Control of Ice Formation Ching-Wen Lo, Venkataraman Sahoo, and Ming-Chang Lu* Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan 300 S Supporting Information *
ABSTRACT: Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, deicing methods are costly, inefficient, and environmentally unfriendly. Recently, the use of superhydrophobic surfaces has been suggested as a potential passive anti-icing method. However, no surface is able to repel frost formation at a very cold temperature. In this work, we demonstrated the abilities of spatial control of ice formation and confinement of the ice-stacking direction. The control and confinement were achieved by manipulating the local free energy barrier for frosting. The V-shaped microgroove patterned surface, which possessed these abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The insight of this study can be applied to alleviate the impact of icing on our daily life and in many industrial systems. KEYWORDS: spatial control, icing, frosting, anti-icing, deicing
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transition from a Cassie to a Wenzel droplet during the freezing process on the superhydrophobic surface.25 The ice adhesion can be potentially reduced by the ion-specific heterogeneous nucleation.32 Generally, ice can be formed on a solid surface either by a condensation−freezing process or by a frosting process. Frosting refers to the formation of crystalline ice from water vapor on a solid surface. It occurs when water vapor is in contact with a solid surface whose temperature is below both the dew point and the freezing temperature of the water vapor. Although superhydrophobic surfaces can be applied to antiicing in the condensation−freezing process,33,34 frosting is expected at a very cold temperature. Thus, indiscriminate ice formation is found everywhere on the solid surfaces through the frosting process, eliminating the ice-phobic function on the superhydrophobic surfaces.29 A way to alleviate the impact of frosting on a solid surface is to spatially control the frost formation at its early stage and remedy the ice issue locally and immediately on the solid surface. There are a few works demonstrating the spatial control of ice nucleation on the surfaces.35,36 Zwieg et al.35 have demonstrated the control of ice nucleation on the alumina surfaces patterned by a hydrophilic ice nucleating coating in the frosting process. However, the hydrophilic coating would result in an increased ice adhesion strength.18 Mischenko et al.36 have demonstrated the spatial control of ice nucleation in the condensation−freezing process. Nevertheless, the spatial control of ice nucleation in the frosting process has not been demonstrated. Here, we demonstrated the spatial control of ice
lobal warming has caused extreme weather to be more frequent and extreme. A massive snowstorm hit the eastern U.S. and record-low temperatures led to snow in several tropical areas in early 2016. Snowflakes falling on the ground will eventually turn into ice as the mass of ice increases. Icing affects our daily life in a number of ways and are also catastrophic for many natural and industrial systems. For example, ice formed on the surface of a heat exchanger reduces the efficiency of a refrigeration system by 50−70%.1 Serious accretion of ice on an insulator will eliminate its electrical insulation and consequently can threaten the security of a power transmission system or a telecommunication system.2−4 Ice formation on the wings of an aircraft can affect flow distribution, eliminating lift force and affecting its safety.5,6 Ice covering on a plant surface may cause crop injury, leading to agricultural disasters.7 Several deicing methods have been developed, for example, electrothermal heating, mechanical removal, and chemical modification.8 However, these methods are often costly, inefficient, and environmentally unfriendly.9 In recent years, utilizing micro/nanostructure-induced superhydrophobic surfaces for anti-icing (i.e., preventing ice formation) on solid surfaces is getting more attention as a result of emerging methods of surface micro/nanoengineering.10,11 Superhydrophobic surfaces can be potentially icephobic because of ultralow adhesion of liquid water on the surfaces. Superhydrophobic surfaces exhibit longer liquid freezing-delay time,12−16 lower ice adhesion,17,18 and shorter solid−liquid contact time during droplet impact.19−24 However, superhydrophobic surfaces lose their ice-phobic property when ice covers the whole surface area.25−31 It was found that ice adhesion on a superhydrophobic surface is approximately the same as that on a superhydrophilic surface because of the © 2017 American Chemical Society
Received: November 1, 2016 Accepted: February 28, 2017 Published: February 28, 2017 2665
