Bio-coated surfaces with micro-roughness and micro-porosity: Next generation coatings for enhanced energy efficiency

: Due to growing cooling demands as well as emerging global warming and climate change issues, cooling systems should be more efficiently utilized. Boiling is an effective heat transfer mechanism, which has a critical role in many cooling systems. Surface modification is considered as the major approach for boiling heat transfer enhancement. In this study, we developed a microbial bio-coating surface modification technique for phase change cooling applications. Thermoacidophilic Sulfolobus solfataricus coating was implemented using a facile dip coating method on different metallic and non-metallic surfaces. Controlled by drying conditions, the coating exhibited rough and porous morphologies. When tested in a boiling heat transfer setup, bio-coated surfaces offered enhancements up to 76.3% in Critical Heat Flux (CHF). Next, a


Introduction
Global warming, as a direct consequence of the increase in greenhouse gas emission, has remained the main worldwide concern during the last decades. Heat loss is the predominant form of the energy lost in energy systems/processes. One of the major sources of energy losses is linked with the constant utilization of air conditioning, refrigeration, and heat pipe systems. According to the International Energy Agency (IEA), the cooling demands in the air conditioning market is predicted to be tripled by 2050 [1]. Thus, many attempts have been made to enhance the energy efficiency of these systems. Due to its ability to remove large quantities of heat, boiling phenomenon is used as a cooling method in air conditioning and refrigeration systems [2], [3].
Despite its superiorities compared to the conventional methods, boiling has a limit called critical heat flux (CHF), where the vapor columns create a vapor blanket film on the surface, which drastically decreases the heat transfer rate from the surface. Figure 1 shows the typical boiling regimes. At first, natural convection corresponding to single phase flow happens. The boiling process starts with the inception of bubbles from the nucleation sites on the heated surface (cavities and surface impurities). Here, the required temperature for the onset of nucleate boiling (ONB) depends on the surface morphology. As the surface temperature increases, nucleate boiling regime starts, a region characterized by more activated nucleation sites and discrete departed bubbles.
Further surface temperature increment results in more generated bubbles leaving the surface in the forms of vapor columns and larger bubbles. The end of the nucleate boiling region is described by excessive generated vapor columns and blankets, which block the coolant path to the surface, thereby leading to boiling crisis or critical heat flux (CHF). As indicated in Figure 1, boiling heat transfer enhancement can be achieved by i) reducing ONB temperature, ii) improving heat transfer coefficient (HTC), and iii) increasing CHF [4]. Many surface modification techniques [5], [6] have been proposed and investigated to achieve these goals. These techniques include implementation of nanowire arrays [7], nanorods [8], graphene coating [9], polymer coating [10], [11], virus coating [12], biphilic surfaces [13], porous surfaces [14], laser-engineered surfaces [15], and plasma spray coating [16]. Based on the application and the range of the applied heat fluxes, these three aspects should be considered in the design of thermal-fluids systems to maximize the heat removal from the surface in nucleate boiling and film boiling regimes.
As an example for surface modification, Ma et al. [17] investigated surface area enhancement in flow boiling via micro-pin-fins. The surface revealed the effect of fin height, fluid velocity and liquid sub-cooling on heat transfer. Sadaghiani et al. [18] explored the effect of copper based micro-and nano-structured surfaces on flow boiling. Alam et al. [19] studied the effect of surface roughness on flow boiling. These studies showed that the roughness elements about the size of 1 and 1.6 μm considerably affected the nucleation site density and boiling inception.
Roughness elements can majorly be introduced via mechanical modification such as sanding, etching, and deposition [6]. In the study conducted by Jung et al. [20], the effects of treated surfaces by anodization in two acids as the electrolyte (deposition) and with submicron-roughness on subcooled flow boiling of FC-72 were investigated at ambient pressure. Their observations marked an increase in HTC and CHF. Sisman et al. [21] applied roughness with the size of 1 to 4µm on aluminum sheets with mechanical sanding technique and reported that smaller roughness could create more active nucleation sites at the same heat fluxes.
In addition to roughness, tuning the porosity is one of the widely used modification techniques in boiling heat transfer. Porous foams generate a high surface area-to-volume ratio.
Rainey et al. [22] investigated the effect of micro-porosity using the ABM coating method (Aluminum particles, Devcon Brushable Ceramic epoxy, and Methyl-Ethyl-Keytone) on subcooled flow boiling of FC-72. Compared to non-coated surfaces, boiling inception took place at lower wall superheats and consequently increased heat transfer coefficients. However, the development of microporous coating using Al2O3 particles with size of less than 10μm resulted in 25% CHF enhancement [23]. Further investigation on R-134a flow boiling in tubes with a metallic porous surface exhibited increased heat transfer coefficient (up to 200%), as the cell size of the metal foam was changed from 20 PPI (pore number per inch) to 40 PPI [24]. In the study conducted by Pranoto and Leong [25], high thermal conductivity graphite foams with average pore diameter of 350 and 650 µm were used in flow boiling experiments. As a result, more nucleation sites on the porous surface could be observed, which raised the heat transfer coefficient by 2.5 times.
As can be seen from the abovementioned studies, the available enhancement methods require expensive infrastructure, have a complex process flow for their fabrication/preparation, and most importantly, cause environmental issues due to released hazardous materials as by-products.
On the other hand, according to the latest market report published by fortune business insights, the global paints and coatings market size in 2019 was 154.6 billion USD and is expected to reach an estimated value of 249.7 billion USD in 2027 [26]. This huge budget is mostly based on the demand for new generation coatings and should be wisely managed in order to meet global energy needs.
To address these issues, we used Sulfolobus solfataricus in this study to modify the surfaces by providing organic porous as well as roughened surfaces. Previously, this technology was formulated by our team [27]. Here, we propose the next generation bio-coating, which is highly durable, environmentally friendly, and cheap and has a practical coating method, for the implementation to thermal-fluids systems. This study not only offers new types of coating morphologies, but also the applicability of the proposed bio-coating is proven with its implementation to a miniature air conditioning system.

