Fabrication And Dynamic Motions Of Asymmetric Particles

With the development of nanoµ fabrication, people start to seek the uniquebehavior of materials at micro, or even nanoscale. Yet for years the spotlight was onthe static state of those properties. The dynamics of microparticles remain unexplored,restrained by the limited dynamic resolution. But revealing the dynamic ofmico/nano-particles is one of the keys to future research. It not only depicts themechanism of nucleation and growth of nanoparticles, but also offers the possibilityof energy transfer within microscale, bringing microfactory and nanorobot to the table.Of all the systems to dig into this topic, electrohydrodynamics shows greater potential.Electrohydrodynamics is to use extra electric field as working horse to drive thedielectric fluid and colloidal particles inside it to move. This is the most effectivemethod to explore the dynamics, because electrohydrodynamics can influence bothmotions of the fluid and the particles.Yet despite all those advantages listed above, there are questions remained in thisarea. Electrohydrodynamic motions of colloidal particles still lack of experiments andthe theoretical conclusions need improvement. Answering these questions can bringmore possibilities to this area.Under this consideration, we choose colloidal particles with well-definedasymmetric features as observed objects to reveal the deep principle ofelectrohydrodynamic of certain particles and the fluid around them. First to concern isto fabricate asymmetric colloidal particles as we design with high monodispersity,well-defined shape and modification.In chapter two, we focused on the design and fabrication of asymmetric colloidalparticles with controlled fashion. Starting from colloidal crystal monolayer, we firstlydeveloped angled colloidal lithography to modify the upper hemisphere of colloidal spheres. A15~80%coverage range was achieved as we control the lattice space ofcolloidal crystals, incidence angle, and azimuthal angle of metal vapor. Then in orderto expend our asymmetric particles to wider modifying materials we introduced themasking/de-masking strategy coupled with colloidal lithographic strategy. Usingpolymer film to cover the colloidal crystal and reactive ion etching (RIE) to controlthe exposed area, we successfully grafted polymer brushes onto the exposed pole ofcolloidal sphere through surface-initiated atom-transfer radical polymerization(SI-ATRP). Then we further developed a brand new strategy that can modify bothpoles of colloidal spheres. Using colloidal spheres as mask to etch the substrate andthen subject the samples to thermal treatment we created the binary asymmetricparticles with modified flat pole at lower hemisphere; then a simple depositionprocess will form the ternary asymmetric particles as we designed. Last but not least,for the first time colloidal crystal mask with asymmetric feature was created just byaltering the working gas feed in RIE treatment. This is the simplest strategy to createasymmetric feature to the masks and effectively helps us broaden our asymmetricparticles to non-spherical ones. Furthermore, combining both the symmetric andasymmetric colloidal crystal masks we successfully fabricated sandwichedasymmetric particles, which is a typical building block for metamaterial. These variedfabricating strategies bring a broader future to asymmetric particles and mostimportantly, they present the possibility in searching their dynamic motions inelectrohydrodynamic system.In chapter three, we start from the basis of electrohydrodynamics. The simplestobject in dynamic of asymmetric particles is the Janus spheres. AC electric field alongvertical direction was used here. Experimental results show that once the Janusspheres were introduced into DI-water and electric field applied, the Janus sphereswill re-orient their symmetric axis perpendicular to the field direction and then movestraight with their polystyrene end forward. The speed and direction of Janus spherescan be controlled by altering the frequency of extra electric field, revealing twocompeting parties to drive the Janus sphere—induced-charge electro-osmosis andelectrohydrodynamic flow along the electrode. Under lower frequency we found theJanus spheres experiencing assembling, fusion, reconfiguration, re-location,de-assembling and re-assembling motions, indicating that this is by nature a typical“active colloidal crystal” system. With these motions we can seek the philosophical question that how alive can a non-living system be and how far away are we fromartificial intelligence.In chapter four, more complicated electrohydrodynamic system was investigated.Thanks to our deep acquisition in fabricating asymmetric nanostructures, we manageto fabricate a series of asymmetric colloidal particles with complex componentswithin “one pot”. Also, starting from colloidal crystal monolayer, we use RIEtreatment to fabricate the classic “sphere-on-cone” structure arrays. Followed byphysical vapor deposition, the asymmetric particles were selectively coated withmetal. Then selectively disperse part of the resulting structure we will obtain Janusspheres, or truncated cones, or halma pawns. They are combinable both in shape andmodifying fashion. Then we also choose vertical AC electric field to drive the particlemotions and interesting conclusions were revealed. We found out that the truncatedcones will rotate without horizontal motions, despite the shape asymmetric axis ispointing to the vertical direction. We drew the conclusion that thickness gradientcoating the truncated cones played the vital part that forces the particle to rotateinstead of experience linear motions. While when we subject the halma pawns into thesystem we created spiral motions, which clearly is the combination of linear motion ofJanus sphere and the rotation of truncated cone. This result indicates that theelectrohydrodynamic motions of asymmetric particles in dielectric fluid are alsocombinable following the same principle of combination in shape and surfacemodification. It will make the electrohydrodynamic system more suitable for real-lifeapplication, such as full control of particle motion in three dimensions, motion controlof bio-materials, targeted detections and transportations.