|Ph.D Student||Fakher Eldeen Eman|
|Subject||Self-Assembly of Rigid Particles at a Liquid/Gas|
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Abraham Marmur|
|Full Thesis text|
Self-assembly is a process wherein disordered systems are organized spontaneously by internal interactions between the components that are affected by the surrounding conditions, without any external direction. It is a phenomenon that exists in various scales and forms in our surroundings; for example, it can be seen in the formation of material structures, such as crystals and colloids, in biological structures, such as in the formation of lipid bilayers, the folding of polypeptide chains into proteins, and the folding of nucleic acids into their functional forms, and even in the formation of planets and stars.
Self-assembly has attracted considerable attention in the field of micro-fabrication, because of the simplicity of this process and the wide opportunities it offers to build sophisticated structures from very small components that are too small to be manipulated robotically. One of the main topics in this field is the aggregation of rigid particles at liquid/gas interfaces. Because of the thin patterns that are obtained in this process, it has many industrial potential applications.
In this study, we analyzed the behavior of two identical and small boxes at the water/air interface in a two-dimensional (2D) system, For two different hydrophobic contact angles, 95̊ and 110̊. The results show that, when two semi-infinite horizontal boxes float at the liquid/fluid interface, they bend the interface surrounding them and as a result capillary forces arise that lead to the mutual attraction of the boxes, even when they are located at a long distance from each other. As the boxes approach each other, by satisfying the balance of vertical forces, the total horizontal forces between them increase, while at some point the moments’ balance is violated and the two boxes incline, when their inner side rises, with a slope angle, β. In this case, it was found that for a specific distance between the two boxes, the balance of the vertical forces may be satisfied for various values of β, but not the moment balance, which indicates that there may be several quasi-equilibrium states along the path to complete assembly. In order to determine the most stable states, it was decided to focus in this study on the minimum moment for each specific distance. It was found that at long distances the boxes are at their horizontal state and as they approach one another they are inclined and rotate around their center of mass; they attach at their inner lower corner and do not rotate down to their horizontal state. For a contact angle of 95̊ and for small masses, 0.002 g/mm, the total moment is not balanced, which indicates that the system is dynamic and not stable. However, for a contact angle of 110̊ and for larger masses, 0.0096 g/mm, it was obtained that the balance of moment is satisfied, and yet the boxes do not rotate to their horizontal state. It can be concluded that maybe there is a need to take into account other interactions that have been neglected in the calculations for such small systems, where small change in system causes to fundamental changes in boxes' behavior .