Inkjet Printing of Magnetic Particles Toward Anisotropic Magnetic Properties.

Unique properties of one-dimensional assemblies of particles have attracted great attention during the past decades, particularly with respect to the potential for anisotropic magnetism. Patterned films can be created using inkjet printing; however, drying of particle-laden colloidal droplets on solid surfaces is usually accompanied by the well-known coffee-ring effect, deteriorating both the uniformity and resolution of the printed configurations. This study examines the effect of externally applied magnetic field on particle deposition patterns. Ferromagnetic Gd5Si4 particles were formulated in terpineol oil and directly deposited via magnetic field-assisted inkjet printing on a photopaper to generate patterned films with suppressed coffee-ring effect. The particle deposition morphology is determined by both solvent imbibition and particle-magnetic field interactions. Three characteristic times are considered, namely, the critical time for solvent imbibition into the substrate (tim), the time it takes for particles to form chains in the presence of the magnetic field (tch), and the time in which the particles reach the substrate in the direction normal to the substrate (tpz). The characteristic time ratios (tpz/tim) and (tpz/tch) determine the final deposition morphology in the presence of magnetic field. The ability to control particle deposition and assembly, thus tuning the magnetic anisotropic properties of nanostructured materials is a promising approach for many engineering applications.

Probing the Colloidal Particle Dynamics in Drying Sessile Droplets.

Particle deposition and assembly in the vicinity of contact lines of evaporative sessile droplets have been intensively investigated during the past decade. Yet little is known about particle arrangement in the contact-line region initiated by the self-assembled particles at the air-liquid interface and how the particle pinning behaves differently compared with that when particles are transported from the bulk of the sessile droplet to the three-phase contact line. We utilized the dual-droplet inkjet printing process to elucidate the versatility in particle deposition and assembly generated near the contact-line region and demonstrated the influence of such printing technique on particle pinning at the contact line after solvent evaporation. Wetting droplets containing sulfate-functionalized polystyrene (sulfate-PS) nanoparticles were jetted over the supporting droplets with carboxyl-PS nanoparticles, where the interplay between the solvent evaporation and particle transport dictates the final morphology of particle deposition. Depending on the particle size and concentration used in the supporting droplet, different morphologies of particle depositions near the periphery of the supporting droplet have been obtained such as stratified rings, blended rings, and rings of particles mainly from the air-liquid interface. Three characteristic times are considered in this study, namely, total time for solvent evaporation ( tevp), time required for the colloidal particles in the supporting droplet to reach the contact line and form the first layers of deposition ( tps), and time needed for the particles at the interface to reach the contact line ( tpw). The ratios of characteristic times ( tps/ tevp) and ( tps/ tpw) determine the final particle assembly near the contact-line region. The ability to control such particle deposition and assembly could have a direct implication on developing facile, cost-effective technologies essential for patterning heterogeneous structured coatings and devices.

pH-modulated self-assembly of colloidal nanoparticles in a dual-droplet inkjet printing process.

HYPOTHESIS: Interfacial self-assembly has been demonstrated as a powerful driving mechanism for creating various nanostructured assemblies. In this work, we employed a dual-droplet printing process and interfacial self-assembly mechanism to produce deposits with controlled assembly structures of colloidal nanoparticles. We hypothesize that pH modulation of the droplet will influence the interfacial self-assembly through the multibody interactions, e.g. particle-particle, particle-interface, and particle-substrate interactions, correspondingly affecting the deposition morphology of the colloidal nanoparticles. EXPERIMENTS: During the dual-droplet printing, a wetting droplet, containing colloidal nanoparticles, was jetted over a supporting droplet that contains water only. pH modulation was carried out to the supporting droplet. The self-assembly of two kinds of functionalized polystyrene (PS) nanoparticles (carboxyl-PS and sulfate-PS) was systematically investigated under various pH conditions. FINDINGS: Depending on the pH level of the supporting droplet, deposits of carboxyl-PS particles ranging from clear ring-like patterns to nearly uniform monolayer depositions have been obtained. On the other hand, the sulfate-PS particles, even at extreme basic and acidic environments, successfully assemble into nearly monolayer depositions. The multibody interactions are discussed. Such findings can be harnessed in manufacturing high-performance optical and electronic devices.