Electrically Driven Interfacial Evaporation for High-Efficiency Steam Generation and Sterilization.

Electrically driven steam generation is a critical process for many heating-related applications such as sterilization and food processing. Current systems, which rely on heating up the bulk water to generate steam, face the dilemma in achieving a large evaporation flux and fast thermal response. Herein, we report a self-floating electrically driven interfacial evaporator for fast high-efficiency steam generation independent of the amount of loaded bulk water in the system. Through localized heating of the wicked water at the air-water interface, the evaporator has achieved an electrical-to-steam energy conversion efficiency of ∼90% at a heating power density of 10 kW/m2 and a fast thermal response of 20 s. The interfacial evaporation design not only achieves a high evaporation efficiency within a broad range of heating power densities by using different wicking materials, but also enables attaining a high evaporation temperature under low heating power densities by tuning the ratio of the vapor outlet area and the evaporation surface area. By integrating an interfacial evaporator within a sanitizer, the resultant system has demonstrated a faster steam temperature rise and superior steam sterilization performance than the commercial bulk heating-based approach.

Introduction

Steam has played a vital role in advancing technology development of human society since the invention of steam engine in the industrial revolution.[1] Even now, steam is widely used to propel gas turbines to generate electricity in power plants.[2] In daily human life, steam is most often known for its household usage in cooking,[3] sterilization,[4] humidification,[5] cleaning,[6] drying,[7] and many other heating-related applications.[8] For example, a sterilizer that uses hot steam to decontaminate baby bottles and toys has become an indispensable appliance for almost every family with children. In a typical sterilization process, a pot of water needs to be intensively heated by an electric heating element until reaching the boiling point to generate the steam. It often takes quite a while before the system can generate hot steam and the thermal response of the steam generation system is strongly dependent on the amount of loaded water. The bulk water heating also causes inevitable heat losses and thereby lowers the energy conversion efficiency of the system. Therefore, bulk heating-based evaporation systems are not well suited for steam sterilization applications, where instant generation of large-flux hot steam is desired.[9,10]

In recent years, interfacial evaporation that relies on localized heating at the air–water interface has emerged as a superior way for steam generation.[11−14] The localized heating design suppressed the heat losses and thereby helped improving energy conversion efficiency without complex thermal insulation engineering, and the reduced thermal mass helped achieve a fast thermal response of the evaporation system.[11] Previous research efforts were focused on solar steam generation,[15−21] which utilizes solar-absorbing materials to convert sunlight into heat to drive water evaporation. Although great progress in improving evaporation performance[22−28] and exploration of steam sterilization applications[29,30] have been made, generation of hot steam often requires concentrated solar fluxes, which involves the usage of expensive optical concentrators. While a new system design has enabled steam generation under one-sun illumination,[31−33] the achieved solar-to-steam conversion efficiency is much lower. Additionally, the intermittent, diffuse, and weather-dependent nature of sunlight prevents the application of the solar steam generator for indoor sterilization at night or during cloudy days.[34−36] By comparison, the steam generator powered by electricity, which can be converted through photovoltaic technology from the Sun or other renewable energy sources, would be a more practical solution.[37−39]

Herein, we report an electrically driven interfacial evaporation system for high-efficiency steam generation and household portable sterilization applications. As shown in Figure , the evaporation structure consists of a water-wicking membrane on the top, an electrical heater in the middle, and a self-floating thermal insulator at the bottom. The floating insulator suppresses the downward heat losses and helps localizing the Joule heating to the wicked water. The localized interfacial electrical-heating (LIEH) evaporation system has shown a fast response time for steam generation within 20 s and achieved a high electrical-to-steam energy conversion efficiency of ∼90% at a heating power density of 10 kW/m2. We further demonstrate that the interfacial evaporation structure allows for the facile tuning of the evaporation efficiency and vapor temperature, and enables high-efficiency steam generation under a broad range of heating power densities. The efficient and fast-response LIEH-based steam generator can be readily integrated within a commercial sanitizer for rapid and effective sterilization under low-heating power densities.

Figure 1

Electrically driven interfacial evaporation system for high-efficiency steam generation and portable steam sterilization. Schematic structure of the evaporator consisting of a water-wicking membrane, a polyimide-sealed electrical heater, and a silicone foam-based floating thermal insulator.

