Article pubs.acs.org/Langmuir
Electrofreezing of Water Droplets under Electrowetting Fields Katherine Carpenter and Vaibhav Bahadur* Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Electrofreezing is the electrically induced nucleation of ice from supercooled water. This work studies ice nucleation in electrowetted water droplets, wherein there is no electric field inside the droplet resting on a dielectric layer. Instead, there is an interfacial electric field and charge buildup at the solid−liquid interface. This situation is in contrast to most previous electrofreezing studies, which have used bare electrodes, involve current flow, and have a volumetric electric field inside the liquid. Infrared and high-speed visualizations of static water droplets are used to analyze surface electrofreezing. Ultrahigh electric fields of up to 80 V/μm are applied, which is one order of magnitude higher than in previous studies. The results facilitate an in-depth understanding of various mechanisms underlying electrofreezing. First, it is seen that interfacial electric fields alone can significantly elevate freezing temperatures by more than 15 °C, in the absence of current flow. Second, the magnitude of electrofreezing induced temperature elevation saturates at high electric field strengths. Third, the polarity of the interfacial charge does not significantly influence electrofreezing. Overall, it is seen that electrofreezing nucleation kinetics is primarily influenced by the three-phase boundary and not the solid−liquid interface. Through careful electrofreezing measurements on dielectric layers with pinholes to allow current flow, the individual role of electric fields and electric currents on electrofreezing is isolated. It is seen that both the electric field and the electric current influence electrofreezing; however, the physical mechanisms are very different.
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While many studies show than an external electric field raises the freezing temperature, other studies16,17 have shown no such effect. These disagreements result from both the manner in which the electric field is applied and various secondary phenomena that can trigger ice nucleation. The most common experiment to demonstrate electrofreezing consists of immersing bare metal electrodes in a small pool of water; an applied voltage sets up a volumetric electric field within the water. However, this also creates a small but finite current flow, as is demonstrated in our experiments. It is therefore not clear if electrofreezing is the result of the electric field, whether freezing is being promoted by the current flow, or by possible chemical reactions at the water−electrode interface. Multiple studies9,18−21 have clearly shown elevated freezing temperatures with applied voltage pulses using bare electrodes. Hozumi et al.19 discovered that the freezing temperature depends on the electrode material, which suggests that surface reactions are important. It is also important to note that current flow implies Joule heating, which will work against nucleation; however, no studies have accounted for this effect. Very few electrofreezing studies have been conducted in the absence of current flows, and several unanswered questions remain. Wei et al.22 used two parallel plate electrodes with a small volume of water between the electrodes but not in direct
INTRODUCTION There is widespread interest in the mechanisms of ice nucleation on surfaces since ice mitigation has widespread applications in the aviation, energy, and infrastructure sectors. Significant efforts1−8 have been undertaken to isolate the effect of surface chemistry and texture on ice formation kinetics and ice adhesion. The goal of all such efforts is to prevent or reduce ice buildup. In contrast, there has been much less effort to understanding phenomena that promote ice nucleation. Accelerating ice nucleation kinetics has applications in appliances, cryopreservation, and pharmaceutical freeze drying.9,10 The benefits of accelerated nucleation can also be exploited to promote the synthesis of clathrates, such as methane hydrates, which have an ice-like structure consisting of a methane molecule in a cage of water molecules. Such accelerated hydrate production methods would significantly benefit applications, such as natural gas transportation. Electrofreezing is the elevation of the freezing temperature of water by applying an external electric field and was first reported in 1861 by Dafour.11 In 1951, Rau12 observed supercooled water droplets instantly freezing when an electric field was applied between two bare electrodes in contact with water droplets. In the 1960’s and 1970’s, Pruppacher13−15 conducted experiments to show that freezing could be initiated by electrical discharges in water. More detailed studies on electrofreezing have been conducted in the last two decades. However, there are significant unanswered questions regarding the basic mechanisms underlying electrofreezing. © XXXX American Chemical Society
Received: December 9, 2014 Revised: February 2, 2015
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DOI: 10.1021/la504792n Langmuir XXXX, XXX, XXX−XXX
