RHEO-COOLING IS…COOL!

RHEO-COOLING  IS…COOL!

        "Rheo-cooling" stems from the idea to use Rheology to add another dimension to the process of cooling.  For example, a liquid becomes a solid if it is cooled below its solidification temperature: water becomes ice below its freezing temperature.  Ice can be shaped into ice cubes by cooling water which has been poured into an ice compartment.  This is really the basis of any molding process: take a liquid, pump it into mold cavities, cool it until it solidifies, and then eject it from the mold; an object with a given shape has been produced.

      I want to demonstrate Rheo-cooling with an example: the solidification of one liter of water contained in a round flask in about 5 seconds! I actually made the experiment and it worked. The video below is amazing to watch and illustrates the idea.

you can also watch it on YouTube by clicking the following link:
Rheo-cooling 1 l of water in a round flask in less than 5 seconds!
      
     In the process of cooling a liquid into a solid using a mold, the mold is really used for two purposes: the mold shapes the object, but it also cools the liquid by heat transfer from the mold surfaces until the liquid becomes a solid.  The classical method to cool or to heat an object is to place it in contact with a cooler or warmer surface.  Most modern molding technologies apply this process of cooling by conduction via surface contact.  Calories of heat are transferred out of the warm surfaces to the cold ones, and in that process the warm liquid, a plastic for instance, becomes a solid by conductive cooling, that is, by transfer of calories. In the bulk volume of the part being cooled, convection cooling occurs. Although it is function of the conductive cooling taking place at the surfaces of the part, convective cooling is generally very slow.

      It should be emphasized that there is a considerable advantage in cooling a material rapidly.  The technology is called "quenching". For example, when water is cooled quickly, as in a freezer, to produce ice cubes, it creates a translucid solid, the ice cube.  Snow is another form of water solidification, and of course the crystals appear quite differently in snow than in the solid water obtained as an ice cube.  This is because the rain drops have had time to fall through several layers of cold atmospheres where the cooling process is much slower than in a deep freezer.

      As a general rule, the rate of cooling materials from the liquid temperature to the solid temperature has a tremendous effect on the material's morphology, the structure, and therefore on the properties of the solid which is formed.  For example, a snow ball and an ice cube don't have the same mechanical resistance.  All the properties of solids are influenced by the rate of cooling. 

    If one were capable of cooling materials extremely quickly, say at an infinite rate, it would be possible to preserve in a solid the properties of the liquid state, that is to say one would get the same homogeneity for the solid as for the liquid.  To illustrate this point further, one should visualize a metal without a single crystal, a metallic glass, it would have novel and extraordinary properties: no more corrosion around the spherulites, and new electrical and magnetic properties.  The implications of a bulk super-quenching process opens up a completely new era of material performance.

      Rheo-cooling was born to circumvent some of the limitations of conductive/convective cooling.

The difficulty in conventional heat transfer by conduction is that it is a relatively time consuming process, unless one works with highly conductive materials, like metals, or with very thin samples, as in the splat cooling super-quench technologies.  Plastics have a large heat capacity and a bad conductivity, two factors which do not favor heat transfer.  So one problem with plastic materials is that if one wants to cool them quickly one must work with films.  Furthermore, it is very difficult to apply very fast cooling treatments, even to thin films, because of the poor efficiency of the process of transferring calories by conduction for plastic materials.

     Cooling by conduction/convection yields inhomogeneous and anisotropic morphologies.  For example, if one takes an ingot of steel which has started to cool, the core of the ingot is still hot when the layers near the edges are solid as a result of contact with the ambient air.  So the whole ingot, which, when cold, represents the whole object, has parts which have solidified before others.  Some crystals have had more time to grow than others, because the longer they stay at a given temperature, the more they grow.  As a consequence, there is a distribution of the sizes of the spherulites inside the structure, resulting in internal stresses, gradients of some sort, and microcracks around the spherulites' boundaries, yielding an object which has weak resistance to corrosion and mechanical stress.