Results and Discussion

A hydrophilic air-laid paper was used as the water-wicking membrane and placed on top of the electrical heater to pump water from beneath for continuous evaporation (Figure a). Its porous structure allows the generated vapor to permeate from the top surface. Compared with normal tissues or wet-laid paper, air-laid paper is flexible and durable,[40,41] and thus is used as the wicking material. To characterize the water-wicking performance, air-laid paper was placed in direct contact with a red-colored dye solution. As shown in Figure b, air-laid paper was quickly wetted by the dye solution with a sorption rate of 0.11 kg/h, and fully wetted paper could store 0.482 kg/m2 of water. Such a quick water sorption rate ensures sufficient water supply during the evaporation process.

Figure 2

Characterization of the evaporator structure. (a) Top-view photograph and SEM image of the air-laid paper water-wicking layer. (b) Characterization of water sorption rate and capacity of air-laid paper. The arrow shows the direction of the water-wicking path. (c) Photograph and SEM image of the silicone foam thermal insulator. (d) Photograph of a hydrophobic hot electrical heater with bubbles formed on the surface. The inset shows a water contact angle of 90° on the heater surface. (e) Photograph of a plasma-treated hydrophilic hot electrical heater with few bubbles. The inset shows a water contact angle of 10° on the plasma-treated heater surface. The heater was electrically heated at 10 kW/m2 for 30 min.

To keep the evaporator floating at the interface, thermally stable porous polydimethylsiloxane (PDMS) foam was used as the floater and thermal insulator. The scanning electron microscopy (SEM) image in Figure c presents that closed pores with an average diameter of ∼150 μm are uniformly distributed within the silicone matrix. Thermal conductivity measurement in our previous work[34,40] indicates that silicone foam has a low thermal conductivity of ∼0.03 W/m K so it can effectively suppress the downward heat leakage and localize Joule heating at the evaporative interface. Compared to polystyrene foam,[28] carbon foam,[13] and carbonized wood[20] thermal insulators that had been used in solar-driven evaporation systems,[11] here silicone foam has the combined advantages of good thermal stability, low thermal conductivity, mechanical flexibility, and biomedical compatibility. These features are desired for fast-response, high-efficiency hot steam generation under high heating power densities for medical sanitization and sterilization applications. Commercial polyimide-sealed Fe–Cr–Al thin-film electrical heating plates were employed as the heater to provide Joule heating and drive the evaporation due to its high electric-to-heat conversion efficiency (∼99%, Note S1). Figure d displays that the as-received heater is hydrophobic with a water contact angle of 90° and many bubbles are formed on the hot surface of the heater when it was immersed within water. After surface plasma treatment, the heater became hydrophilic showing a water contact angle of 10° and very few bubbles were observed (Figure e). Considering the intimate contact between the hydrophilic water-wicking layer and the hydrophilic heater, we chose the plasma-treated hydrophilic heater in the evaporation system.

The evaporation test was performed by recording the real-time mass change of the evaporation system with an analytical balance. The evaporation experiment was carried out on an open water body without complicated thermal insulation. We comparatively studied the evaporation performance of three different systems: evaporation driven by bottom electrical heating (BEH), interfacial electrical heating (IEH) that only has the electrical heater at the air–liquid interface, and LIEH (Figure S1). Figure a shows that under the same input heating power density of 10 kW/m2 the LIEH system has achieved an evaporation mass loss of 7.27 kg/m2 within 30 min, which is much larger than that in the BEH system (0.92 kg/m2) and IEH (3.35 kg/m2) systems. Another observation is that the evaporation mass loss linearly increases with the heating time for the LIEH system while the mass change gradually increases in the other two systems. The obvious difference in evaporation rates is clearly demonstrated in Figure b. The LIEH evaporation system has reached a steady evaporation rate of ∼14.53 kg/m2 h within 50 s. By contrast, the evaporation rate for BEH and IEH systems continuously increases during the whole heating period meaning that they did not arrive at the steady state. The fast response of the LIEH evaporation system was further demonstrated by periodically switching on and switching off the heater, which shows a stepwise mass change with a response time as short as ∼20 s (Figure S2).

Figure 3

Electrically driven evaporation performance. (a) Time-dependent evaporation mass loss for three different evaporation systems under a heating power density of 10 kW/m2. (b) Evolution of evaporation rates. (c) Comparison of evaporation efficiency under different heating power densities. (d) Temperature evolution profiles at the air–water interface and the bottom of the container under a heating power density of 10 kW/m2.