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15 min before conducting the frosting experiments to ensure no residue liquid was left on the solid surfaces. Frosting on V-shaped microgrooves with the same groove width of 7 μm but different groove spacings of 125, 165, and 250 μm (denoted as VMG-125, VMG-165, and VMG-250 surfaces hereafter) was investigated for evaluating the groove density effect on nucleation. Frosting on trapezoid-shaped microgrooves having a groove width and spacing of 7 and 125 μm, respectively (denoted as TMG-125 surface hereafter) was also conducted for assessing the effect of groove shape on icegrowing dynamics. The scanning electron microscope (SEM) pictures of the SiNW, VMG-125, and TMG-125 surfaces are shown in Figure 1B−D, respectively. The fabrication of the SiNW array followed the same procedure as that in our previous works.37−39 The fabrication procedure of the microgroove surfaces is described in the Experimental Section. The synthesized nanowires were vertically aligned having a height of approximately 10 μm with diameters and spacings in the range of 50 to 300 nm. A thin layer of Teflon of approximately 20 nm thick was coated on top of the surfaces to render the surfaces hydrophobic. The contact angles on the plain Si surface, the SiNW surface, and VMG-125, and TMG-125 surfaces were 100°, 150°, 140°, and 140°, respectively (see Experimental Section for the experimental details). The corresponding contact angle hysteresis on these surfaces was 80°, 5°, 60°, and 60°, respectively. The insets in Figure 1B−D are the contact angles on SiNW, VMG-125, and TMG-125 surfaces. The contact angles on the VMG surfaces with other groove densities can be found in Table S1. It is worth noting that the contact angle was independent of groove density on the VMG surfaces. Spatial Control of Ice Formation. The observation of the initial ice nucleation process on the surfaces was realized by a stepwise increasing of the vapor pressure from 0.6 Torr to 1.2− 2.1 Torr with a step size of 0.1 Torr. Different structured surfaces had different thresholds of supersaturations (supersaturation is defined as the ratio of vapor pressure above the surface to the saturation pressure corresponding to the surface temperature). However, the relation between structure and threshold of nucleation was not definite (see Figure S1 in the Supporting Information (SI)). Figure 2 shows the ESEM images of the dynamics of the ice nucleation processes on the plain Si, SiNW, TMG-125, VMG125, VMG-165, and VMG-250 surfaces. As can be seen in Figure 2, a random distribution of ice embryos was found on the plain Si surface and SiNW surface (Figure 2A,B), whereas the spatial control of ice nucleation was observed on the microgroove-patterned surfaces (Figure 2C−F). Both TMG and VMG surfaces demonstrated the ability of spatial control of ice nucleation (Figure 2C−F). Furthermore, the NSD on the microgroove-patterned surfaces could be tailored by manipulating the groove density (Figure 2D−F). In principle, heterogeneous nucleation is an intrinsically random process because the energy barriers for nucleation on an ordinary surface are randomly distributed. The nucleation rate J is exponentially decayed with the free energy barrier of nucleation ΔG (i.e., J = J0 exp(−ΔGkB−1T−1)), and the free energy barrier is a function of surface topography through the surface nucleation cavity:
nucleation on a solid surface through manipulating the free energy barrier for nucleation by engineering the regional roughness scale on a solid surface. To the best of the authors’ knowledge, this method has not been reported before. It was found that ice preferentially nucleated on patterned microgrooves on the surfaces. The ice nucleation site density (NSD) could also be manipulated by tailoring the number of microgrooves on the surfaces. Moreover, the growth kinetics of ice could be altered by adjusting the shape of the microgroove. Ice stacked along the direction of the V-shaped microgroove, whereas it grew in random directions on the trapezoid-shaped microgroove. The spatial control of frost formation and the confinement of ice-growing kinetics were found to be advantageous for anti-icing and deicing. The Vshaped microgroove surface possessed the longest ice-covering time (the time required for ice to cover the whole surface area in the icing experiments) and the shortest dwell time (the time period during which ice covered the whole surface area after switching off the Peltier cooler in the deicing experiments) among various kinds of surfaces. We anticipate that the concept learned from this work can be applied to relieve the impact of icing on our daily life and in many industrial systems.
RESULTS AND DISCUSSION Frosting and deicing processes on a plain Si surface, a Si nanowire (SiNW) array-coated surface, and V-shaped and trapezoid-shaped microgroove patterned surfaces were systematically investigated under an environmental scanning electron microscope (ESEM). The test section and operation conditions for conducting experiments in ESEM are provided in the Experimental Section. Experimental conditions in the ESEM chamber were carefully controlled for in situ microvisualization of the dynamics of the frosting and deicing processes on the surfaces. The thermodynamic states for frosting and deicing processes were carefully adjusted in a region lower than the triple point of water to prevent formation of liquid water (see Figure 1A). Points A and B in Figure 1A represent the initial states of frosting and deicing processes, respectively. Besides, the studied surfaces were preheated at 25 °C under 0.6 Torr for
Figure 1. Initial thermodynamic states of the frosting/deicing processes and surface characterization. (A) Initial thermodynamic states for frosting and deicing processes. Points A and B represent initial states of frosting and deicing processes, respectively. SEM images of (B) the SiNW surface, (C) the VMG surface, and (D) the TMG surface. The insets in (B), (C), and (D) are the contact angles on the SiNW, VMG-125, and TMG-125 surfaces. Scale bars are 15 μm.
