Evaporation Of Colloidal Droplets Research Articles

Pinning and Depinning Dynamics of an Evaporating Sessile Droplet Containing Mono- and Bidispersed Colloidal Particles on a Nonheated/Heated Hydrophobic Substrate

The present study is primarily focused on the coupled effects of substrate heating, colloidal dispersion, and particle size variation on the contact line (CL) pinning-depinning dynamics of evaporating droplets containing mono- (3/4.5 μm) and bidispersed (3 and 4.5 μm) polystyrene colloidal particles on poly(dimethylsiloxane) (PDMS) substrates. Experimental techniques such as high-speed visualization, optical microscopy, infrared thermography, and scanning electron microscopy are implemented to discover the plausible causes dictating the underlying physics. In the case of the nonheated substrate, there exists a significant delay in the CL depinning for the evaporating droplets containing bidispersed particles, as opposed to the monodispersed cases. A first-order model is illustrated for the determination of the net horizontal force acting on the particles near the CL. Interestingly, the model's findings revealed that due to the difference of particle size in the case of the bidispersed suspension, the interparticle contact force gets modified, thus enhancing the CL pinning. For the heated substrate cases, droplets with monodispersed particles (3 μm) exhibit a substantial delay in the CL depinning, whereas a nearly complete pinning of the CL is witnessed for the case of bidispersed colloidal suspension droplets. It is mainly due to the augmentation of particle deposition near the CL because of the circulatory thermal Marangoni and outward capillary flows. Thus, the mobility of the CL is inhibited, which is further reinforced by the presence of different-sized particles. Eventually, a ring-like deposition is observed, as opposed to an inner deposit commonly observed from the evaporation of colloidal droplets on hydrophobic substrates.

Surfactant Control of Coffee Ring Formation in Carbon Nanotube Suspensions.

The coffee ring effect regularly occurs during the evaporation of colloidal droplets and is often undesirable. Here we show that adding a specific concentration of a surfactant can mitigate this effect. We have conducted experiments on aqueous suspensions of carbon nanotubes that were prepared with cationic surfactant dodecyltrimethylammonium bromide added at 0.2, 0.5, 1, 2, 5, and 10 times the critical micelle concentration. Colloidal droplets were deposited on candidate substrates for printed electronics with varying wetting characteristics: glass, polyethylene terephthalate, fluoroethylene propylene copolymer, and polydimethylsiloxane. Following drying, four pattern types were observed in the final deposits: dot-like, uniform, coffee ring deposits, and combined patterns (coffee ring with a dot-like central deposit). Evaporation occurred predominantly in constant contact radius mode for most pattern types, except for some cases that led to uniform deposits in which early stage receding of the contact line occurred. Image analysis and profilometry yielded deposit thicknesses, allowing us to identify a coffee ring subfeature in all uniform deposits and to infer the percentage coverage in all cases. Importantly, a critical surfactant concentration was identified for the generation of highly uniform deposits across all substrates. This concentration resulted in visually uniform deposits consisting of a coffee ring subfeature with a densely packed center, generated from two distinct evaporative phases.

Evaporation of colloidal droplets from aluminum–magnesium alloy surfaces after laser-texturing and mechanical processing

Evaporation-induced self-assembly on solid surfaces is widely used for the production of photonic devices, circuit boards of electronic devices, and cooling technologies. There are numerous factors affecting self-assembly. However, the state of a solid surface is one of the most important. In this work, we analyzed the effect of mechanical processing methods (widely used in mechanical engineering) and promising pulsed laser texturing of aluminum-magnesium alloy on the characteristics of self-assembly of polystyrene (PS) particles during the evaporation of colloidal solution droplets. The patterns were visualized under a scanning electron microscope. The profile of an evaporating droplet was obtained using the optical shadow technique. It was found that during the evaporation of water droplets loaded with PS particles on aluminum-magnesium alloy samples, the ring-like patterns were formed. Such a pattern type was explained by the evaporation of a droplet in the constant contact diameter mode during almost the entire evaporation time (more than 90% of the total evaporation time). The dried patterns formed on aluminum-magnesium alloy surfaces after satin-finishing and laser texturing were found to be stretched parallel to the vector of laser beam or polishing tool movement due to capillary force. In addition, we determined the conditions when the particles formed spot-like patterns on laser-textured surfaces. These results provide new knowledge in understanding the effect of different processing methods of a material on the formation of dried patterns and are important for many applications.