The evaporation efficiency (η) was calculated by η = ṁhLV/qelectrical, where ṁ is the average mass change rate per unit area over the evaporation period (1800 s), hLV is the evaporation enthalpy of water that contains both liquid-to-vapor phase change enthalpy and sensible heat, and qelectrical is the input electrical power density, which can be tuned by changing the applied power onto the electrical heater. Figure c shows that the LIEH system has a much higher evaporation efficiency than the IEH and BEH systems within the tested heating power densities. The low evaporation efficiency for the IEH and BEH systems could be understood from the fact that most of the input thermal energy is used for sensible heating of bulk water rather than driving the evaporation. Because of the large thermal mass and serious heat loss, the time to reach a steady evaporation state of the IEH and BEH systems is much longer than the LIEH system. The evaporation efficiency of the LIEH evaporator reached the highest evaporation efficiency of ∼90% under a heating power density of 10 kW/m2 and further increasing heat power density leads to dropped efficiency (Figure c). The decreased efficiency is related to the changed evaporation mode from thin-film evaporation to boiling, which generates large bubble and increases the convective heat losses in the LIEH system. By establishing a COMSOL model (Note S2), we simulated the temperature distribution of the evaporation system, which confirms the overheating of the LIEH evaporator above 100 °C when the power density increases from 10 to 12 kW/m2 (Figure S3). Based on the simulated surface temperature of the evaporator under different heating densities, we quantitatively estimated the heat losses and the evaporation efficiency for the IEH and LIEH evaporation systems (Note S3). Four types of heat losses are identified for the LIEH evaporation system, namely, convection loss, radiation loss, downward conduction loss to silicone foam, and sensible heating of the water-wicking layer. The calculation shows that the percentage of heat losses with respect to the input heating power decreases as the power density increases from 2 to 10 kW/m2, which explains the increasing evaporation efficiency in this range. When the heating power density increases to 12 kW/m2, the percentage of convection heat loss increases from 4 to 8% because of the increased heat transfer coefficient under boiling. In the IEH evaporation system, the major heat loss comes from the download heat leakage toward the bulk water body due to the lack of the floating thermal insulator. By deducting various forms of heat losses in the system from the heating input, the calculated evaporation efficiency profiles are in good agreement with the experimentally measured results (Figure S4).

The lost heat is also reflected in a temperature distribution difference among three evaporation systems. Figure d shows that the LIEH evaporator has achieved the highest interfacial temperature of ∼81 °C and the lowest bottom temperature of ∼24 °C. In the LIEH system, the bottom temperature of the floating insulator remains the same temperature with the surrounding environment implying that almost no heat is leaked from the evaporation surface to the bulk water during the evaporation process. On the other hand, the IEH system could not reach the high temperature as the LIEH system because part of heat is leaked into the bulk water. As shown by the higher bottom temperature than the interfacial temperature, the electrical heating in the BEH system mainly causes the increase of the bottom water temperature, and the low temperature at the air–water interface limits vapor generation. It was noted that the evaporation rate profiles in Figure b and the interfacial temperature curves in Figure d showed similar trends. This is because the evaporation rate has a strong positive relation with the evaporation temperature based on the Langmuir evaporation equation.[42] It should be noted that although the evaporation temperature, which was measured by the thermocouple in direct contact with the water-wicking layer, was lower than 100 °C, COMSOL simulation indicates that the temperature at the surface of the electrical heater has reached 100 °C. This implies that steam was generated at a heating power density of 10 kW/m2, but it was quickly cooled down when the steam enters into the air.[26,28]

To achieve high evaporation efficiency under the broad range of heating power densities, we further investigated the usage of filter paper (with a thickness of 155 μm) and hemp paper (with a thickness of 303 μm) as the water-wicking layer in addition to the air-laid paper (with a thickness of 222 μm) (Figure S5). The SEM images in Figure a show that the filter paper has a semi-permeable structure composed of tightly weaved fibers and only a few pores with a diameter of ∼10 μm are left. The cellulose fibers in air-laid paper are randomly distributed rendering good breathability of water vapor. Hemp fabric weaved by thick fibers that consist of many fine fibers, and large pores (∼100 μm) are observed. The corresponding three-dimensional (3D) microscopy images in Figure b show that filter paper has the smallest thickness variation while the holes in the hemp fabric are thoroughly penetrated. Figure c presents that the filter-paper-based evaporation system has a highest evaporation efficiency of ∼80% under a low power density of 2 kW/m2 and the efficiency continuously decreases with increasing heating power density. The high evaporation efficiency observed in the filter-paper-based system under 2 kW/m2 is attributed to the fact that filter paper has the lowest amount of absorbed water (∼0.6 g), thus the evaporation surface could reach a higher temperature. However, filter paper has a small vapor permeability of 1.33 g/m2 s, which is between the evaporation rate of the evaporator under heating powers of 2 kW (0.78 g/m2s) and 4 kW (1.47 g/m2 s). When the evaporation rate is larger than the vapor permeability of water-wicking membranes, the escape of vapor is impeded. Therefore, under higher heating power densities, the small shallow pores in filter paper impede the escape of vapor, which in turn tends to cause overheating and serious heat losses. The hemp fabric-based evaporation system has shown an opposite trend and has achieved the largest efficiency of ∼85% at 12 kW/m2. The larger amount of wicked water within the hemp fabric wicking layer (∼1.0 g) restricts the rise of the evaporation surface temperature, which suppresses the evaporation efficiency at low heating power densities. On the other hand, the large deep holes facilitate the release of vapor bubbles through the hemp fabric and ensure stable evaporation under high heating power densities.