ΔG = 2666
4 2 πre σsvF − 4πσsv(rc − re)2 F 3
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Figure 2. Spatial control of ice nucleation. ESEM images of ice nucleation on (A) plain silicon, (B) a SiNW surface, (C) a TMG surface with a groove spacing of 125 μm, and VMG surfaces with groove spacings of (D) 125 μm, (E) 165 μm, and (F) 250 μm. Scale bars are 200 μm.
Figure 3. Control of ice growth kinetics. (A) Hexagonal ice composed by two basal facets (c-axis) and six prism facets (a-axis). (B) Random and aligned orientations of c-axes were found on TMG and VMG surfaces, respectively. (C) Ice embryos appear on the side walls, the edges, and the valleys of groove on TMG surfaces, resulting in different orientations of ice crystals. On the other hand, an ice embryo forms only at the valley of grooves on the VMG surface, leading to the confined ice orientation. Scale bars are 15 μm.
where F = 0.25(2 − 3 cos θ + cos3 θ) and J0, kB, T, re, σsv, rc, and θ are a proportional constant, Boltzmann constant, temperature, equilibrium radius of ice, surface energy between solid/ vapor, cavity radius, and contact angle, respectively.40 The observation of the random distribution of nucleation on the plain Si and SiNW surfaces was because of the random nature of the roughness on these two surfaces. The large NSD observed on the SiNW surface (see Figure 2B) was a result of the large number of microscale cavities formed during the nanowire synthesis process. The plain Si surface is relatively smooth. In principle, the required supersaturation for ice nucleation on a smooth surface is larger than that on a rough
surface. As a result, the ice embryo on the plain Si was correspondingly larger. The ice embryos were the nucleation seeds on the plain surface, and the ice crystals grew with time on the plain Si surface. On the other hand, the patterned microgrooves locally lower the free energy barrier for nucleation at the microgrooves on the surfaces, resulting in the preferential ice nucleation on the microgrooves (see Figure 2C−F). Control of Ice Growth Kinetics. An ice crystal evolves into a nearly isometric simple prism (i.e., hexagonal structure) composed by two basal facets and six prism facets as shown in Figure 3A.41,42 The yellow and red arrows in Figure 3A indicate 2667
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Figure 4. Dynamics of the frosting processes under a high supersaturation condition. Selected snapshots of ice-covering evolution during frosting on (A) plain Si, (B) SiNW, (C) TMG-125, (D) VMG-125, (E) VMG-165, and (F) VMG-250 surfaces. (G) Quantitation analysis of ice-covering ratio as a function of time on studied surfaces. Yellow solid symbols represent ice-covering times on the studied surfaces. (H) The open symbols represent experimental data of ice growth velocity, whereas the lines represent the theoretical predictions of eq 3 with different kinetic constants (A). Scale bars are 200 μm.
the directions of the c-axis and a-axis of the ice crystal, respectively. In addition to the spatial control of ice nucleation on the microgroove-patterned surfaces, the ice basal facet (caxis) was also confined to be aligned along the direction of the V-shaped groove on the VMG surfaces, whereas a random distribution of the c-axis was found on the TMG surface (Figure 3B). Ice embryos only formed at the valley in the Vshaped microgroove, resulting in the confined ice growing direction on the VMG surfaces (Figure 3C). On the other hand, ice embryos appeared randomly at sidewalls, edges, and valleys in the trapezoid-shaped microgroove, leaving the
random ice growing direction in the trapezoid-shaped microgroove (Figure 3C). The reason behind the confined crystal growth on the VMG surfaces is presumably due to the corner effect.43 It is shown that the kinetics of crystal nucleation can be adjusted by the shape of nanopores.43 The corners at TMG and VMG surfaces served as a promoter for nucleation. If the nucleation happens in a corner formed by two flat planes making an angle ϕ, the critical energy barrier needed to be surmounted for spontaneous ice crystal growth is44 2668
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Figure 5. Dynamics of the deicing processes. Selected snapshots of the deicing processes on (A) plain Si, (B) SiNW, (C) TMG-125, (D) VMG-125, (E) VMG-165, and (F) VMG-250 surfaces. (G) Ice-covering ratio as a function of time during the deicing processes. (H) Sublimation velocities on the studied surface during the deicing processes. Yellow solid symbols represent dwell times on studied surfaces. Scale bars are 200 μm.
