Fundamental study on adhesion of ice to cooling solid surface

Many technological troubles are caused by ice adhesion to a cooling solid wall (surface). Therefore, it is urgent to clarify a mechanism of ice adhesion.It is thought that ice adhesion to the cooling wall is governed by heat transfer and interfacial phenomena between ice and the wall. In this study, shearing stress corresponding to adhesion force per unit area to remove ice from the wall surface and some reagents' contact angles on the wall and ice were measured, varying the wall material and its surface state. Moreover, shearing work to remove ice from the wall surface and surface energies of the wall and ice were calculated by the shearing stress and contact angles, respectively. And adhesion energy at an interface between ice and the wall was also calculated by the calculated surface energies. And then, influence of heat transfer and interfacial phenomena on ice adhesion was discussed to clarify the mechanism of ice adhesion.     

Measurement on nano scale by scanning probe microscope for obtaining real ice adhesion force

It is important to know a real ice adhesion force on a cooling solid surface. When an ice adhesion force is measured by giving a shearing force at the interface between the solid surface and ice, there is a possibility that a measured ice adhesion force is an apparent value including a force that destroys ice due to unevenness of the surface. Thus, to measure the ice adhesion force without influence of the surface unevenness, one of the authors developed a method for measuring the ice adhesion force on the nano scale by using a scanning probe microscope. In this paper, ice adhesion forces to copper oxide and hard glass test plates were measured at −5 °C on the nano scale by this method, and the real ice adhesion forces could be measured. Moreover, the representative value of proper shearing stresses obtained by real ice adhesion forces divided each ice adhesion area was given.     

Suppression of ice adhesion force to cooling wall due to voltage application

For ice storage, one of the authors has previously reported on the ice slurry formed by cooling water–silicone oil mixture with stirring. When the mixture is stirred in a vessel, the oil is charged due to static electricity. If the vessel can attract the charged oil, suppression of ice adhesion force to a cooling wall may be possible. In this study, therefore, a certain voltage was applied to the vessel filled with the mixture with cooling and stirring simultaneously, and water was frozen in the vessel. Then, the ice adhesion force to the cooling vessel wall was measured under a constant apparent adhesion area between the ice and cooling vessel wall. From the measurement results, optimal conditions of the oil viscosity, rotation speed and applied voltage to suppress the ice adhesion force effectively were clarified. Moreover, the factors governing suppression of the ice adhesion force were clarified.     

Formation of high performance ice slurry by W/O emulsion in ice storage (effective method to propagate supercooling dissolution)

This study focused ice slurry formation in an ice storage system using W/O emulsions with 70 and 80% water contents. Emulsions consisted of a silicone oil–water mixture with a small amount of amino-group-modified silicone oil additive. Ice slurry was formed by cooling the emulsion without ice adhesion to the cooling wall, as water in the emulsion did not directly contact the cooling wall. As the structure of W/O emulsion slowed the propagation rate of supercooling dissolution, voltage and ultrasonic wave were applied to the W/O emulsion to propagate dissolution more quickly and decrease maximum supercooling degree, respectively. Thus, the effects of voltage and ultrasonic wave applications on propagation rate were clarified.     

Investigation of method for measuring adhesion force of ice in nano/micro scale by using SPM

Ice adhesion to a cooling solid surface occurs in many situations, and it causes many serious problems that can lead to economic losses. Therefore, it is necessary to clarify the mechanism of ice adhesion to a cooling surface. In past studies, the adhesion force of ice was reported to be strongly governed by the surface energy of the cooling solid surface. In consideration of this surface energy, it is essential to investigate the ice adhesion force on a nano/micro scale. However, very little research has been conducted in nano/micro scale. Thus, in this paper, by using a scanning probe microscope (SPM), the methods for measuring the ice adhesion force and contact area between the cooling solid surface and formed ice are proposed, and the optimal measurement conditions of the SPM are determined. Then, the validity and effectiveness of the method are clarified.     

