Phase behavior of colloidal monolayers on one-dimensional periodic and quasiperiodic light fields

by Lamiss Zaidouny

Institution: University of Stuttgart
Department: Fakultät Mathematik und Physik
Degree: PhD
Year: 2015
Record ID: 1100116
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2015/9840/


We experimentally investigate under real space conditions, crystalline and quasicrystalline colloidal assemblies in the presence of one dimensional light field potentials. By using scanning optical tweezers, such potential landscapes can – in contrast to interference patterns – be controlled in situ without realignment of optical components. In this thesis we have reported on the creation of colloidal crystalline phases in one-dimensional periodic potential landscapes. We use a suspension of silica particles and bromobenzene in order to realize a wide range of line spacings relative to the lattice constant. Such a suspension is characterized by low electrical polarity and therefore the Debye screening length of the system is about 4.6 µm. This large distance as compared to aqueous suspensions, results in the formation of crystals with lattice constants up to 20 µm. In the presence of the laser pattern, a rich variety of crystal-crystal and crystal-disordered structures are observed. The observed phases are stabilized by the competition of optical and electrostatic forces and thus strongly depend on both the periodicity and the laser intensity of the light field. In the second part of the thesis our experiments demonstrate that a smooth transition between one-dimensional crystals to quasicrystals is possible by the formation of a periodic average structure (PAS) on 1D quasiperiodic substrate potential. This intermediate phase is robust with respect to parameter variations. Contrary to atomic quasicrystals, the formation of PAS does not require complex or even anisotropic pair potentials. Obviously, PAS can be generally expected in situations where periodic and quasiperiodic ordering principles compete against each other. PAS of higher-dimensional quasicrystalline structures, can be experimentally achieved e.g. in decagoconal colloidal monolayers formed in the presence of five interfering beams. In contrast to the 1D case, two-dimensional systems have no simple 1-1 mapping between periodic and quasiperiodic structures, hence experimental colloidal studies will provide direct information how crystal to quasicrystal transitions are achieved under such conditions. Finally, it should be mentioned, that in addition to the phases observed here, other structures can form. For example, at higher particle concentrations and smaller Debye screening lengths, phases with interstitial particle strings between the quasiperiodic or periodic lines should be observed. In addition, inverse tweezing with quasiperiodic light potentials is also applicable for such colloidal systems. In dieser Arbeit beschäftigen wir uns mit dem Verhalten zweidimensionaler, repulsiv wechselwirkender kolloidaler Systeme in Anwesenheit von eindimensionalen Potentialen mit periodischen sowie quasiperiodischen Linienabständen. Experimentell werden diese Potentiale durch eine gerasterte optische Pinzette realisiert. Dabei lassen sich neben der Potentialtiefe auch der Abstand der Linien kontinuierlich variieren. Für den Fall von eindimensional periodischen…