ΔGc,crit =
4 2 πre σsvQ 3
surfaces were 125° and 71°, respectively. If 90° − ϕ/2 < θ < 90° + ϕ/2 (which is the case for the current investigation for ice nucleation in the TMGs and VMGs), the nucleation occurs at the apex of the corner. In addition, Q(θ, ϕ) = 4−1π−1{cos θ sin2 θ sin ω − cos θ(3 − cos2 θ)ω + 4 sin−1 [sin(ω/2) sin(ϕ/ 2)]}, where ω = 2 cos−1[cot θ cot(ϕ/2)].44 The equilibrium radius (re) in eqs 1 and 2 can be determined using re =
(2)
The minimum and maximum intrinsic ice contact angles were measured during the initial phase of ice nucleation, and the values were found to be 70° and 100°, respectively (see Figure S2A). The measured corner angles ϕ on the TMG and VMG 2669
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ACS Nano 2vsσsvT(Tsat − T)−1hsv−1,40 where vs, Tsat, and hsv are ice-specific volume, saturation temperature, and latent heat, respectively, and an re of 12 nm could be obtained. The ratio of the critical free energy barrier at a corner to the critical free energy barrier on a flat surface (ΔGc,crit(ϕ)/ΔGc,crit (ϕ = 180°)) as a function of corner angle satisfying the condition 90° − ϕ/2 < θ < 90° + ϕ/2 is plotted in Figure S2B. The two solid lines in Figure S2 represent the calculated results with the intrinsic ice contact angles of 70° and 100°, respectively. The critical free energy barrier decreased on reducing the corner angle and reducing the ice contact angle. Besides, the critical free energy barrier at the apex in a corner is less than the critical free energy barrier on a flat surface. Thus, the nucleation is more likely to happen at the apex of the corners. Given that only one clear corner is present at the V-shaped microgroove, one can observe the confined crystal growth direction on the VMG surfaces (Figure 3B). Anti-Icing Performance on the Surfaces. Frosting is inevitable at a very cold temperature. To evaluate the anti-icing performance on the surfaces, frosting dynamics was investigated by instantly forming a thick layer of ice on the surfaces at a high supersaturation condition through increasing the ESEM chamber pressure directly from 0.6 Torr to 2.5 Torr. Figure 4 reveals the dynamics of the frosting under a high supersaturation condition on the surfaces. All the experiments have been conducted at least three times, and the data shown in Figure 4 and Figure 5 exhibit representative experimental results. The whole set of the experimental results can be found in Figure S3 in the SI. As depicted in the second row in Figure 4A−F (t = 2 s), ice embryos primarily formed at the microgrooves on the microgroove-patterned surfaces (Figure 4C−F). However, ice embryos were randomly distributed on the plain Si and SiNW surfaces (Figure 4A,B). Thus, the microgroove-patterned surfaces again demonstrated the ability of spatial control of nucleation. The ice-covering ratio (defined as the ratio of the projected area covered by ice to the overall projection area) varied with time during the frosting process and is shown in Figure 4G. The corresponding supersaturation as a function of time during the frosting process is shown in Figure S4A in the SI. Ice covered the whole area of the SiNW surface in a very short time due to the large number of ice crystals nucleating at an early stage. In contrast, the ice-covering ratio increased gradually with time on the plain Si and microgroove-patterned surfaces. The yellow solid symbols in Figure 4G indicate the time required for ice to cover the whole area of the surfaces (i.e., the ice-covering time). The smallest ice-covering time ( VMG-165 > VMG-125 ∼ TMG-125 > plain Si > SiNW. The microgroove-patterned surfaces exhibited the lowest ice growth velocity, as well as the longest ice-covering time. Thus, the utilization of spatial control of frost formation on a solid surface could provide a superior anti-icing performance. 2670
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increased to a peak value of 27 μm s−1 after the dwell time (denoted as solid yellow symbols in Figure S6A) and subsequently reduced to zero. For a very rough ice surface, the condensation coefficient of 1 is usually assumed.41 The calculated sublimation velocity from the Hertz−Knudsen equation with α = 1 was 31.25 μm s−1, which is close to the measured peak velocity on the plain Si surface. A rough ice surface provides more kink sites and consequently a large sublimation velocity.49 The relatively low sublimation velocity at an early stage was due to the flat topography on the sublimating surface (Figure S6B). The sublimation tended to start at the grain boundary due to the presence of defects serving as the kink sites (Figure S6F). As time increased, the ice surface became roughened and subsequently tapered ice was formed (Figure S6C). The tapered ice could provide abundant kink sites for enhancing sublimation. At the end, the density of kink sites decreased, and consequently the sublimation velocity decreased as the ice-covering ratio diminished (Figure S6D,E). Although the total time required to completely empty the ice on the surfaces showed no significant difference among the studied surfaces, the aligned parallel receding ice wavefronts observed on the VMG surfaces were beneficial for deicing because they could largely reduce the area covered by ice at an earlier time. The VMG-250 surface showed the smallest dwell time and therefore exhibited the best deicing performance among the studied surfaces.