Figure 4

Tunable evaporation performance. (a) SEM images of different water-wicking membranes: filter paper (left), air-laid paper (middle), and hemp fabric (right). (b) 3D microscopy photographs of different water-wicking membranes: filter paper (left), air-laid paper (middle), and hemp fabric (right). (c) Evaporation efficiency of the LIEH-based evaporator with different water-wicking membranes. (d) Tunable vapor temperature of the air-paper evaporator with different top-surface coverages.

Vapor temperature is another important performance indicator for the evaporation system. To tune the evaporation temperature, we covered the air-laid paper evaporation surface with a silicone film (2 mm thick) punched with holes (Figure S6). By varying the ratio of the vapor outlet area to the evaporation surface area (σ = Aoutlet/Aevaporation), we comparatively measured the steady-state evaporation temperature. The punched cover reduces the evaporation area and the evaporation mass flow, thus the same Joule heating input can induce a higher evaporation temperature. Additionally, the coverage of the air-laid paper wicking layer decreases the convection heat loss from the evaporation surface, which can also help increase the evaporation temperature. Figure d confirms that the punched cover indeed boosts the evaporation temperature and the cover with a smaller σ shows stronger enhancement. When the electrical heating power density is 6 kW/m2, the evaporation temperature could be continuously increased from ∼70 to ∼100 °C by using the punched cover with a decreasing σ from 1 to 0.01. By contrast, without the coverage of the punched cover the evaporation temperature only reached ∼85 °C even when the heating power density was doubled to 12 kW/m2. Such a strategy enables the generation of hot steam under ambient pressure with low heating power densities.

It should be noted that the fast response and high electrical-to-vapor conversion efficiency of the evaporator are from the unique localized interfacial heating design. Besides the demonstrated flat 2D heaters, such an evaporator can be driven by 1D and 3D heaters as well. In previous solar-driven evaporation systems, it has been demonstrated that the evaporator constructed by folded three-dimensional absorbers can further improve the evaporation flux and evaporation efficiency than the two-dimensional flat absorbers.[43,44] Compared with the solar-driven evaporators, the configuration of heating sources can be more versatile in electrically driven interfacial evaporation systems. By connecting the thin film heater with a 3D metallic heat sink (Figure S7), we investigated the evaporation performance of the pin fin-structured evaporator. The evaporation efficiency of such a system was determined to be 66.4, 69.8, and 72.7% under the heating powers of 5, 10, and 15 W, respectively. Compared to the flat configuration, the evaporation efficiency is slightly decreased in the pin fin-structured evaporator, which could be related to the large thermal mass of the pin fin metallic heat sink. In this case, a portion of Joule heating was consumed to heat up the heat sink. However, unlike the flat configuration that leads to overheating and decreased evaporation efficiency under a high heating power, the evaporation efficiency of the pin fin-structured evaporator continuously increases with the tested heating power. The enlarged evaporation surface areas from the three-dimensional pin fin evaporation structure is beneficial for achieving large evaporation fluxes without causing overheating problems, which usually occur in the evaporators with a flat configuration under high heat power conditions. It should be noted that the pin fin structure is only one implementation for the 3D evaporator design and there are other possible ways for improving the evaporation performance. For example, the recovery of evaporation latent heat is another effective approach to boost the evaporation flux, which has been demonstrated in multistage solar-thermal desalination devices.[45]