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films.

We present results on the photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm-sized crystalline Si nanoparticles (NP) deposited on glass substrates. Simulations of the absorbed light intensity and photo-induced temperature distribution across the Si NP films were carried out by using the Finite difference time domain (FDTD) and finite element mesh (FEM) modeling and the obtained data were compared with the local temperatures measured by micro-Raman spectroscopy and then was used for determining the heat conductivities in the films of various thicknesses. The cubic-to-hexagonal phase transition in Si NP films caused by laser-induced heating was found to be heavily influenced by the film thickness and heat-conductive properties of glass substrate, on which the films were deposited. The values in drop-casted Si nanogranular films were found to be in the range of lowest of other types of nanostructurely voided Si films due to enhanced phonon scattering across inherently voided topology, weak NP-NP and NP-substrate interface bonding within nanogranular Si films.

Enhanced Coffee-Ring Effect via Substrate Roughness in Evaporation of Colloidal Droplets

The analysis of dried drop patterns has various applications in research fields like archeology, medical practice, printing, and so on. In this paper, we studied the evaporation and pattern formation of polytetrafluoroethylene (PTFE) colloid droplets on smooth substrate and rough substrates with different roughness. We found that the evaporation of droplets shows remarkable coffee-ring effect on smooth substrate and that the cross-section of the ring is wedge-shaped with its thickness decreasing from the edge to the center. However, with increasing roughness, the effect strengthened, with the section of the coffee-ring changing from wedge- to hill-shaped. The contact angle decreased with increasing roughness, leading to an increase in evaporation rate. Moreover, wicking led to additional evaporation, which also enhanced capillary flow, moving more particles to the edge. In addition, the rough structure of the substrate inhibited the back-flow of the capillary flow, preventing the particles’ move to the center. The formation of radial wrinkles on the edge also led to particle retention, preventing them from moving to the center. All these factors contribute to the decreased width and increased height of the coffee-ring pattern after evaporation on rough surfaces. It is an effective method to regulate the deposition pattern of evaporating droplet by changing the substrate roughness.

Influence of nano-scaled roughness on evaporation patterns of colloidal droplets

Evaporation of colloidal droplets often leads to various deposited patterns which are not only interesting but also provide a very simple and useful method to fabricate functional materials. The patterns induced by the evaporation can be tuned via several factors, among which the roughness of the substrate is an important one. However, the effect of nano-scaled roughness is scarcely studied and far from being fully understood. In this work, the evaporation and pattern formation of SiO2 colloid droplets are studied on smooth substrate and nano-rough substrate, respectively. The aim of this work is to clarify how the evaporation dynamics and patterns are influenced by nano-scaled roughness. The roughness of the substrate is analyzed by using a scanning electron microscope and an atomic force microscope, the evaporation process and pattern formation are monitored via an in-situ microscope observation. The obtained deposited patterns are analyzed by using stylus profiling. It is found that the evaporation of droplets is accompanied by an obvious coffee ring effect on smooth substrate and the deposition patterns are bowl-shaped. However, uniform thickness evaporation patterns are obtained through evaporation on rough substrate, moreover, the crack density increases obviously. The analysis shows that nano-roughness is able to inhibit the circumfluence of droplets along the substrate, which greatly weakens the compensation for capillary flow, leading to particles gathering at air-droplet interface and formulating a particle layer. This prevents the coffee ring effect, and eventually results in the formation of evaporation patterns with uniform thickness.

Crack formation and prevention in colloidal drops.

Crack formation is a frequent result of residual stress release from colloidal films made by the evaporation of colloidal droplets containing nanoparticles. Crack prevention is a significant task in industrial applications such as painting and inkjet printing with colloidal nanoparticles. Here, we illustrate how colloidal drops evaporate and how crack generation is dependent on the particle size and initial volume fraction, through direct visualization of the individual colloids with confocal laser microscopy. To prevent crack formation, we suggest use of a versatile method to control the colloid-polymer interactions by mixing a nonadsorbing polymer with the colloidal suspension, which is known to drive gelation of the particles with short-range attraction. Gelation-driven crack prevention is a feasible and simple method to obtain crack-free, uniform coatings through drying-mediated assembly of colloidal nanoparticles.