Self-Assembly of Colloidal Particles from Evaporating Droplets: Role of DLVO Interactions and Proposition of a Phase Diagram (original) (raw)
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Langmuir, 2013
Studies on self-assembly of colloidal nanoparticles during formation of nanostructured particles by spray-drying methods have attracted a large amount of attention. Understanding the self-assembly phenomenon allows the creation of creative materials with unique structures that may offer performance improvements in a variety of applications. However, current research on the self-assembly of colloidal nanoparticles have been conducted only on uncharged droplet systems. In this report, we first investigated the self-assembly processes of charged colloidal nanoparticles in charged droplets during spray-drying. Silica nanoparticles and polystyrene spheres are used as a model system. To induce a positive or a negative charge on the droplets, we used an electrospray method. Repulsive and attractive interactions between charged colloidal nanoparticles and droplet surface are found to control the self-assembly of colloidal nanoparticles inside the charged droplet. Interestingly, self-assembly of colloidal nanoparticles inside charged droplets under various processing parameters (i.e., droplet charge, droplet diameter, and surface charge, size, and composition of colloidal nanoparticles) allows the formation of unique nanostructured particles, including porous and hollow particles with control over the internal structure, external shape, number of hollow cavities, and shell thickness, in which this level of control cannot be achieved using conventional spray-drying method.
The manipulation of colloidal nanoparticles (NPs) in a drying droplet has critical importance not only for several industrial applications but also their assembly into patterns on surfaces. The influence of a tip with hydrophilic or hydrophobic surfaces dipped into a drying droplet on hydrophilic or hydrophobic surfaces on the behavior of 98 nm latex NPs was investigated. The formation of concentric rings on hydrophilic glass surfaces regardless of the surface chemistry of the dipped tip was observed. On the other hand, no pattern formation on hydrophobic surfaces was observed with the insertion of the tip. With a hydrophilic tip, the concentric rings were formed due to stick–slip motion of the solvent contact line resulting from competition between pinning and capillary forces while the capillary effect was not effective until the surface of the tip was changed by adherent NPs making the tip surface available for water adherence with a hydrophobic tip, which results in the pulling of droplet towards the tip. It is also found that the tip thickness and suspension concentration significantly influences the formation of concentric rings on surfaces. This simple procedure can be used to influence the distribution or assembly of NPs in the droplet area.
Coalescence, evaporation and particle deposition of consecutively printed colloidal drops
The particle deposition dynamics of two consecutively printed evaporating colloidal drops is examined using a fluorescence microscope and a synchronized side-view camera. The results show that the relaxation time of the water–air interface of the merged drop is shorter than that of a single drop impacting on a dry surface. It is also found that both morphology and particle distribution uniformity of the deposit change significantly with varying jetting delay and spatial spacing between two drops. As the drop spacing increases while keeping jetting delay constant, the circularity of the coalesced drop reduces. For the regime where the time scale for drop evaporation is comparable with the relaxation time scale for two drops to completely coalesce, the capillary flow induced by the local curvature variation of the air–water interface redistributes particles inside a merged drop, causing suppression of the coffee-ring effect for the case of a high jetting frequency while resulting in a region of particle accumulation in the middle of the merged drop at a low jetting frequency. By tuning the interplay of wetting, evaporation, capillary relaxation, and particle assembly, the deposition morphology of consecutively printed colloidal drops can hence be controlled.
Analysis of Profile and Morphology of Colloidal Deposits obtained from Evaporating Sessile Droplets
Colloids and Surfaces A: Physicochemical and Engineering Aspects
We experimentally investigate the profile and morphology of the ring-like deposits obtained after evaporation of a sessile water droplet containing polystyrene colloidal particles on a hydrophilic glass substrate. In particular, the coupled effect of particle size and concentration are studied. The deposits were qualitatively visualized under an optical microscope and profile of the ring was measured by an optical profilometer. The profile of the ring resembles a partial torus-like shape for all cases of particles size and concentration. The cracks on the surface of the ring were found to occur only at smaller particle size and larger concentration. We plot a regime map to classify three deposit types-discontinuous monolayer ring, continuous monolayer ring, and multiple layers ring-on particles concentration-particle size plane. Our data shows a possible existence of a critical concentration (particle size) for a given particle size (concentration) at which the monolayer ring forms. For the larger particle sizes, the immersion capillary forces between the particles dominate, aiding the formation of a monolayer ring of the particles. The relative mass of the particles accumulated in the ring is lesser in cases of the monolayer ring. We measure the width and height of the ring and show that they scale with particle concentration by a power law for the multiple layers ring. This scaling corroborates with an existing continuum based theoretical model. We briefly discuss the effect of the interaction of growing deposit with shrinking free surface on the ring dimensions and profile. The present results aid understanding of the ring formation process and will be useful in guiding the design of self-assemblies of the colloidal particles formed by the evaporating droplets.
