Doctor of Philosophy
Mechanical and Nuclear Engineering
Inkjet printing has attracted much attention in recent years as a versatile manufacturing tool, suitable for printing functional materials. This facile, low-cost printing technique with high throughput and accuracy is considered promising for a wide range of applications including but not limited to optical and electronic devices, sensors, solar cells, biochips, and displays. The performance of such functional devices is significantly influenced by the deposit morphology and printing resolution. Therefore, fabrication functional devices with precise footprints by inkjet printing requires deep understanding of ink properties, material interactions, and material self-assembly.
In conventional inkjet printing process, where sessile droplets are directly printed on substrates, particle depositions are usually associated with the well-known, undesirable coffee-ring effect due to the high solvent evaporation rate at the edges of the printed droplets. Such particle accumulation phenomenon in vicinity of the three-phase contact lines of sessile droplets is considered detrimental to inkjet printing applications. This study investigates the material interactions and self-assembly of colloidal inks in inkjet printing applications at different length scales. The potential of inkjet printing has been exploited through employing the dual-droplet inkjet printing of colloidal particles to investigate the self-assembly of colloidal nanoparticles at the air-liquid interface and at the three-phase contact line of sessile droplets, which provide better understanding of the particle deposition morphologies after solvent evaporation. Different from conventional inkjet printing, the dual-droplet printing involves jetting wetting droplets, containing colloidal nanoparticles dispersed in solvents with high vapor pressure, over supporting droplets composed of water only. By tuning the surface tensions and controlling the jetting parameters of the jetted droplets, monolayers with closely-packed deposition of colloidal nanoparticles are demonstrated. Various solutions are proposed to totally suppress or mitigate the coffee-ring effect in inkjet printing applications through tuning the pH value of the supporting droplets in the dual-droplet inkjet printing to control the multibody interactions (i.e., particle-particle, particle-interface, and particle-substrate interactions) or by applying magnetic field to direct the self-assembly of colloidal particles in conventional inkjet printing. In addition, the influence of various forces such as drag force, van der Waals force, electrotactic force, and capillary force on the particle deposition and assembly in vicinity of the three-phase contact line area were investigated for both the conventional and dual-droplet inkjet printing techniques.
Finally, fabrication of functional devices such as stretchable conductors have also been demonstrated by inkjet printing of silver nanowires into elastomer substrate, where the viscous liquid elastomer layer shaped the printed silver wire lines into tens of micrometers in dimeter. The silver nanowires align along the printing direction during solvent evaporation, resulting in wires with good mechanical stability and electrical performance. The printing techniques and the outcomes presented in this study can be harnessed in engineering and manufacturing a wide range of technological applications ranging from high-performance optical and electronic devices to stretchable conductors and sensors.
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