Sulfolobus solfataricus culture
Archaeal cells have a unicellular structure with circular chromosome. They resemble to eukaryotic cells due to their metabolisms involving DNA replication and transcription. However, they remarkably exhibit the same structure and morphology and other macromolecule metabolisms as of prokaryotes [28]. Nevertheless, archaeal domains harbor distinctive complex traits at molecular level that enable them to endure extreme physiological conditions such as low or high temperatures (−2 °C to 15 °C or 60 °C to 122 °C), high salinity (2 M to 5 M NaCl), and low or high pH (<4 or >9) [29], [30], [31]. Sulfolobus solfataricus is a hyperthermophilic sulfur metabolizing archaeon, which belongs to Crenarchaeota phylum. This species grows best at temperatures around 80° C and highly acidic pH (2)(3)(4). Sulfolobus solfataricus essentially metabolizes sulfur as a main cellular metabolism. Sulfolobus solfataricus cells generally have a spherical shape processing flagellums, which generate frequent lobes [32]. Highly thermostable properties of this species make it a strong candidate for biotechnological applications regarding heat transfer enhancement.
The initial batch of Sulfolobus solfataricus was purchased from American Type Culture Collection (ATCC® 35092™), and the growth medium was prepared according to the ATCC® Medium 1304 recipe. The medium composition is provided in Table S1. Since Sulfolobus solfataricus prefers anaerobic growth, the initial cultivation rate was relatively slow during the preparation. In order to reach maximum growth rate, the culture volume was stepwise increased.
Conical flask containing 30mL of pre-warmed medium was first inoculated using aliquot of the cell vial. Then, the cell growth was monitored using UV Spectrophotometer at OD600 for 72 -96h.
The culture volume was gradually increased up to 100 mL, 250 mL, and 500 mL by adding prewarmed medium. To generate the mass production of the archaeon in a sustainable manner, the growth medium was supplied with 2 g/L D-sucrose, and the cells were grown at constant temperature and PH of 84°C and 3.9, respectively.
When the OD600 of culture reached to 1, 250 ml of the culture was harvested after cooling down on ice and further harvested using centrifugation at 4500 ×g for 15 min. The remaining culture was added with the fresh medium up to 500 ml and was kept for further incubation. Then, the supernatant was discarded, and the pellet was stored at −80 °C until use.
In order to prepare bio-coated surfaces, the pellet was washed once with 250ml of phosphate buffer (20% (v/v) in H20) at room temperature and then re-suspended with 250ml of fresh phosphate buffer. Finally, Poly-L-lysine (0.1 % (w/v) in H2O) was added into the suspension with a ratio of 1:5 to increase the adhesion of the biomaterial to the desired surface. Then, the coating mixture was applied on the silicon wafer with thickness of 500µm and interior side of an evaporator made of aluminum. Subsequently, the coated samples were incubated at room temperature as well as at 60°C to initiate the evaporation process. Figure S1 summarizes the culture, coating and drying processes. More information about the coating process of the aluminum tube is provided in the Supporting Information ( Figure S2).

