# Magnetic Nanofluid Droplet Impact on an AAO Surface with a Magnetic Field
## 1 Introduction
[20] used photolithography to fabricate a hydrophobic microgrooved surface. It was reported that the phenomenon of droplet bouncing with breakup is dependent on the pitch of the grooved surface and the Weber number. [PERSON] et al. [21] showed that convex superhydrophobic surfaces could result in a 28.5% reduction in droplet-surface contact time compared with flat superhydrophobic surfaces. In fact, the effect of nanostructure on the droplet impact on the surface is also investigated in some studies [22]. [PERSON] et al. [23] introduced water droplet impact dynamics onto micro- and nanostructured superhydrophobic surfaces. The results indicated that the nanoscale on multiscale surface roughness plays an important role in the impact process for higher Weber numbers (\(\mathrm{We}\geq 120\)). [PERSON] et al. [24] investigated the dynamic behavior of microliter droplets impacting on hydrophobic surfaces with hierarchical roughness. The number of droplet bouncing events was exponentially increased by increasing the hydrophobicity of the surface, which was coated with gold nanoparticles. [PERSON] et al. [3] fabricated a micro/nanostructure on aluminum alloy surfaces. The experiment displayed an excellent anti-icing property and droplet rebounding activity. Recently, a suspension of nanoparticles in a base fluid, the so-called nanofluid, has been used in a number of studies due to the unique thermal-fluid properties [25], like spray cooling [26; 27], solar collector [28], and engine oil [29]. In droplet impingement studies, [PERSON] and [PERSON] [30] reported that the spreading characteristics of TiO\({}_{2}\)/ethylene glycol nanofluid droplet impinged on aluminum surfaces. It was seen that the spreading diameter increases as the volume fraction of nanoparticles also increases. [PERSON] et al. [31] experimentally observed the dynamic and heat transfer performances of TiO\({}_{2}\)/water nanofluid droplet impact on a silicon surface with no coating, a superhydrophobic coating, and a superhydrophobic coating. It has shown that the cooling effectiveness could be enhanced by dispersing TiO\({}_{2}\) nanoparticles in water droplets. However, the superhydrophobicity results in a reduction in cooling energy due to the smaller wetted area and the shorter contact time. Compared to conventional nanofluids, magnetic nanofluids possess magnetically controllable properties under magnetic fields [32; 33; 34]. In order to accurately control the magnetic nanofluids in fluidic devices and energy systems, it is very important to understand the static and dynamic magnetowetting [35]. The shape and contact angle of a magnetic nanofluid sessile droplet in magnetic fields were investigated experimentally and numerically in some recent studies. [PERSON] et al. [36] observed the magnetowetting of a sessile magnetic nanofluid droplet on a planar surface. The experimental results showed that the shape of the sessile droplet is flattened and the contact angle is reduced as the magnetic flux increases. [PERSON] and [PERSON] [37] further investigated the magnetowetting of an oil-based magnetic nanofluid and a water-based magnetic paint on a hydrophobic flat surface. The elongated shape of a magnetic nanofluid droplet and the flattened shape of a magnetic paint droplet were observed under an external magnetic field. In the experiment, it was observed that as the magnetic field increases from 0 to 20 mT, the magnetic nanofluid droplet contact angle decreases first and then increases; however, as the magnetic field increases from 100 to 300 mT, the magnetic paint droplet contact angle increases first and then decreases. [PERSON] et al. [38] investigated the effects of a magnetic field and nanoparticle concentration on the magnetowetting of water-based magnetic nanofluids. They found that the shape of a magnetic nanofluid droplet can be changed by applying a magnetic field gradient. The above magnetic field effect can be further enhanced by increasing the nanoparticle concentration.