We proceeded to employ the high-efficiency LIEH-based evaporator for fast steam generation within closed systems. In this case, the evaporator (5 cm in diameter) was placed within a cylindrical polystyrene vessel with a diameter of 8 cm and a height of 10 cm. Figure a presents that the LIEH-based evaporator successfully generated steam at 100 °C under a heating power density of 18 kW/m2 within 2 min, while it takes ∼10 min for the BEH-based system to generate steam. We further demonstrated that such a fast-responsive advantage is almost independent of the amount of bulk water loaded in the evaporation system. We compared the time for the evaporation system to generate 100 °C steam when it is loaded with three different volumes of water (50, 100, and 200 mL). Figure b presents that the required heating time for the LIEH-based evaporator to generate steam under a fixed heating power density is almost the same, but the steam generation process for the BEH-based evaporator is much slower and is strongly dependent on the amount of loaded water in the system. This difference can be understood from the fact that the LIEH-based evaporator only heats up the amount of water stored within the wicking layer, which is almost independent of the water body beneath, whereas the BEH-based evaporator needs to heat up the bulk water body in order to generate steam. We also carried out theoretical analyses on the response time for the generation of 100 °C steam in both the BEH-based and LIEH-based steam generator (Note S4). In the BEH-based system, the response time is inversely proportional to the heating power density, and the proportionality is mainly determined by the mass of loaded water (Figure b). This means that the bulk heating-based steam sterilization faces the dilemma of achieving large evaporation masses, which is determined by the amount of loaded water, and the fast thermal response in steam generation. In contrast, the response time of the LIEH-based evaporation system is independent of the amount of loaded water as evidenced by the overlapped profiles. Compared to the experimentally measured results, the slight underestimation of the required thermal response time is due to the neglected heat losses from the evaporation system during the theoretical modeling.

Figure 5

Electrically driven interfacial evaporation for fast-responsive steam generation and sterilization. (a) Evaporation temperature evolution of the LIEH-based and BEH-based sterilization device with the same heating power density of 18 kW/m2. (b) Response time to generate the saturated steam (100 °C) by sterilization devices with variable volumes of water. The dash lines are from theoretical calculation. (c) Representative temperature evolution profiles at the evaporation interface and the top of the commercial sanitizer. The inset image shows the locations for temperature measurement. (d) Photographs of ager plates containing bacterial cells before and after LIEH-based and BEH-based sterilization.

To test the steam sterilization performance, the LIEH-based evaporator was placed within a commercial steam sanitizer containing 50 mL of water. Two thermocouples were utilized to monitor the temperature evolution of the generated steam. One is located at 1 cm above the evaporation interface (Tinterface) and the other is located at the top surface of the sterilization chamber (Ttop). As shown in Figure c, the Tinterface immediately increased and reached a steady temperature close to 100 °C within 5 min under the heating power of 18 kW/m2, and the Ttop profile showed a similar quick rise and reached a stabilized temperature at ∼90 °C due to heat dissipation from the cover surface. Under the same heating conditions, the evaporation temperature rise in the BEH-based system was much slower. The Tinterface and the Ttop only reached 85 and 64 °C after heating for 15 min, respectively. The Escherichia coli bacteria were used as the indicator in the steam sterilization experiments.[46] The sterile tubes containing the bacterial solution were exposed to hot steam within the sanitizer for 15 min under a heating power density of 18 kW/m2 (Figure S8). Figure d presents that all the E. coli bacterial cells were killed by the LIEH-based steam sanitizer, but many bacteria cells were still alive after treatment in the BEH-based steam sanitizer due to the low vapor temperature. The high evaporation temperature and large evaporation flux achieved in the LIEH-based steam sterilization enable rapid effective sterilization under a low electrical heating supply.

Conclusions

In summary, we demonstrated a localized heating-based electrically driven interfacial evaporator as an alternative to conventional bulk heating-based evaporation systems for high-efficiency steam generation and fast-responsive steam sterilization. The unique interfacial evaporation system design enables achieving high evaporation efficiency under a broad range of heating power densities and facile tuning of the evaporation temperature by varying the ratio of the vapor outlet area to the evaporation surface area. The resultant floating evaporator is capable of rapidly generating high-temperature steam under low-heating power densities, and the steam generation rate is independent of the amount of loaded bulk water. Besides the demonstrated fast steam sterilization application, it is anticipated that the reported evaporator could be integrated with other systems to explore high-performance cooking, steam cleaning, humidification, and other vapor or steam-enabled applications. In addition, the electrically driven interfacial evaporators can make use of the electricity converted by the photovoltaic systems during daytime to continuously drive the evaporation at night or under cloudy weather conditions. Thus, they can be a complementary system to the widely explored solar-driven interfacial evaporators that operate in situ only during daytime.