Surface characterization
Once coated surfaces were completely dried, they were processed for the surface characterization. To avoid disparity, which might generate as a result of surface charge formation of biomaterial, and to develop homogeneous surface for high resolution image processing, a thin layer (3-4 nm) of gold-palladium alloy was employed using SEM sputter coater. A scanning electron microscope (SEM Leo Supra 35VP, Oberkochen, Germany) was used to capture the microscale images of the coated samples. The Secondary Electrons (SE) mode was used for the imaging due to its high spatial resolution. The images were taken using the SE mode at low voltage (3KV and 10KV) in 0 and 45 degree titled visions to obtain both full images of the surface area and the cross sectional area. The wettability of the bio-coated surfaces was measured using the water contact angle (WCA) method [33]. A 5μL of the water drop was applied on the five various spots of each sample, and their relevant average contact angle was determined.  In addition to silicon and glass surfaces, the bio-coatings were prepared and characterized on metallic surfaces including copper, aluminum and stainless-steel ( Figure S3). Similar to silicon and glass substrates, metallic surfaces also have rough and porous morphologies under dip coating conditions.

New surface morphology for phase change systems
The bio-coating surface is inspired by microbial biofilm formation. Generally, the biofilm formation is considered as a significant survival mechanism in microbial life, which protects the microorganism from austere conditions such as nutrient scarcity or unfavorable temperature.