Interaction of bi-dispersed particles with contact line in an evaporating colloidal drop
The deposition behavior of an inkjet-printed aqueous colloidal mixture of micro-and nanoparticles onto a glass substrate with systematically varied wettability has been investigated using fluorescence microscopy. Real-time bottom-view images show that particles inside an evaporating drop rearrange themselves near the drop contact line according to their sizes, where smaller particles tend to deposit closer to the contact line compared to the larger ones. By increasing substrate wettability, particles in the bi-dispersed mixture can be further separated compared to those on substrates of poor wettability. This is primarily because, during different stages of evaporation, the interplay of surface tension, drag due to evaporative flow, and particle–substrate interactions rearrange particles inside a colloidal drop near the contact line region. Forces acting on particles determine the extent to which particles enhance contact line pinning, which ultimately determines the final deposition morphology of particles from a bi-dispersed colloidal mixture.
New Journal of Physics, 2009
An efficient way to precisely pattern particles on solid surfaces is to dispense and evaporate colloidal drops, as for bioassays. The dried deposits often exhibit complex structures exemplified by the coffee ring pattern, where most particles have accumulated at the periphery of the deposit. In this work, the formation of deposits during the drying of nanoliter colloidal drops on a flat substrate is investigated numerically and experimentally. A finite-element numerical model is developed that solves the Navier-Stokes, heat and mass transport equations in a Lagrangian framework. The diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet-air interface. Laplace stresses and thermal Marangoni stresses are accounted for. The particle concentration is tracked by solving a continuum advection-diffusion equation. Wetting line motion and the interaction of the free surface of the drop with the growing deposit are modeled based on criteria on wetting angles. Numerical results for evaporation times and flow field are in very good agreement with published experimental and theoretical results. We also performed transient visualization experiments of water and isopropanol drops loaded with polystyrene microsphere evaporating on respectively glass and polydimethylsiloxane substrates. Measured evaporation times, deposit shape and sizes, and flow fields are in very good agreement with the numerical results. Different v velocity vector v = (u.v) V dimensionless velocity vector V = (U.V) X Concentration of particles [kg of particles/kg of solution] z axial coordinate [m] Z dimensionless axial coordinate [ 0 max, / r z ]
Journal of Colloid and Interface Science, 2006
Line evaporation of dense nanoparticle suspensions is studied theoretically and experimentally. The 2-D lines are drawn by a pen-like nozzle continuously dispensing a commercially available concentrated organic suspension (50 wt%; 4.3 vol%) of 5 nm gold nanoparticles in toluene solvent. Such particle-containing lines show promise for industrial applications where circuits are inkjet-printed and heat-treated to dry off the organic solvent and sinter the nanoparticles, thus producing a continuous electrically conducting path. The employed nanosuspension displays spontaneous thickening upon contact with a solid surface in the ambient atmosphere, and thus does not dry according to the well-established coffee-stain forming mechanism applicable to dilute particle suspensions. In the present work, model lines (∼1 mm width) are studied to elucidate the drying peculiarities of such nanoparticle slurries. These scaled-up lines allow detailed spatial measurements of their topography throughout their prolonged evaporation period, and make possible direct comparisons between experiment and theory. The results show the particle deposits formed by evaporative drying of these lines to be of non-uniform thickness with a dent in the middle of the lateral cross-section. Formation of this practically undesirable landscape is attributed to the highly non-uniform evaporative character of sessile or pendent liquid lines, which results in a non-uniform consolidation of the porous phase formed upon contact with the solid surface. The formulated description of the shape changing process, as done in the framework of the consolidation theory, yields predicted deposit speck shapes that compare favorably with the temporally resolved experimental data.
Self-organization of colloidal particles during drying of a droplet: Modeling and experimental study
Advanced Powder Technology, 2018
Formation of structurized micro/nanoparticle aggregates in spray drying process is analyzed theoretically and experimentally. Colloids of mono-and bimodal particle size distribution are used as the precursors to demonstrate different patterns of particle self-organization inside the drying droplet. In case of monodisperse primary particles their self-organization in the final aggregate results in either a hollow or a full (packed) spherical structure. For primary particles with bimodal size distribution, either the layered structure of aggregates is formed (with smaller particles forming outer layer and the bigger particles captured inside) or the ordering of bigger particles on the aggregate surface is observed, depending on process parameters. Numerical investigations allow to predict and explain the conditions at which selfassembling of particles within powder aggregates takes place.