Continuous ice slurry formation by using W/O emulsion with higher water content

A W/O (water-in-oil) emulsion was made from a water–lamp oil mixture with higher water content and a small amount of an additive of amino group-modified silicone oil, and the emulsion could be changed into an ice slurry by cooling with stirring. By using a new continuous ice formation system proposed by one of the authors of this paper, the ice slurry could be formed continuously and stably in an ice formation vessel made of stainless steel. From the experimental results, the conditions were clarified for realizing continuous ice formation for 10 h without ice adhesion to the cooling wall. Moreover, in order to propagate supercooling dissolution of the emulsion effectively and to decrease viscosity in the ice slurry, voltages were applied to the emulsion and ice slurry formed, respectively, and it was clarified that the voltage impression was effective for both.     

Fundamental study on adhesion of ice to solid surface: Discussion on coupling of nano-scale field with macro-scale field

In a previous study, it was reported that the removal process (behavior) of ice from a cooling solid surface depended on the characteristic of the solid surface, and that the ice adhesion force to the cooling surface was mainly governed by the surface energy of the surface. In the present study, using the method of surface analysis, the ice removal process and the ice adhesion force clarified in a macro-scale field were also discussed in a nano-scale field to couple these scale fields. Simultaneously, another surface analysis of the solid surface was carried out to identify the distributions of hydrophilic and hydrophobic groups on a solid surface. Attempts to couple the nano-scale field with the macro-scale field revealed that the results for both scale fields were in approximate qualitative agreement.     

Influences of number of hydroxyl groups and cooling solid surface temperature on ice adhesion force

It is important to quantitatively clarify factors governing ice adhesion force to a cooling solid surface. Thus, one of the authors considered hydrogen bonding between hydroxyl groups of the ice and solid surface. Several additives with different numbers of hydroxyl groups were selected. The ice adhesion force of ice which is made from an additive–pure water mixture to a cooling hard glass surface was measured at a fixed surface temperature. Similar measurements were conducted with various molar-concentrations of several additives and with surface temperatures. Since the hydrogen bonding on the surface occurs after adsorption of the additive and ice adhesion force depends on surface energy of ice, measurements of the amounts of additives adsorbed to the SiO₂ and the surface energy were similarly conducted. And influences of the number of hydroxyl groups in the adsorbed additive and the surface on ice adhesion force to the glass surface were quantitatively clarified.     

Continuous ice slurry formation using a functional fluid for ice storage

A functional fluid composed of an oil–water mixture with an additive is transformed into an ice slurry by cooling while stirring. This paper describes a new continuous ice slurry formation method. Experiments were carried out by varying conditions such as the supply time of functional fluid, the stirrer torque, brine temperature and degree of supercooling. As a result, the characteristics of the ice formation and recovery processes were clarified. It was found that the ice particles gradually became uniform in size and spherical, and grew to 3.5 mm in diameter during about 10 h. The factors influencing the size of formed ice particles were discussed because the larger ice particles were expected to melt more rapidly. The ice particle size was found to increase with decreasing degree of supercooling and cooling rate, and with increasing stirrer wing diameter.     

Measurements of correct ice adhesion forces to metal test plates in nano-scale by using SPM

To prevent many problems caused by ice adhesion, its control technology is required. To settle this technology, obtaining the correct ice adhesion force is a key point. However, ice adhesion forces obtained by current conventional methods are apparent values including a force breaking ice. Thus, authors developed a method to measure correct ice adhesion force and accurate ice diameter in a nano-scale by SPM and both for copper were measured. To construct a database to store the correct ice adhesion forces and diameters for various metals, first, the ice adhesion forces and the ice diameters for aluminum, titanium, carbon steel, and stainless steel are measured at −5°C by the SPM. Additionally, to easily compare our results with other works, shearing stresses of the metals are also given by the ice adhesion forces and ice adhesion areas. Validities of our results are proven by comparing them with the surface energies and other works.     

Active control of ice adhesion force to copper surface by varying surfactant concentrations

Adhesion of ice to a solid surface has been investigated by many researchers including one of authors. While, authors group clarified that it was possible to actively control a supercooling degree due to adsorption of surfactant molecules to a solid surface by varying surfactant concentrations. In this study, control of the ice adhesion force through the adsorption to a copper surface and its active control by varying surfactant concentrations are investigated. A surfactant-pure water mixture is frozen on a copper surface at a fixed surface temperature, and the adhesion forces are measured by a SPM, varying surfactant concentrations. While, the amounts of surfactant adsorbed to a copper surface are also measured by a QCM, varying surfactant concentrations. The possibility of active control of the adhesion force is evaluated based on the relationship between the ice adhesion force and the amount of adsorbed surfactant.     