Similar to bacterial cells, archaeal cells form biofilm, where cells initiate the self-immobilization
and build up the stacked layers to adapt to the temporary mutual lifestyle. In the biofilm process, cells inevitably produce multilayer microbial community, which leads to irreversible attachment, subsequent development of micro colonies and a further process for the biofilm maturation. In response to various stress states, death signals induce autolysis of some microbes to provide nutrition for colony survival known as "die for community" [34].
The proposed bio-coating process employs Sulfolobus Solfataricus to form a biofilm on the surface using poly-l-lysine to act as an extracellular polymeric substance (EPS). The structure of the biocoated layer developed under the room temperature displays roughness mainly due to smooth autolysis of the surface microbial biofilm. However, the porous shape structures form due to the rapid release of the gasses such as H2S, CO2 and SO2, which consequently generate a distinct morphological structure on the biofilm ( Figure 3). These porous structures identified as micro and nano structures reflect the trace of the released gasses and demolished cells. In addition, the rapid evaporation withdraws more dissolved gases to the bio-layer/air interface and aligns the microorganisms in dispersed fashion and thus establishes the porous structure. On the other hand, evaporation at room temperature corresponding to the steady state form of evaporation allows micro-organisms to slowly descend on the substrate and forms the roughened coating.
Most of the Sulfolobus species display aerobic growth. However, Sulfolobus Solfataricus has a higher growth rate at low oxygen concentrations [35]. Aerobic microorganisms grow in the presence of the oxygen when the O is the final electron acceptor and water and CO2 are the final products. Additionally, Sulfolobus species are known to be chemosynthesis microorganisms due to their ability in oxidizing sulfur to metabolize various carbon sources and consequently direct the energy for carbon fixation. Our carbon source to grow Sulfolobus Solfataricus is D-sucrose.
Sucrose is a disaccharide consisting of glucose and fructose. Aerobic metabolism generates CO2 as a result of the glucose oxidation. Basically, they gain energy from growing on sulfur or various organic compounds. Sulfur metabolism pathways in archaea are part of the food chain survival strategy, when there is a lack of energy source, sunlight, nearby hydrothermal vents, with abundant hydrogen sulfide and carbon dioxide. The SO4 2is the prerequisite in sulfur metabolism pathway of Sulfolobus Solfataricus, which produces H2S gas through three sequential reactions [36] as indicated in Figure 3. Moreover, these microorganisms could metabolize organic sulfur and emit H2S under aerobic conditions [37]. It was reported that in some studies the rate of oxidation is determined by the biogeochemical characteristics of the sediment in sulfur-oxidizing microorganisms [38], [39]. According to these studies, the rate of the sulfur oxidation in such microbes is more on the sediment surface, where enormous amount of the oxidants is available as a result of high rate sulfur reduction [40]. Most of the sulfide oxidizing microorganisms grown in lab cultures are mostly derived from surface sediments [41].
Temperature discrepancy between two samples is the main reason to generate various morphologies defined as roughness and porous coatings. This temperature difference has two main effects on the evaporation rate and resulting cell sedimentation: i) Surface evaporation occurs as a result of vapor transport at the bio-film/air interface. The vapor transport is highly dependent on the diffusion coefficient of the dissolved gas, where the diffusion coefficient exponentially increases with the temperature [42]. For example, the diffusion coefficient of air is almost five times higher at 60℃ compared to the room temperature. ii) The second reason is derived from the differences in solubility of the generated gases within the biomaterial at different temperatures. At higher temperatures, the solubility of the generated gases such as H2S, SO2, CO2 and air decreases exponentially and results in major difference in gas release from the bio-film (Table 1). This rapid release of the gasses at 60℃ contributes to the alteration in sedimentation profile and also the biofilm rupture.  Figure 3. Generation of CO2, H2S and SO2 as a result of Sulfolobus sulfactorius biological metabolism [36]. Figure depicting evaporation of bio-layer under two different conditions (rapid and smooth). The rapid evaporation results in formation of the porous structure, whereas smooth evaporation causes roughened bio-film on the substrate. The schematics represent two various biocoated morphological surfaces and their function once boiling occurs. The experiments on non-coated and bio-coated surfaces were performed under the atmospheric pressure at the mass flux of ~100 kg/m 2 s (corresponds to average mass flux of miniature airconditioning systems [44]). The wall superheats (ΔTsat)defined as the temperature difference between the surface (Ts) and saturation temperature (Tsat)were obtained for the applied heat inputs. The comparisons of the heat transfer performance between non-coated and coated surfaces are presented in Figure 4. It should be noted that each test was repeated for three times to ensure the repeatability of the experiments. The local heat transfer coefficient is calculated as:

Testing and analysis method
In equation 1, is the net heat input (W), ∆ is the wall superheat (K), and is the surface area (m 2 ). Four temperature measurements were used to calculate the average temperature of the surface in the aluminum block (at 4 mm beneath the substrates). The average of the measurements was used as the surface average temperature.
The net heat flux, heat loss, and surface temperatures are calculated using equations (3) to (5): where I, V, and A are the applied current (A), voltage(V), and the heated area (m 2 ), respectively.
Heat loss is the difference between input power and the amount of heating needed at a specified mass flow rate at a constant ∆ . The percentage of the heat loss in the boiling setup was calculated less than 8%. LAl is the vertical length between surface temperature measurement location and substrate, and LSilicon is thickness of the silicon wafer, which is 500 µm. K is the thermal conductivity, and = 6 × 10 −6 2 is the thermal resistance of the thermal paste used to reduce the thermal resistance between two surfaces.   Figure 5a). This is due to presence of abundant nucleation sites on the rough-coated surfaces. However, on the non-coated surfaces, many slugs (large vapor bubbles) can be observed.

