The Research On Foam Evolution Dynamics Under Magnetic Gravity Compensation

With the rapid development and wild application of material science, demands for new functional materials have become more and more urgent. Among them, porous foam materials, which possess properties of “green-energy, light-weight and high-performance”, have attracted lots of attention all over the world. During the preparation of foam materials, foam evolution dynamics, which exists throughout the whole process, has played a vitally important role. However, in the current industrial process of improving materials’ performances, most of the attention was paid to the manufacturing equipment and technological parameters, so the evolutionary dynamics has not been systematically researched and controlled. Meanwhile, at the theoretical level, the study of foam dynamics mechanism can help to settle the space-filling problem and figure out the structure evolution rules in other physical systems.Based on the present study, we take the aqueous foam and magnetic foam as the research object, and experimentally analyze several key scientific problems, including the foam stability, coarsening mechanism and structure evolution law. Whereafter, the “Magnetic Gravity Compensation” method, which we have newly designed, was applied to further study the evolutionary dynamics in magnetic foams.In the first part, we design a brand new method to carry out Magnetic Gravity Compensation, which is realized by combining the magnetic fluid and two pair Helmholtz-Maxwell coils. After that, we analyze the distribution of magnetic field and the gravity level of magnetic fluid respectively.At first, a finite element model and precise teslameter was used to measure the magnitude of magnetic field separately. The results show that the field distribution is linear along the vertical direction, and uniform in the center area. Also, the experiment are consistent with the numerical simulation accurately. Subsequently, by calculating the gravity compensation level of magnetic fluid, we found that a uniform gravity compensation region, which consists a cylindrical area with diameter of 60 mm and height of 60 mm, exists in the center of the external coils. And the gravity could be totally compensated while the current in Maxwell coil reached a specific value.In the second part, we set the surfactant concentration and gravity compensation level as experimental variables, and quantitatively analyze the foaming ability as well as foam stability. All the experiments were carried out in the same device which we designed to test the foam performances.During certain concentration range, the foam performance was significantly improved by adding surfactant agent, and gradually reaching the saturation state. After that, excess additive agent would cause overmuch surfactant molecules attaching to the foam membrane, thus lowering the foam stability instead. Simultaneously, we further analyze the foam performances by means of magnetic gravity compensation. The results show that the liquid fraction increases by applying the magnetic field, this is due to the restraining of drainage and thus foam imbibition process worked. Also, with the progressive enhancement of gravity compensation level, foam stability was markedly improved, this result indicates that the foam decay was suppressed by reason of the increasing liquid phase.In the third part, we keep track of the coarsening dynamics process in two dimensional(2D) foams over several hours, and analyze two fundamental dynamics mechanism, which refers to the topological evolution and bubble growth rule.On one hand, two topological process could be observed. Firstly, there is the neighbor-switching process, which we shall call a T1 process. It can be pictured as taking place as an edge shrinks to zero, to be replaced by another one in such a way that connections to vertices are rearranged. This justifies the statement in Plateau’s Law, which points out that only vertices with 3(in 2D)are topologically stable. Secondly, bubbles(with 3,4,5 sides) may vanish gradually along with the coarsening process——the T2 process. We confine this to the vanishing of a three-sided cell, while a cell with more than three sides can vanish through a series of T1 processes to make it three-sided, followed by a T2 process.On the other hand, on both normal gravity and magnetic gravity compensation conditions, the 2D bubble growth rule conforms well to the Von Neumann’s Law. Meanwhile, the growth of magnetic bubbles was restrained under magnetic gravity compensation, this is due to the properties of liquid membrane was changed with the external magnetic field, as a consequence of making the foam itself more stable.In the fourth part, a numerical method was used to simulate the evolutionary process of 2D monodisperse foam structure, so as to provide deeper insight to the formation mechanism of six-sided honeycomb, and directly guide the preparation technology of honeycomb core.Firstly, we use the two-phase system composed of mercury and air to simulate the 2D monodisperse foam, the results show that six-sided honeycomb can be constructed through a liquid equilibrium process, this is the energy minimization process coupled with the effect of surface tension. Whereafter, by separately analyzing the foam patterns at different moments, we find that the gap between the bubbles decreased gradually under the action of a certain force caused by the membrane junctions. While the gap disappear completely, the six-sided pattern was achieved in the meantime.