    In the case of water in the video, I used the fact that the temperature of solidification, Tc, was a function of the concentration of CO₂ dissolved in it. Even with only 2 bars of CO₂ on top of a closed volume of water, and a good shake to dissolve the CO₂ in the liquid, I could depreciate Tc by about 3 to 4 ^oC. In other words, Tc was no longer 0 ^oC (the freezing temperature for water), but -3.5 ^oC.  In the video you see, at the beginning, two handheld thermometers, one reading the temperature of the refrigerant liquid, outside the flask   (-6.5 ^oC), and one reading the temperature of the liquid inside the flask, the one with the dissolved CO₂ ( -2.5 ^oC). The water is “plasticized” by the amount of dissolved CO₂, which is function of the pressure of CO₂, which is read via a manometer. Rheo-cooling, in this case, consists of opening the valve of CO₂ to the atmospheric pressure, which “instantaneously” releases the gas from the top of the liquid, raises the Tc of solidification from -3.5 ^oC to 0 ^oC, i.e. above the temperature of the liquid, -2.5 ^oC, so that now, at each point of the liquid in the flask, the solidification should thermodynamically occur. In the video, we observe that the wave front of solidification diffuses down very quickly, starting at the surface between the liquid and the CO₂ , as pressure drops to atmospheric pressure. 

      The initial state of the water before the release of pressure was a true liquid, as we can see in the video from the motion of the liquid surface when the technician raised the flask outside the refrigerant bath as the flask was tilted before it stood still. We clearly see crystallization occurring everywhere and at all at once throughout the entire volume of water. After about 5 seconds, the ice has frozen the entire flask and we observe a solidified immobile translucent material, not a transparent liquid. One also sees, in the video, that during solidification the heat of crystallization “instantaneously” heats up the material which is solidifying. The final temperature was below 0, so that the ice remained stable at the end. Note that many parameters can be changed during the Rheo-cooling procedure: the refrigerant temperature (we used a mixture of ethylene glycol and water, and a cooling coil was immerged in the bath), the efficiency of the shaking method to dissolve the CO₂ in the water, the use of vacuum to jump-pump the gas out to trigger Rheo-cooling etc. In summary, what we did in this example is show that solidification can occur by raising the value of the phase transition, here the solidification temperature. Normally, solidification is induced by lowering the actual temperature of the material across the solidification temperature, which remains constant. 

      There is a famous sequence in the film “le Bossu” where chevalier Laguardere (Jean Marais) warns his enemy by saying :” Si tu ne vas pas a Laguardere, Laguardere ira a toi”. Strangely enough, I always think of that sentence when I consider how to decrease (T-Tc), Tc being Laguardere and T being the enemy. Solidification occurs when they see each other and fight (T=Tc)!.

  

      Through this video illustrating how the concept of Rheo-cooling works, one sees that Rheo-cooling could be adapted to other situations, other materials, making use of other parameters to vary the value of phase transition, yet it may always be considered as a new powerful way of controlling the state and the microstructure of materials through the simulation of cooling rates across transitions, providing effective cooling rate values much beyond the values obtainable by conductive cooling.

     Suppose that a substantial enhancement of a property by the application of a rapid cooling rate were possible, Rheo-cooling may add a new degree of freedom to enhance cooling rate effects.  This could be especially interesting if this technology could be applied to bulky or thick materials. In particular, the technique could produce more homogeneous products after cooling through large thicknesses.

      Cooling by conductive transfer of calories often does not yield high enough quench rates to substantially modify the physical behavior.  With poor thermal conductors, such as plastics, it is impossible to predict what kind of property improvements would result if it were possible to apply such quench treatments.  

       In my next blog I will show another movie to illustrate how to apply Rheo-cooling to plastic molding and discuss what kind of improvement benefits we can obtain.