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is used to bias the droplet, and the bottom surface of the dielectric layer is grounded. The applied voltage is entirely expressed across the dielectric layer, which is much more insulating than water. There is no electric field inside the water, but there is a strong electric field in the dielectric layer. It should be noted that this experimental setup is commonly used for electrowetting studies.25−30 The dielectric layer prevents current flow; therefore, any electrofreezing effects can be attributed to the near-surface electric field alone. It should be noted that an electrical double layer (EDL)31 will also form at the surface. However, the capacitance of the nanometer-thick EDL is significantly higher than that of the dielectric layer; the overall interfacial capacitance is the same as the dielectric layer capacitance. It should also be noted that all experiments were conducted using dc voltages. The conditions of zero electric field inside the liquid and no current would be invalid for ac voltages. Figure 2a shows a schematic of the experimental setup. All experiments were conducted in a custom-made environmental chamber with humidity control. To obtain statistically significant data, each experiment was done with five static droplets (5 μL). For each test condition, the experiment was repeated at least twice (see Supporting Information). Infrared thermography and high-speed visualizations were used to detect the onset of freezing. A germanium window was added to the environmental chamber for IR visualization. More details regarding the experimental setup are provided in the Supporting Information. Figure 2b shows the cross-section of the devices used in the present experiments. Polished aluminum wafers that were 0.8 mm thick were covered with a thin dielectric layer. Kapton tape (25 μm thick) was added between the aluminum wafer and a liquid nitrogen cold plate to electrically isolate them. This setup could test five droplets simultaneously and used five copper electrodes (0.25 mm diameter) to bias the droplets. The aluminum wafer was grounded, and the droplets were biased (positive or negative) with a high-voltage dc power supply. The aluminum wafer was attached to the cold plate using thermal grease. An ammeter was added to measure current flow during the experiment. Two different dielectric layers were used in the present work. The first was 25 μm thick polyimide (Kapton film from DuPont), and the second was a CYTOP film (Asahi chemicals) with thicknesses ranging from 2 to 4 μm. It should be noted that the thermal resistance of these thin films is extremely small despite the poor thermal conductivity of polymers. The experiments involved depositing five deionized water droplets (5 μL) on the surface in contact with the prepositioned wire electrodes. It should be noted that the wires were used even for the control experiments (at 0 V). The chamber was then dehumidified with a nitrogen flush to eliminate moisture, which could condense or form frost. When the relative humidity decreased below 1%, the voltage was switched on and the plate temperature was ramped down at 5 °C/min until the onset of freezing. The time interval between closing the chamber door and the beginning of the temperature ramp down was less than 2 min. The IR camera and the high-speed camera detected the onset of freezing, which can also be seen visually. It should be noted that the evaporation-induced mass loss in the droplet was measured to be very small in the time interval over which these experiments were conducted. Figure 3 describes the methodology to detect ice nucleation by tracking the temperature of a droplet by an IR camera.32 The onset of nucleation is accompanied by a sudden temperature spike due to the latent heat release. The temperature stays uniform, while the freeze front propagates, and the entire droplet freezes, after which the frozen droplet cools to the substrate temperature. It should be noted that the IR camera measures the temperature at the top of the droplet, which is close to 0 °C during freezing, as reported in another experimental study.32 This measurement was used in a 1-D thermal model to estimate the solid−liquid interface temperature at which freezing occurred (Supporting Information). It should be noted that ice nucleation depends on and can be triggered by many factors including surface chemistry, presence of nucleating agents, and vibrations. The present experiments are
contact with either electrode. With this configuration, they observed a supercooling temperature increase of 1.6 °C at electric field strengths of 0.1 V/μm. However, this conclusion is based only on three measurements and does not capture the stochastic nature of nucleation. A similar but statistically significant study with over 100 measurements was conducted by Wilson et al.,16 which showed that the freezing temperature was unaffected at electric fields up to 0.05 V/μm. Similarly, Doolittle and Vali17 found no difference in nucleation rates in electric fields as high as 0.6 V/μm applied to droplets on a grounded plate, where a wire grid located above the droplets acted as the high-voltage, noncontact electrode. More recently, Orlowska et al.23 demonstrated that electric fields approaching 1 V/μm accelerate nucleation without current flow; notably, this electric field is one order of magnitude stronger than previous efforts. It should be noted that the maximum applied electric field reported in electrofreezing studies is 6 V/μm;23 this restriction emerges from the need to stay beyond the breakdown limit of the air gap in such experiments. Overall, there are significant unanswered questions on the mechanism of electrofreezing (electric-field induced or current-flow induced) and the influence of the electric field strength on the freezing temperature elevation. Additionally, it is likely that surface polarity plays an important role. Recently, Ehre et al.24 demonstrated that negative surface charges lower the freezing temperature, whereas positive surface charges elevate the freezing temperature. This study provides insights into various possible mechanisms underlying electrofreezing via statistically significant measurements of electrofreezing using infrared (IR) thermography and high-speed imaging. The focus of this work is an indepth study of surface electrofreezing, wherein the electric field in the interior of the liquid is zero, and the electric field is concentrated near the solid−fluid interface. Specific objectives of this work include (a) quantifying the electric field and freezing temperature relationship at ultrahigh field strengths exceeding 10 V/μm (more than one order of magnitude higher than in previous studies), (b) isolating the influence of current flow and chemical reactions on electrofreezing, and (c) studying the role of surface polarity.