Deicing Performance on the Surfaces. Since ice covering on a surface is inevitable at a sufficiently low temperature, deicing performance on the surfaces is of practical concern in industrial and natural systems. The dynamics of deicing on surfaces was also investigated in the ESEM after the surfaces had been covered by a layer of ice through switching off the Peltier cooler at the stage. The deicing method adopted in this work was classified as an electrothermal heating method. The initial ice layer thicknesses on the studied surfaces could not be measured directly using the original 45 deg tilted stage. However, the measured cross-sectional ice thickness on a 90 deg tilted stage shown in Figure S5 in the SI suggested that the initial ice thicknesses were approximately the same among the studied surfaces on the 45 deg tilted stage. Figure 5 shows the dynamics of deicing on the surfaces. No preferentially directional receding of ice was observed on the plain Si, SiNW, and TMG-125 surfaces (Figure 5A−C). On the other hand, VMG surfaces exhibited aligned parallel receding ice wavefronts (Figure 5D−F). This suggested that the confinement of the ice-stacking direction at the V-shaped microgroove resulted in the aligned in-plane crystal growth direction and gave rise to the aligned receding ice wavefronts during deicing. The deicing processes were activated at central lines between V-shaped microgrooves on the VMG surfaces (Figure 5D−F), whereas they were randomly activated on plain Si, SiNW, and TMG-125 surfaces (Figure 5A−C). The ice-covering ratio during the deicing processes as a function of time is shown in Figure 5G. The corresponding undersaturation (the ratio of the saturation pressure corresponding to the surface temperature to vapor pressure above the surface) is shown in Figure S4C in the SI. The dwell time, defined as the time period during which ice covered the whole area of the surfaces after switching off the Peltier cooler, was utilized to quantify the deicing ability. A smaller dwell time suggested a better deicing performance. The yellow solid symbols in Figure 5G indicate the dwell times on the surfaces. The dwell times on the plain Si, SiNW, and TMG-125 surfaces were approximately 78 ± 11, 85 ± 5.7 and 67 ± 8.7 s, respectively. Furthermore, the dwell times on the VMG-125, VMG-165, and VMG250 surfaces were 72 ± 10, 62 ± 13, and 37 ± 7.0 s, respectively. The groove density-dependent dwell time on the VMG surfaces was due to the fact that ice in the regions between the initial nucleation spots was much easier to remove presumably due to a weaker bonding between the ice and the solid surface in those regions (see Figure 5D−F). Thus, the spatial control of frost formation is beneficial for deicing. Since the surface of ice is very dynamic during melting and the condensation coefficient (α) is very sensitive to the surface structure,41 the obtained sublimation velocity could not be modeled by the Hertz−Knudsen equation. It was found that the sublimation velocity was not correlated with the undersaturation and was surface morphology dependent. To discuss the effect of surface morphology on sublimation velocity, the sublimation velocity as a function of time is shown in Figure 5H. The yellow solid symbols in Figure 5H indicate the dwell times on the surfaces. The methods for determining the sublimation velocity can be found in the Experimental Section. The sublimation velocities on the surfaces gradually increased to a peak value and then decreased to zero. The sublimation velocity on the plain Si surface shown in Figure 5H is highlighted in Figure S6A. The corresponding topographies at points b′, c′, d′, and e′ in Figure S6A are shown in Figure S6B−E in the SI, respectively. The velocity gradually
CONCLUSION In conclusion, we demonstrated the ability to spatially control ice nucleation. This control was achieved by manipulating the free energy barrier of nucleation on the VMG surfaces. Moreover, the growth kinetics of ice can also be altered by adjusting the shape of the microgroove: Ice stacked along the direction of the V-shaped microgroove, whereas it grew in random directions on the trapezoid-shaped microgroove. The spatial control of frost formation and the confinement of icegrowing kinetics improved the anti-icing and deicing performances. EXPERIMENTAL SECTION Test Section and Operation Conditions in ESEM. The microscopic dynamics of frosting and deicing processes on the studied surfaces were imaged using an FEI