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Precise and smart nanoengineered surfaces: Impact resistance, icephobicity and dropwise condensation

Periodic Reporting for period 2 - NICEDROPS (Precise and smart nanoengineered surfaces: Impact resistance, icephobicity and dropwise condensation)

Reporting period: 2018-09-01 to 2020-02-29

Phase change phenomena of water such as icing and condensation are ubiquitous and plays vital role societally and technologically to various applications from aerospace and maritime industries for transportation to household and industrial heat exchangers for heating, ventilation and air conditioning (HVAC) systems or energy production and harvesting. Especially, condensed water droplets or films from vapour in the air and icing of the water droplets may impose significant decrease in energy efficiency or operational limitations. Here, in NICEDROPS, we are endeavouring to seek fundamental understanding on icing and dropwise condensation and to develop rational surface designs improving phase change heat transfer and energy efficiency. Thus, we have focused on studying metallic nanoengineered surfaces with a sustainable and scalable method, which are applicable to the industrial applications. We chose aluminium as the principal material considering its extensive usage on aircrafts and heat exchangers. Clearly the problem has strong social relevance and a potential to make deep positive impact.
The design of icephobic surfaces and anti-icing performance have gained attention in terms of delaying the formation and accumulation of ice on heat transfer surfaces as well as reducing the adhesion of ice layer on the surface. Rationally (nano)engineered surfaces retard the ice formation process more in comparison to the smooth and slippery surfaces. Additionally, by applying these structures on metallic surfaces we can also open up potential applications for flow condensation of steam. Here, we adopted electrochemical anodization on aluminium, to fabricate surface morphology favourable for delaying the freezing of water on the surface and reducing the ice adhesion.
It is inferred from the available literature that during droplet and solid particle (such as ice crystal) impact as well as in flow condensation phenomena, nanotextured surfaces may experience intense shear wave propagation. Therefore, emphasis is placed on the improvement of the impact resistance of the developed surface. Along the way we have also developed a new class of piezoelectrics materials which have broad ranging applications from self-powered sensing to smart surface designs.
We have conducted research by work plan and work packages that we proposed in the project proposal. This is a midterm report for the project (total duration: 5 years). The followings are the key achievements of the project in line with the proposed task.
In WP1, we proposed the design and fabrication of hierarchical anodised aluminium oxides (AAO) surfaces with the controlled porosity and nanoscale dimension to apply to WP2 (Nanomechanical Behaviour of Shear Thickening Suspensions) and WP3 (Phase Change Phenomena). We designed and built an experimental setup for anodisation of aluminium. Figure 1 describes the schematics of the anodisation setup. The polished surfaces are anodized in acidic electrolytes (0.3M H2C2O4 (oxalic acid) or 0.3M H2SO4 (sulfuric acid)) for achieving different surface nanotextures. In both the cases, aluminium acts as anode and platinum is used as cathode. After anodisation, we etched the aluminium surface to widen the nanopores as we desired to fabricate. The pore size of the anodized aluminium can be controlled by varying the etching duration time. The pore widening time is optimized to about 120 min and 15 min for oxalic acid and sulfuric acid respectively. Figure 2 shows atomic force microscopy (AFM) profiles of the fabricated surfaces with different morphologies such as sparse-fiber (sf), dense fiber (df), coarse-pore (cp) and fine pore (fp). We successfully fabricate hierarchical anodised aluminium surfaces having micron-scale fibres (~100 µm) and nanoscale pores (down to ~50 nm). First anodisation step fabricates irregularly grown aluminium oxide structures, however; after the removal of aluminium oxide by an acidic solution, the surface will have regularly dimple-like patterned bare aluminium surface. The second step anodisation will grow oxides based on this pattern, which will result in regular nanopores and oxide layer grown up to 100-101 µm. However, it is inevitable to utilise harsh chemicals such as acids for anodisation and solvents for cleaning. Thus, to reduce the usage of hazardous chemicals and simplify the anodisation process, we have attempted to replace the first anodisation step as other process which can produce the regular pattern for the anodisation. To better control the porosity, we have developed a block copolymer (BCP) templated aluminium anodisation process as shown in the Figure 3. BCP templated anodisation process not only provides well arranged nanopores but also eliminates two-step anodisation process. We have focused on well-arrayed BCP patterning process with PS-b-P4VP block copolymer, resulted in ~ 50 nm patterns which corresponds the nanopores from two step anodisation process.