Study on continuous ice slurry formation using functional fluid for ice storage – Discussion of optimal operating conditions

A functional fluid was made by adding a small amount of additive to a water–silicone-oil mixture with 90 vol% water content, and the functional fluid was transformed into an ice slurry by cooling while stirring. The new ice formation system, which authors proposed for ice storage based on the results of previous studies, demonstrated that the ice slurry could be formed continuously for 10 h. In the current paper, experiments were carried out, varying operating conditions, and an optimal operating condition was determined to improve performance of the present system still more. From the experimental results, the conditions necessary to increase the amount of recovery ice were clarified. The time-dependencies of the shape and size of formed ice particles were also shown. Moreover, the reason why the freezing temperature of the functional fluid rose due to repetition of ice formation was clarified, and its measure was discussed. The present study then found that it was possible to form and recover a larger amount of ice than in previous attempts, given the rise in freezing temperature.     

Study on prevention of ice adhesion to cooling wall due to voltage impression in ice storage—discussion on possibility of attraction of oil to wall

For ice storage, one of authors has studied new ice slurry formed by cooling a water–oil mixture with stirring. When the mixture is stirred in a vessel, oil is charged by static electricity due to friction. If the vessel wall can attract charged oil, prevention of ice adhesion to the wall may be realized. Therefore, in this paper, in order to observe behavior of charged water–oil droplet or mixture in electric field by a high speed camera or video camera, two types of experiments were carried out. One was that the water–oil droplet charged by static electricity was made to fall plumb down between two electrodes with electric field or without electric field, varying the water content of droplet. The other was that a constant voltage was applied on the vessel filled with the water–oil mixture stirred. From experiments, it was confirmed that attracting force between the charged wall (electrode) and charged oil acted.     

Ice storage system using water–oil mixture Discussion about influence of additive on ice formation process

Ice storage is one technique for effective use of thermal energy. So, many studies on slush ice as a thermal storage material have been done. We have also been studying a suspension (slush ice) made from an oil-water mixture by cooling and stirring. From our study results, it was found that an additive having both an amino group (-NH₂) and a silanol group (-SiOH) was essential to form a suspension with high IPF without adhesion of ice to the cooling wall. Moreover, ice particles formed in the suspension were dispersed and granular, and did not stick to each other. In the present paper, we carried out experiments to clarify the characteristics of the suspension formation process. From a thermal analysis of the substance formed in the suspension by difference scanning calorimeter (DSC), it was found that the substance was not ice but a compound of ice and additive. Then, at a very small depression of freezing point (about 7°C) all water in the mixture could be frozen by using the additive.     

Non-uniform melting in packed beds of fine ice slurry

The melting process of packed beds of ice slurry consisting of aqueous solution and ice particles with a mean diameter of 0.2 mm, so-called liquid ice, was studied. The packed ice layer was melted with aqueous solution sprayed on top of it directly and indirectly. Experiments of the melting process were done by changing the mass of ice, the inlet temperature and the flow rate. The time history of the outlet temperature of the bed was measured and the melting phenomenon was observed. During the melting, it was found that passages through the ice layer were formed and the aqueous solution sprayed on the top of the ice layer hardly permeated the ice layer, but flowed through the passages. Assuming that the flow rate of the solution through the passages was known, the melting process was analyzed. In the case of direct spraying, the effects on the outlet temperature and the formation of passage were clarified by changing the initial mass of ice, the flow rate and the inlet temperature. When the solution was sprayed on top of the ice layer directly (direct spraying), the outlet temperature was kept constant because the heat exchange was enhanced by the mixing effects of spraying. On the contrary, when an obstacle was placed on top of the ice layer (indirect spraying), the outlet temperature was higher than that in the case of direct spraying and was not kept constant.     