Quanta 200 FEG ESEM with a Peltier cooling stage. The absolute uncertainty in chamber pressure and Peltier cooling stage temperature was estimated to be ±0.1 Torr and ±0.2 °C, respectively. The samples with dimensions of 5 mm × 5 mm × 500 μm (width × length × height) were attached directly on the 45° tilted copper stage by using a silver paste (SPI Supplies, USA). The silver paste, which possesses a high thermal conductivity (∼10 W m−1 K−1), reduces the interfacial thermal resistance. Figure S7 in the SI shows a schematic of the test section in the ESEM chamber. The copper stage was surrounded by a 5 mm thick layer of polydimethylsiloxane with a low thermal conductivity of ∼0.1 W m−1 K−1 to prevent excess heat from the ambient. The temperature of the sample surface was assumed to be equal to the temperature of the Peltier cooling stage, whose temperature is measured by a T-type thermocouple. The Peltier cooling stage was cooled by a water flow pumping system. The working conditions in the ESEM chamber were as follows: acceleration voltage of 15 kV, working distance of approximately 10 mm, and emission current of approximately 10 μA. A relatively large magnification (scanning area of approximately 534 μm × 718 μm) was adopted to diminish the heating effect by the electron beam. The dynamics of the icing and deicing processes were imaged with a 512 pixel × 512 pixel frame size. The images were saved every 2−4 s. 2671
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that the saturation vapor temperature (approximately −7.2 °C) was larger than the surface temperature initially. The surface temperature became stable after a certain period of time because of the competing effects between the heat absorption of the deicing process and the heat release from the environment. Methods for Determining Ice Growth Velocity during Icing Experiments. The experimental ice growth velocity shown in Figure 4 was determined by dividing the diameter change of ice crystals (yellow dash lines in Figure S9 in the SI) between two snapshots by the corresponding time interval. Figure S9A−F in the SI depict the selected snapshots of ice-covering evolution during icing on plain Si, SiNW, TMG-125, VMG-125, VMG-165, and VMG-250 surfaces, respectively. It is noted that the diameter used in determining the growth velocity was always perpendicular to the wavefront of ice (red solid lines in the figure). The supersaturation as a function of time is shown in Figure S4A in the SI. At the early stage of ice formation, the supersaturation was high and a large number of ice crystals were randomly formed on the plain Si and SiNW surfaces in a very short time (see Figure S9A,B in the SI). As a result, large ice islands immediately formed on the two surfaces by combining the small ice crystals. Once the islands developed, the diameters of the islands were measured to determine the growth velocity. The extremely high growth velocity found on the SiNW surface shown in Figure 4H was due to the large number of combinations occurring in a very short period of time on the surface. The microgroove-patterned surfaces provided the ability to spatially control the ice nucleation as shown in Figure S9C−F in the SI. When the ice crystals forming at the microgrooves filled up the grooves on the microgroove-patterned surfaces, the growing lengths of ice depicted as yellow dashed lines in Figure S9C−F in the SI were used to determine the growth velocities on the TMG-125, VMG-125, VMG-165, and VMG-250 surfaces. Thickness of the Ice Layer. A customized vertical copper stage shown in Figure S5A in the SI was used to characterize the crosssectional ice thickness. Ice growth velocity normal to the surface was obtained by dividing the thickness variation of the ice layer (yellow dashed lines in Figure S5A in the SI) between two snapshots by the corresponding time interval. Figure S5B−D show the measured crosssectional ice thicknesses on studied surfaces from three repeated experiments. The supersaturations as a function of time on the surfaces (obtained from the experimental data shown in Figure S5B) are also depicted in Figure S5E. Increasing the vapor pressure from 0.6 Torr to 2.5 Torr for growing the ice layer resulted in the increment of surface temperature from −25 °C to −9.5 °C, and the corresponding supersaturations on the surfaces decreased from 6.26 to ∼1.2. Figure S5F depicts the vertical ice growth velocity on the surfaces from the data shown in Figure S5B, in which the solid