Heat transfer performance
These slugs prevent the surface rewetting path and results in a high surface temperature beneath the large vapor bubbles. On the other hand, on the rough-coated surface, these tiny bubbles move towards the surface, thereby rewetting the surface and increasing the cooling efficiency of the surface (marked in red in Figure 5a) [45].
On the porous-coated surfaces, as a result of the presence of many interconnected channels (as shown in SEM images in Figure 2c), transportation of coolant within the bio-coating takes place during the boiling process, especially at higher heat fluxes. These interconnected channels act as liquid supply paths when large vapor blankets form at high temperatures. As a result of this effect, the flow morphology (distribution of vapor phase inside the two-phase flow) changes dramatically.
As can be seen in Figure 5b, compared to the non-coated surface, the porous-coated surface houses a more distributed vapor phase, which is more evident at high heat fluxes (q≥40 W/cm 2 ).

System performance analysis
Based on the obtained promising results from the boiling experiments, a miniature air conditioning system was assembled to examine the performance of the bio-coated evaporators.
First and second laws of thermodynamic are used to analyze the performance of the system. In the energy analysis, when the first law of thermodynamics is considered, all types of energy are taken into account and the quality of energy types is neglected. For instance, it is not possible to show the change in the quality of thermal energy from the hot ambient air (heat source) to the evaporator surface (cold source). On the other hand, the exergy analysis involving the second law of thermodynamics leads to a better understanding about the energy transfer process. Basically, the exergy of the system represents the maximum amount of the work when the system comes back to its equilibrium with the environment. This is especially important for cooling applications, where the system will have a waiting period for the next cooling cycle.  According to the obtained results, it can be concluded that the proposed coating has the potential to be used in different applications. Rough-coated refrigerator shows higher enhancement at low wall superheats (ΔT<2.5K) compared to porous-coating, which agrees with the obtained results from fundamental studies. This kind of coating is suitable for small-scale refrigerators with low cooling capacities. On the other hand, porous coating offering a superior COP enhancement at medium and high wall superheats is suitable for larger refrigerators with high cooling capacities.

Exergy analysis, second law
In this section, the second law analysis is presented to give more insight into the energy recovery (exergy availability) for non-coated and coated evaporators. The following expression can be written based on the general exergy balance [46]: where ̇ and ̇ are the total exergy (transferred by heat, work and mass) at the inlet and outlet, respectively, and ̇ is the rate of exergy destruction. For the ambient temperature of 0 and assuming that the change in kinetic and potential energies is negligible, the exergy is given as: The component-wise analysis for the air conditioning system including evaporator, condenser, preheater and pump are as follows:  Pump:  Preheater:  Total: Exergy destruction permits the evaluation of exergy destroyed per component. The quality of the vapor was obtained using a transparent channel at the outlet of the evaporator, and the images was post-processed using ImageJ software. FC-72, with given properties in Table 2, was used as the working fluid in the closed-loop system. The first law analysis highlights that the rough-and porous-coated evaporators work more efficiently at low and high temperatures, respectively. Thus, the exergy analysis was performed on rough-and porous-coated surface at wall superheats of 0.5 K and 5 K, respectively. Vapor qualities, cooling powers, COPs and evaporator exergy destruction rates are tabulated in Table 3.  The larger exergy losses between evaporator inlet and outlet in the non-coated evaporator are due to i) higher wall superheats required for the onset of nucleate boiling, ii) non-uniform surface temperature due to lower number of active nucleation sites, iii) larger frictional pressure drop due to the presence of larger vapor bubbles [47], [48], [49]. Generated larger bubbles ( Figure 5) on the non-coated evaporator results in larger bubble slip velocity and higher deformability, which all eventually increase the frictional pressure drop of the boiling flow inside the 2-m long evaporator [50].
By improving the phase change process, the coated specimens generate more vapors inside the evaporator. Therefore, the inlet vapor quality of the condenser is higher for the coated evaporator compared to the non-coated one. As a result, the exergy destruction rate for the condenser is higher for the cycles that utilize enhanced evaporators. Figure 7. a,b) Thermodynamic cycle of two phase air conditioning system with rough-and porous-and non-coated evaporators. Here, the rough-coated evaporator is tested and compared to the non-coated evaporator at wall superheat of 0.5 K and porous-coated evaporator was tested and compared to the non-coated evaporator at wall superheat of 5 K. c,d) Percentage of exergy destruction in different components of the lab-made air conditioning system with the mentioned evaporators Air conditioning systems -most important energy systems -are one of the largest energy consumers in residential and commercial buildings. In the United States alone 400 terawatthours of electricity are consumed annually for air conditioning purposes [51]. Setting higher efficiency standards for cooling devices is one of the approaches, which can dramatically reduce the need for new power plants, cut gas emissions, and reduce the costs. Our proposed bio-coating to be implemented to air conditioning systems will not only significantly reduce the green-house gas emission but it will also assist in saving billions of dollars.