WP1 also suggested to develop conformal polymer coatings to functionalise the nanoporous AAO surfaces as improving the surface impact resistance by cushioning the shear waves upon jet/droplet impacts. These polymer coatings are also included in WP2 for smart surface design to delay freezing and intercalate with shear thickening suspensions to tune yield stress and nanomechanical behaviour of the suspension. These polymers (PVDF (poly(vinylidene uoride)) and P(VDF-TrFE) (poly(vinylidene uoride-trifluoroethylene)) for coatings are known to be piezoelectric generating electricity by mechanical stresses. From the beginning of this project term, we developed two composites namely molybdenum sulphide (MoS2) nanoflower and copper nanowire (CuNW) doped PVDF (shown in Fig. 4), which showed spontaneous piezoelectric β phase crystallization in the polymer. The packaged nanocomposites exhibit significant piezo-voltage (up to 70 V) under gentle human finger tapping force and were able to light up LEDs (shown in Fig. 5). Interestingly, they also showed piezocatalytic behaviour by degrading dyes in water under ultrasound vibrations as well, shown in Figure 6.
As proposed in WP2, we developed the fabrication method for oil-based, discontinuous shear thickening suspensions into anodised pores as a way to develop smart, impact resistant surfaces. We used silica based porous nanoparticles and activated hydroxyl group before being functionalised by APTES ((3-Aminopropyl)triethoxy-silane), C9H23NO3Si). The mesoporous nanoparticles have approximately nanopores with 3.5 nm of diameter in average. The nanoparticles will be tested for shear thickening behaviour along with other nanoparticles.
We designed experimental protocol and tested for freezing delay and anti-icing for the fabricated surfaces (refer to Figure 2) in WP1 as proposed in WP3. First, to characterise the surface wettability before anti-icing experiments, advancing as well as receding contact angle are measured. Furthermore, to assess the impalement resistance and dynamic stability of the developed surfaces, drop impact tests (maximum impact velocity achieved 3 m/s, We=300) are also conducted. The droplet impalements were observed when We>80 for most surfaces except the surface with sparse fibres. We built a custom-made icing chamber to conduct experiments for anti-icing characteristics, enabling us to cool droplets and surroundings altogether. We carried out freezing delay measurements on different surfaces resulted in that the nanoengineered surfaces exhibited relatively lower nucleation temperatures compared to untreated aluminium surfaces as shown in Figure 9. However, the polished surface (s) shows the lowest nucleation temperature among all surfaces as the nucleation rate is strongly dependent on the interplay between contact angle and roughness. We also tested ice adhesion on the surfaces in the same testing chamber by measuring the loads to dislodge the same size of the ice cube. We observed nanoengineered surfaces with hierarchical structure reduced ice adhesion approximately 50% compared to the original aluminium surfaces as shown in Fig. 10.
As mentioned in the summary above, piezoelectric and piezocatalytic effects of self-poling behaviour of PVDF nanocomposite showed remarkable results and a pioneering work to be applied not only in the proposed work here but also further in micro-nanoscale energy harvesting.
Until the end of the project, we will focus on achieving milestones proposed in each work package. For WP1, we expect BCP templated AAO surface can allow us to fabricate well-arrayed nanoporous surfaces having an average pore diameter close to 10 nm. Such controlled nanoengineered surfaces will further enable us to continue to explore nanomechanical behaviour of shear thickening suspension (STS) driven by a surface shear from droplet movements or jet impinging. We expect to fabricate and test the oil-based discontinuous shear thickening suspensions (DSTS), which will be one of other major state-of-the-art results produced from this project, because it could potentially have dramatic influence on impact and vapour shear resistance while also maintaining nucleation control capability. Once surface with oil-based DSTS are fabricated, rheological measurements and AFM based nanomechanical characteristics will be studied as proposed in WP2.
With all the surfaces and coating developed in WP1 and WP2, to better understand and test the condensation and anti-icing performance, we will design and build two environmental chambers to control the ambient temperature and humidity. Figure 11 shows the environmental testing chamber already in production. The chamber has ability to remove the presence of non-condensable gases such as nitrogen and oxygen and to control the surface temperature on a liquid-pass cold plate enabling condensation or freezing experiments to fundamentally understand heat transfer enhancement and anti-icing performance in pure saturated vapour environment on the fabricated surface. The surfaces with ~10 nm pores can dramatically change nucleation behaviour of ice and droplet as their size is similar with the critical size of ice and condensed vapour nucleus. Also, we expect the exploitation of ultra-high speed camera (~5 Mfps) and laser induced forward transfer (LIFT) technique can help us to better understand droplet and jet behaviour on the nanoengineered surface by capturing instant moments and very fast (10-200 m/s) droplet/ice crystal impact simulating aerospace applications.