symbols and lines were the measured velocities and the theoretical predictions of eq 3, respectively. The obtained kinetic constants on the surfaces were between 1 and 10. The cross-sectional ice growth could also be explained by the nucleation-limited kinetics, as can be seen from Figure S5F. The cross-sectional ice thicknesses on the surfaces exhibited no significant variation, and the obtained averaged ice thickness on the studied surfaces from all 18 sets of data points at 180 s was approximately 64.96 ± 6.92 μm. The vertical ice growth velocities were approximately the same among the surfaces, and the kinetic constants were on the same order of magnitude on studied surfaces. This suggested that structure effects on vertical ice growth were negligible. Methods for Determining the Sublimation Velocity in the Deicing Processes. Depicted in Figure S10 in the SI, the experimental sublimation velocities were determined by dividing the length change between receding ice wavefronts (the yellow dashed lines in the figure) between two snapshots by the corresponding time intervals. The wavefronts on the plain Si, SiNW, and TMG surfaces were difficult to identify due to irregular receding of ice observed on these surfaces (see Figure S10A−C in the SI). Thus, arbitrary parallel receding wavefronts (with fixed angles of Φ, Φ1, and Φ2 on the plain Si, SiNW, and TMG surfaces, respectively) were employed to determine the sublimation velocities on the surfaces. On the other hand, as shown in Figure S10D−F, the receding wavefronts could be
Fabrication Procedure of the Microgroove Surface. The fabrication procedure of the microgroove surfaces is shown in Figure S8 in the SI. First, a thin layer of low-stress Si3N4 with a thickness of about 450 nm was deposited using low-pressure chemical vapor deposition, and a layer of photoresist (PR) was spin-coated and patterned on top of the low-stress Si3N4 layer. Second, the exposed low-stress Si3N4 was anisotropically etched away using reactive ion etching (RIE), followed by the removal of the PR layer. Third, the remaining low-stress Si3N4 layer served as an etching mask for silicon etching. The V-shaped microgroove-patterned surfaces were obtained using 30% KOH etching at 80 °C for 4 min, whereas the trapezoidshaped microgroove-patterned surface was manufactured using ∼26% KOH etching at 60 °C for 12 min. Fourth, the residual low-stress Si3N4 layer was subsequently removed by RIE, and a superhydrophobic surface having microgrooves was obtained by fully covering with a thin layer of polytetrafluoroethylene (Teflon, Dupont) of about 20 nm by using inductively coupled plasma reactive ion etching. The thickness of the film was measured by an M2000 ellipsometer, which showed a mean value of 20 nm and a standard error of 2.5 nm. Contact Angle Measurement. Table S1 shows the contact angles on the plain Si, SiNW, TMG-125, VMG-125, VMG-165, and VMG250 surfaces. The contact angle measurements were carried out using the sessile drop method (DSA-100, Oldinburgh Co., Germany). The contact angles on the VMG and TMG surfaces were measured in the direction along the microgrooves. The values of the contact angles were determined using Krüss drop shape analysis with an accuracy of 0.1°. The tested fluid was deionized water (surface tension ∼72.8 mN· m−1), and all contact angles were measured at room temperature. The equilibrium contact angles on the solid surfaces were examined by gently depositing a liquid droplet of 10 μL volume on the surfaces through a microsyringe pump at the same injection rate of ∼190 μL min−1. Experimental Methodologies for Studying Ice Nucleation, Anti-Icing, and Deicing Performances in ESEM. To study the initial ice nucleation shown in Figure 2, the temperature of the Peltier cooling stage and the chamber pressure were first set at −25 °C and 0.6 Torr, respectively. The observation of ice nucleation on the surfaces was realized by stepwisely increasing the vapor pressure from 0.6 Torr to 1.2−2.1 Torr with a step size of 0.1 Torr. The threshold supersaturations for ice nucleation on the surfaces were between 1.1 and 1.76. The threshold supersaturations corresponded to the supersaturations at which initial ice embryos were observed on the surfaces. The icing experiments shown in Figure 4 were carried out through directly increasing the vapor pressure from 0.6 Torr to 2.5 Torr at the same surface temperature of −25 °C. At this point, abundant ice crystals immediately formed on the surface. Ice