Tuning Of Self-assembly Patterns Induced By The Evaporation Of Colloidal Droplets

The evaporation of liquid droplets is one of the most common and simplephysical phenomenon in everyday life. However, it is of essential importance for theself-assembly of nanoparticles into expected orders. Furthermore, tuning theevaporation depositions into desired patterns is still a great challenge and manyscientific questions are still remaining open. Therefore, it has aroused extensiveresearch interest in the field of materials science and technology.In the present dissertation, the effects of polymer, inorganic salt and surfactanton the evaporation of colloidal droplets have been investigated. The roles played bythe wettability of both substrate and colloidal particles have been clarified. Moreover,how the evaporation patterns were tuned by the substrate vibration and externalmagnetic field has been studied.The evaporation process and the induced cracking of SiO2colloidal dropletshave been investigated by in-situ observation. The evaporation of silica colloidaldroplet forms obvious coffee ring pattern with cracular cracks. However, once PEO isintroduced, a homogeneous deposition is obtained, indicating the inhibitation ofcoffee ring effect. The crack propagation direction changes from arc-path toradial-like. Furthermore, its velocity is lowered by3orders of magnitudes by thepresence of PEO which leads to the formation of larger-sized aggregates. Theenhanced inhomogeneity decreases the nucleation barrier of crack and in turn thedriving force of its growth. This may be responsible for the sluggish crack growthwith PEO additives.The evaporation of PTFE colloidal droplet leads to the formation of radializedcracks. Its quantity decreases with increment of particle volume fraction. Radicalizedwrinkling have been observed, which may be induced by the stretch effect of thereceding liquid zone by its surface tension. Therefore, similar macroscopic stress fieldis built, eventually leading to the formation of radicalized cracks. It is found, bothexperimentally and theoretically, the crack propagation velocity satisfies H3/5scaling.In addition, we have accomplished the tuning of crack patterns by usingsurfactant. Radicalized wrinkling is suppressed by the presence of SDS additives.This is because the lowered surface tension can not build macroscopic stress field inthe gel zone. With0.5%SDS, the propagation direction of the cracks is reversed. Thecracks are completely suppressed when adding3%SDS to the system. The results suggest the capillary force between colloidal particles is of great importance in thebuilding and relishing of the drying stress.The coupling between salt and colloidal particles plays important role in theformation of evaporation patterns. With the presence of NaCl, the stress field may bechanged and the outward capillary flow is restrained. Therefore, it is favoring theformation of homogeneous deposition. Interestingly, a rough dendrite structurecontaining both salt and PTFE particles is formed, which may be resulted from therelease of stress induced by evaporation.To study the influence of substrate roughness and wettability, and the wettabilityof colloidal particles, the evaporation experiments were performed on differentsubstrates and by using particles of different hydrophobicity. When the substrate ishydrophilic, the contact line is always pinned during evaporation, leading to obviouscoffee ring stain. However, when the substrate is hydrophobic, the contact line isun-pinned. The particles form a continuous shell and eventually broken into severalbig blocks. With the increase of substrate roughness, the crack numbers increase. It isalso observed the coffee ring effect is greatly restrained by using hydrophobicparticles.The effect of substrate vibration and magnetic field on the evaporation patternshave been studied tentatively. It is found that the drying patterns of colloidal dropletcan be significantly changed by choosing appropriate frequency and vibrationamplitude of the substrate, implying substrate vibration may alter the capillary flow ofthe droplet and in turn the particle distribution and transportation inside. The magneticfield can increase the interaction between Fe3O4particles, thus changing the dryingpatterns which depend on the direction of the magnetic field.