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EXPERIMENTAL METHODS
The proposed work studies surface electrofreezing in the absence of electric fields within the water. This is achieved by applying an electrical potential difference across a dielectric layer with a water droplet on top of the dielectric layer (Figure 1). A thin wire electrode
Figure 1. Schematic illustrating the experiment to study surface electrofreezing under electrowetting fields. B
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Figure 2. Schematic of the experimental setup for the electrofreezing experiments.
Figure 3. (a) Illustrative temperature−time curve showing the onset of freezing, and (b) details of the temperature spike after the initiation of nucleation (due to latent heat release), followed by a constant temperature until the entire droplet freezes. targeted at isolating the effect of an electric field on electrofreezing. The experimental setup and procedure were therefore designed to keep other variables that affect nucleation constant in all experiments.
triggers, such as vibrations or air drafts. With increasing electric fields, the droplets freeze at higher temperatures. There is a sharp increase in the freezing temperature for electric fields up to 20 V/μm. However, the electrofreezing effect saturates afterward, and stronger electric fields result in only marginal increases in the freezing temperature. Quantitatively, the electrofreezing induced freezing temperature elevation was more than 15 °C, which is significant. It should be noted that the magnitude of the electrofreezing effect will also depend on geometry and fluid volumes; the present work cannot directly predict the freezing temperature for all electrofreezing situations. It is important to note that the effect of polarity on electrofreezing was not very significant. Below 10 V/μm electric fields, positive surface voltages resulted in greater temperature elevations than negative voltages. However, this appears to be a secondary effect; both polarities display the same trend and nearly the same electrofreezing temperatures at higher electric fields. The nature and chemistry of the ions at the surface will definitely affect the local energy barrier for nucleation; however, the present results suggest that polarity and surface chemistrybased factors are less important than the electric field strength. Very high electric fields (80 V/μm) were used in these experiments; these are an order of magnitude higher than in any other study. These fields were still below the breakdown strength of the dielectric material, which is ∼300 V/μm for polyimide. Another notable aspect of this work is that every sample was used only for a single experiment. It is well known that dielectric layers can degrade or trap charges with repeated use at high voltages.25 To avoid confounding the present results with such phenomena, it was decided to use a dielectric layer for one experiment only. It should also be noted that the
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RESULTS Figure 4 shows the electrofreezing curve (freezing temperature versus the electric field) for the polyimide film. It should be
Figure 4. Electrofreezing curve for polyimide film. The electrofreezing effect saturates for fields stronger than 20 V/μm.
noted that each data point is the average of at least 10 measurements. The error bars represent the 95% confidence intervals calculated using the t distribution (Supporting Information). It is clearly seen that interfacial electric fields and surface charge can elevate the freezing temperature in the absence of current flow. In the control experiments (in the absence of an electric field with same surface chemistry, environment, and humidity), the DI water droplets freeze at approximately −30 °C. This is much lower than the thermodynamic freezing point due to the absence of nucleation C
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ROLE OF ELECTRIC CURRENTS ON ELECTROFREEZING This section presents experiments that isolate the influence of electric currents on electrofreezing. To qualitatively validate the polyimide layer results, similar experiments were performed with CYTOP (http://www.agcce.com/index.asp) films. CYTOP is a commercially available amorphous fluoropolymer. The dielectric strength of CYTOP is 90 V/μm, and the highest electric field used was 70 V/μm. Figure 5 shows the
footprint area of the droplet will change after the application of the voltage. However, this area increase occurs before the droplet freezes and will not affect the electrofreezing trends. More insights into electrofreezing emerge from high-speed visualizations. The Supporting Information contains high-speed visualizations (4000 fps) of droplets freezing under an electric field (Video 1). Frame A shows a droplet under no electric field, and Frame B shows a droplet under an electric field of 70 V/μm. It is seen that nucleation always initiates at the threephase contact line. This is expected because it is the preferred nucleation site in the absence of electrical voltages. The sharp liquid wedge near the three-phase line concentrates the electric field and further promotes nucleation at that location. The freezing process consists of two phases and begins with a rapid phase, wherein the entire droplet converts to a gel-like state in 15 °C) by controlling the electric field. However, this freezing temperature elevation eventually saturates at high electric fields (>20 V/μm). In the absence of current flow, the electrofreezing mechanism can be attributed to the reduced nucleation activation energy upon application of an electric field. Heterogeneous electrofreezing can also be triggered by current flow through water; in such cases, the freezing temperatures are higher. The physical mechanisms for electrofreezing in this case are more complex and include the effects of an electric field plus the effects of bubbles, which act as nucleation sites.
* Supporting Information S
Detailed information about sample preparation and procedures, calculations, and error analysis; two videos showing electrofreezing kinetics and bubble generation due to current flow and chemical reactions. This material is available free of charge via the Internet at http://pubs.acs.org.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. E
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