An experimental study on ice formation around horizontal long tubes

The results of an experimental study are presented where the growth rate of ice on the outside of cooled copper tubes was studied. The tubes, which were immersed in water in an insulated vessel, were internally cooled by circulating glycol through them.It was found that axial growth rate of ice is distinct at low values of the coolant Reynolds number and short freezing times. The slope of the ice thickness with axial distance showed moderate dependency on time but varied with coolant flow rate, and with Stanton and Biot numbers.A key result from the experiments is the abrupt ice thickness enlargements on the surface of tube bends. This anomaly may be attributed to internal flow disturbances of the coolant, and creation of local eddies inside the bends that enhance growth of ice. The effect was evident for a low Reynolds number (Re = 251.9 and Bi < 1), and fades out for large Reynolds number flows.     

Temperature-dependency on adhesion force of ice made from surfactant–pure water mixture to copper surface

Since ice adhesion force to a cooling solid surface causes various problems, its reduction is important. Accordingly, after studying reduction methods from various viewpoints, the authors herein have focused attention on adsorption of the hydrophobic groups of surfactant molecules to a copper surface. And, the adhesion forces of ice made from the surfactant-mixtures at a fixed concentration to the copper surface at various temperatures were measured by the scanning probe microscopy. Simultaneously, adsorbed amounts of the surfactant molecules to the copper surface were measured by the quartz crystal microbalance system with varying surfactant mixture temperatures. Furthermore, the ice adhesion force depends on surface energy of ice, and the surface energy also depends on temperature. So, surface energies of ice were also measured with varying temperatures. And the influence of temperature-dependency of the surface energy on the surface temperature-dependency of the ice adhesion force was clarified considering the adsorbed amount of surfactant.     

Force estimation of mechanical removing processes of mushy structure in an aqueous solution

In a system of producing liquid ice with mechanical removing processes of mushy structure in an aqueous solution, a solid layer as thermal resistance does not stack up on a cooling surface. It prevents refrigerator's COP dropping and also prevents ice production rate dropping. However, some extra energy is needed for mechanical removing processes of the mushy structure from cooling surfaces. In this paper, the required force for removing the mushy structure is expressed as a function of experimental conditions based on bending-beam theory. The mushy structure is regarded as an elastic body. The estimated values of modulus of elasticity and values of adhesion force are reasonable compared with the observed results. Parameters that affect on mode difference, maximum force and steady force are shown.     

Nucleation of supercooled water on solid surfaces

Heterogeneous nucleation of water was investigated using molecular dynamics simulation. Solid with fcc (111) surface was placed at the bottom of a cell consisting of 864 water molecules. ST2 model with NPT ensemble was used. The pressure and temperature were set to be 0.1 MPa and 275 K, respectively. The interaction between water and the solid was based on the equations proposed by Spohr. Exception was made on the lattice constant which was slightly modified to fit with that for ice structure. The shape of the solid surface was considered. It was found that the only one layer of water molecules was adsorbed in a case of a flat surface, whereas ice nucleation occurred by removing some of the atoms from the surface. Spohr's interaction was also modified so that the dipole moment of water became anti-ferroelectric. It was found that the modification increased the ice growth, further. The effect of lattice constant of solid on nucleation was also investigated. It was found that the variation on lattice constant with a few percent from that of ice was acceptable for nucleation, especially on shrinking side. On expanding side, however, it gave some gaps for water molecules to fit in other than that for ice structure, and it prevented the growth of ice. Hence, a guideline for the selection of ice nucleus material was obtained.     

Momentum transfer of ice slurry flows in tubes, modeling

In this paper we present results of the studies of ice slurry flow in horizontal tubes. The possibility of treating the rheological parameters of ice slurry as being those of Bingham fluid was confirmed. The values of parameters (mass fraction, flow velocity) corresponding to the laminar, intermediate and turbulent flow were determined which permits to optimize the flow in the systems working with this cooling agent. Critical flow velocity and mass fraction of ice values were determined thereby; they correspond to a change in character of an ice slurry flow from a laminar to turbulent motion. Experimental results were compared to the analytical results, based on the Hedström and Tomita algorithms (the laminar and turbulent flow, respectively). The comparison showed a very good agreement between these data.