Declaration of Interests
The authors declare no competing interests.

Coating process of the aluminum tube
A lab-made long oven was prepared to dry the coated long tube while rotating. The drying system is shown in Figure S2. The air was blown by air compressor to the heat chamber and the temperature of the air was increased to 300℃. The heating part has a resistance in a lamp, which acquires power from the power supply. It passes into five paths to dry the pipe in five regions of the oven. The pipe has 6 ball bearings to keep the pipe straight during the rotation. The inner volume of this 2 meter long tube is about 60 ml and the archaea's mixture volume injected to the tube is about 40 ml to allow the fluid to evaporate better during the rotation. After being sure that the inside of the tube was coated and it was dry, the 2 meter long tube was bent by the Swagelok tube bender to make the evaporator.

Boiling test section
The schematic of the boiling setup is demonstrated in Figure S4. Two cartridge heaters were used for applying heat from the bottom of the channel. A digital power supply was connected to cartridge heaters, and current and voltage were measured at every heat flux. The test section was made of aluminum. To minimize the heat losses, Teflon was used surrounding the aluminum block.
A plexiglass was utilized on top of the channel, which allow the flow visualization from the top.
Two holder plates were used on top and bottom of the setup to sandwich the Plexiglass on top of plastic O-rings. A commercial camera, Canon EOS 70D, was used to visualize boiling during heating. Four horizontal holes were formed in the aluminum block under the surface of the minichannel to measure the surface temperatures using T-type thermocouples and UNI-T UT325 Contact thermometers. The bio-coated silicon was located on top of the aluminum block. Deepcool Z3 paste, a superconductive thermal paste, was used for attaching the silicon surface on the aluminum block. A micro gear pump was used to pump the fluid through the system. Deionized water was used as the working fluid. The working fluid was degassed using a vacuum tank to remove dissolved gas from the water by lowering the pressure inside the tank and was then introduced into the preheater section before entering the boiling test section. The preheater was an immersed heat exchanger in 100℃ silicon oil whose temperature was controlled with a PID controller.

Figure S4. Open loop boiling test section
Lab-made close-loop air condition system Figure S5 depicts the experimental setup of the miniature air conditioning system, which was assembled to measure the cooling performance of the bio-coated aluminum tube. It consists of a condenser, a chiller, a pump, a preheater, a reservoir, valves, and the test section. The temperatures and pressures at different locations were measured using UNI-T UT325 thermometers connected to T-type thermocouples and pressure gauge (Omega). A preheater is placed before the test section to keep the inlet temperature of the coolant at 56°C. Two thermocouples were installed for temperature measurement, one prior to entering the evaporator and the other one at the exit of the evaporator. The test section was a constant temperature chamber, which includes an electrical heater, four thermocouples for air temperature measurements, two thermocouples for coolant temperature measurements, a fan, a spongy pad for heat dissipation and the evaporator. The chamber walls were thermally isolated via 5 mm thick ceramic paper to reduce the thermal losses.
The consumed work by the system is the sum of pumping power, preheater power and the chiller power. The pumping power kept constant at 10.2 W for pumping the fluid at flow rate of 1.66 ml/s and mass flux of 100 kg/m 2 s. The preheater connected to a power supply consumed 110W to increase the fluid's temperature from 23℃ to 56℃. The condenser connected to a chiller use 220W to reduce the temperature of the heated fluid to 23℃. Figure S5. Experimental setup of the lab-made miniature air conditioning system

Uncertainty analysis
Uncertainty analysis was based on the error propagation of experimental data using the methodology proposed by Coleman et al. [1]. The general formulation is expressed as: is the uncertainty in parameter xi. The parameters in our experiments with the corresponding uncertainties are listed in Table S2.