Experimental Section

Chemicals and Materials

PDMS precursor and foaming agent were supplied by Bluestar Silicones Co., Ltd (Shanghai). The two-component silicone elastomer (Sylgard 184) was purchased from Dow Corning. The polyimide-sealed film heater was purchased from Shanghai Songdao Heater Co., Ltd. Air-laid paper was purchased from Dongguan Jia Chong Purification Technology Co., Ltd. Gram-negative bacteria E. coli and nutrient agar plates were purchased from Shanghai Luwei Microbial Sci. & Tech. Co., Ltd.

Fabrication of Evaporator

The LIEH-based evaporator consists of a piece of water-wicking air-laid paper, a hydrophilic polyimide-sealed electrical heater, and silicone foam, which are stacked together. The as-received polyimide-sealed film heater (1.25 cm in radius) was treated within a plasma cleaner (PDC-32G, Harrick Plasma) for 30 s to convert the surface from hydrophobic to hydrophilic. A typical silicone thermal insulation foam was prepared by mixing 2 g of PDMS precursor and 2 g of foaming agent into a uniform mixture within a Petri dish (1.75 cm in radius). The obtained mixture was left at room temperature for 1 h for foam formation. Afterward, PDMS foam was placed within an oven at 60 °C for 12 h to complete the foam formation process. The silicone cover film was prepared by mixing 2 g of silicone elastomer (Sylgard 184, part A) and 0.2 g of curing agent (Sylgard 184, part B) within a Petri dish and curing at 120 °C for 30 min. The cover films with different numbers of holes were prepared through mechanical punching.

Characterization and Property Measurement

The microstructure of different water-wicking membranes was observed by a SEM (JSM-7800F Prime, JEOL). The thickness of different water-wicking membranes was measured by a micrometer (SYNTEK, China). The water contact angle of the polyimide-sealed electrical heaters was measured by a contact angle analyzer (DSA-100, Kruss). The electric-to-heat conversion efficiency of the electrical heaters was measured by heating water in an insulated glass container (Figure S9). A 3D optical microscopy (Keyence VHX-S50) was used to characterize the surface microstructure and the distribution of pores within different water-wicking membranes. To characterize the water-wicking rate, one side of the rectangle-shaped sample (2 cm × 4 cm) was placed in contact with a red water-dye solution and the mass change of the sample was monitored. The water absorption capacity was evaluated by immersing the membranes in water and measuring the mass change.

Electrically Driven Evaporation Experiment

In a typical evaporation experiment, an electrical heater was connected to a dc digital-control power supply (KA3005D, KORAD) with tunable heating power densities. The mass of evaporated water was measured by an analytical balance (BSM-220, Shanghai Zhuo Jing Electronic Technology Co., Ltd.). Natural evaporation weight loss was measured and was deducted when evaluating the evaporation efficiency of different evaporation systems. The temperature evolution of the evaporation system was measured by K-type thermocouples (TT-K, Omega Engineering Inc.), which were connected with a data collector (34972A LXI Data Acquisition/Switch Unit, Agilent). The evaporation temperature was measured by placing the thermocouple in contact with a water-wicking layer.

Steam Sterilization Experiment

In steam sterilization experiments, the evaporators (6 cm in diameter) were integrated in a commercial steam sanitizer (Jin Zhi Ai LS-B321, Lonsun Electrical Appliance Co., Ltd, China). One thermocouple was placed at a distance of 1 cm from the evaporation surface to measure the generated steam temperature, and another thermocouple was attached to the inner surface of the top cover to monitor the temperature of the sterilization chamber. In each sterilization experiment, 1 mL of E. coli bacteria-suspended solution was stored in a 3 mL of sterile centrifuge tube, and the centrifuge tube was mounted on a plastic rack within the sterilizer. The centrifuge tubes were sealed by a piece of the breathable film to allow steam penetration and prevent contamination. The sample was sterilized under an input heating power density of 18 kW/m2. After heating for 15 min, 100 μL of bacterial solution was extracted out and spread onto the agar plates, which were then incubated at 37 °C for 24 h. The sterilization performance of different evaporation systems was evaluated by counting the number of bacterial cells alive on the agar plates after cultivation. As a comparison, the as-received bacterial solution was diluted by 105-fold and the number of cultivated cells was counted by the same method.