formation is an exothermic process. Since the large amount of latent heat dumped on the surfaces could not be quickly removed by the Peltier cooler, the surfaces’ temperatures increased from −25 °C to ∼−9.5 °C in 30 s, and the temperatures were stable at ∼−9.5 °C afterward, as shown in Figure S4B in the SI. In the figure, the symbols represent experimentally measured temperatures on the surfaces, whereas the lines presented in the first 30 s were the fitting lines of the experimentally measured temperatures. The fitting equations were used as the temperature inputs in theoretical predictions of ice growth velocities on the surfaces shown in Figure 4H. For the deicing experiments shown in Figure 5, an initial ice layer was created on the surfaces using the same method as that described in the icing experiments. The time period for forming the initial ice layer was fixed at 180 s on all the surfaces. The obtained initial ice layer thickness was approximately the same among the surfaces (see Figure S5B−D in the SI). After the surfaces had been covered by a layer of ice, the deicing experiments were conducted by directly switching off the Peltier cooler. The temperatures of the surfaces increased from ∼−9.5 °C to ∼−5.5 °C in 30 s after the turn-off of the Peltier cooler, and the temperatures were stable at ∼−5.5 °C afterward (see Figure S4D in the SI). The increment of surface temperature resulted from the heat transferred from the hotter vapor to the solid surface given 2672
DOI: 10.1021/acsnano.6b07348 ACS Nano 2017, 11, 2665−2674
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ACS Nano clearly defined on the VMG-125, VMG-165, and VMG-250 surfaces because the VMG surfaces revealed the parallel aligned receding wavefronts. The large number of activated spots on the SiNW surface shown in Figure S10B resulted in the sharp reduction of ice-covering ratio after the dwell time, as found in Figure 5G.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07348. Additional information on contact angle measurement; corner effect on ice nucleation; threshold supersaturations for ice nucleation; additional experimental data; variations of supersaturation, undersaturation, and surface temperature; thickness of the ice layer; sublimation velocity on the plain Si surface; test section for conducting frosting and deicing experiments in the ESEM chamber; fabrication procedure of the microgroove superhydrophobic surfaces; methods for determining ice growth velocity in the icing experiments; methods for determining the sublimation velocity in the deicing processes (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Ming-Chang Lu: 0000-0002-1186-0191 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS C.L. would like to thank Chien-Chang Lin for discussions on the operation procedure and Tai-Lang Lin for assistance with technical methodology in the ESEM at Academia Sinica. The authors also thank the Nano Facility Center at National Chiao Tung University for the use of their facilities. This work was supported by the Ministry of Science and Technology of Taiwan under grants MOST 105-2221-E-009-007-MY3 and MOST 105-3113-E-009-003. REFERENCES (1) Emery, A. F.; Siegel, B. L. Experimental Measurements of the Effects of Frost Formation on Heat Exchanger Performance. In Heat and Mass Transfer in Frost and Ice, Packed Beds, and Environmental Discharges, Proceedings of AIAA/ASME Thermophysics and Heat Transfer Conference, Seattle, WA, 18−20 June, 1990; pp.1−7. (2) Wei, X.; Jia, Z.; Sun, Z.; Guan, Z.; MacAlpine, M. Development of Anti-Icing Coatings Applied to Insulators in China. IEEE Electr. Insul. Mag. 2014, 30, 42−50. (3) Wang, S.; Jiang, X. Progress in Research on Ice Accretions on Overhead Transmission Lines and its Influence on Mechanical and Insulating Performance. Front. Electr. Electron. Eng. 2012, 7, 326−336. (4) Dehkordi, H. B.; Farzaneh, M.; Van Dyke, P.; Kollar, L. E. The Effect of Droplet Size and Liquid Water Content on Ice Accretion and Aerodynamic Coefficients of Tower Legs. Atmos. Res. 2013, 132, 362− 374. (5) Lynch, F. T.; Khodadoust, A. Effects of Ice Accretions on Aircraft Aerodynamics. Prog. Aerosp. Sci. 2001, 37, 669−767. (6) Marwitz, J.; Politovich, M.; Bernstein, B.; Ralph, F.; Neiman, P.; Ashenden, R.; Bresch, J. Meteorological Conditions Associated with the ATR72 Aircraft Accident near Roselawn, Indiana, on 31 October 1994. Bull. Am. Meteorol. Soc. 1997, 78, 41−52. 2673
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