substrates with hydrophilic, neutral and hydrophobic substrates after 1 ns of relaxation equilibrium. The corresponding contact angle values are shown in Table 3. Among geometric bodies

Conclusions

In this work, the effect of the wettability of a ND on the dynamic spreading on a fence pattern rough substrate was studied using MD. The influence of substrate morphology, surface characteristics of nanoparticles, volume fraction of nanoparticles, gas-liquid surface tension and viscosity on the contact angle and spreading process are discussed. The main conclusions are as follows:

(1)
When the r and the f are the same, the contact states of the ND on the smooth substrate and the rough substrate are

CRediT authorship contribution statement

Zhao Wang: Her main work includes conceptualization, methodology, analysis, and writing original draft preparation.

Ling Li: She is responsible for the reviewing and editing of the paper.

Mo Yang: His contribution is mainly to discuss some issues in the article.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant No. 51476102 and No.51736007.

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Molecular dynamics simulation of the wetting characteristics of a nanofluid droplet on rough substrate

Highlights

Abstract

Introduction

Section snippets

Simulation details

Effects of fence pattern on substrate wettability

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

Microelectron. Eng.

International Journal of Heat & Mass Transfer

Chem. Phys. Lett.

Chem. Phys. Lett.

Advances in Colloid & Interface ence

Colloids & Surfaces A Physicochemical & Engineering Aspects

Adv. Colloid Interf. Sci.

J. Comput. Phys.

J. Comput. Phys.

Droplet evaporation study applied to DNA chip manufacturing

Langmuir : the ACS journal of surfaces and colloids

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Nat. Mater.

Influences of electroosmotic flows in nanopillar chips on DNA separation: experimental results and numerical simulations

Israel Journal of Chemistry

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Nat. Commun.

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A molecular view of Tanner’s law: molecular dynamics simulations of droplet spreading

J. Fluid Mech.

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Langmuir : the ACS journal of surfaces and colloids

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Langmuir : the ACS journal of surfaces and colloids