Coffee-ring Formation Research Articles

Drying-Induced Flash Nanoprecipitation in a Sessile Drop: A Route to Synthesize Polymeric Nanoparticles.

Flash nanoprecipitation is a simple and scalable method to produce nanoparticles by rapid mixing of a polymer solution with an antisolvent. High-speed mixing devices for the continuous synthesis of polymeric nanoparticles and drug-encapsulated nanoparticles have been designed. In this work, we demonstrate a different approach to induce flash nanoprecipitation using the differential evaporation of solvents in a sessile drop. To show proof of concept, we use polymethyl-methacrylate (PMMA) dissolved in a tetrahydrofuran (THF)-water mixture as a model system. A sessile drop of the polymer solution is allowed to dry under controlled conditions. The sessile drops of the PMMA-THF-water ternary mixture are observed to dry in the constant radius mode. As THF in the drop evaporates faster than water, PMMA supersaturates and precipitates as nanoparticles. Although coffee-ring formation is well-studied in the drying of colloidal suspensions, this work demonstrates the formation of nanoparticles in situ due to a change of solvent quality and subsequent deposition of particles at the pinned contact line. Using the theory of drying of binary solutions, we calculate the temporal variation of composition. The drying paths passing through the low-concentration branch of the binodal give rise to nanoparticles, whereas those passing through the high-concentration branch yield porous films. Spherical polymeric nanoparticles in the size range of 250-700 nm were synthesized using this technique starting from drops with different initial polymer concentration. The method is a cost-effective (no high-speed mixing is required) and scalable alternative to conventional flash nanoprecipitation for synthesizing polymeric nanoparticles for potential applications in drug delivery, diagnostics, and polymer recycling.

Electric field-modulated evaporative thin film deposition of bio-particles for piezoelectric applications.

Bio-based functional materials can be used to replace or limit the use of synthetic materials sourced from unsustainable sources. However, the potential of such materials remains largely unexplored. In this study, we demonstrate the use of weak AC electric fields to deposit ultra-thin piezoelectric films from cellulose nanocrystals (CNC). This is the first time electric fields are used to realize <50 nm thick uniform bio-based piezoelectric films wherein the bioparticles exhibit unidirectional arrangement. Interestingly, we found that the use of weak AC electric fields of suitable frequencies completely mitigates the coffee ring effect (CRE), which results in defect-free uniform ultra-thin films. Additionally, the electric fields appear to help in realizing unidirectional alignment of particles in the films, which enhances their piezoelectric properties. The method was also tested for chitin nanocrystals (ChNC), which have a similar aspect ratio but bear opposite polarity surface charges, and the influence of the field on coffee ring formation and particle orientation in CNC thin film deposition was validated. The phenomena can be attributed to the constant spatio-temporal curvature of the evaporating liquid film, the transient state between the three-phase contact (TPC) line, the electric field-dependent contact angle, and the permanent and field-induced dipole moments. These factors lead to particle polarization and alignment. The films have an optimum electrical frequency of deposition at which they are continuous and uniformly thin, have unidirectional alignment of particles, and function as a single dipole.

Biomimetic Surface Engineering to Modulate the Coffee-Ring Effect for Amyloid-β Detection in Rat Brains.

Surface engineering of nanoparticles has been widely used in biosensing and assays, where sensitivity was mainly limited by plasmonic colour change or electrochemical responses. Here, we report a novel biomimetic sensing strategy involving protein-modified gold nanoparticles (AuNPs), where the modulation strategy was inspired by gastropods in inhibition of coffee-ring effects in their trail-followings. The so-called coffee-ring effect presents the molecular behaviour of AuNPs to a macroscopic ring through aggregation, and thus greatly improves sensitivity. The assay relies upon the different assembly patterns of AuNPs against analytes, resulting in the formation or suppression of coffee-ring effects by the different surface engineering of AuNPs by proteins and peptides. The mechanism of the coffee-ring formation process is examined through experimental characterizations and computational simulations. A practical coffee-ring effect assay is developed for a proof-of-concept target, amyloid β (1-42), which is a typical biomarker of Alzheimer's disease. A novel quasi-titrimetric protocol is constructed for quantitative determination of the target molecule. The assay shows excellent selectivity and sensitivity for the amyloid β monomer, with a low detection limit of 20 pM. Combined with a fluorescent staining technique, the assay is designed as a smart sensor for amyloid β detection and fibrillation evaluation in rat cerebrospinal fluids, which is a potential point-of-care test for Alzheimer's disease. Connections between amyloid fibrillation and different courses of brain ischaemia are also studied, with improved sensitivity, lower sample volumes that are required, convenience for rapid detection, and point-of-care testing.

Ink droplet drying analysis for understanding the ink-catalyst layer transition in proton exchange membrane fuel cells

There is still limited understanding of ink structure and the transition process from ink to the catalyst layer (CL) in proton exchange membrane fuel cells despite recent interest in the field. This study proposes a methodology for analyzing the CLs formed after drying single- and multi-ink droplets produced using an ultrasonic spray. Two model inks, Pt/Vulcan Carbon and Pt/Ketjen Black, are prepared using these catalysts, and their characteristics are analyzed through sedimentation behavior, ionomer adsorption quantification, and rheology. The structural properties of the spray-dried CLs are examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy-Auger electron spectroscopy. These analyses reveal distinct differences in the CLs' structures, influenced by physical phenomena such as coffee ring formation and droplet coalescence, which are dependent on the ink properties. Furthermore, the structural characteristics of the CLs produced by the two model inks affect the electrochemical behavior of the 25 cm2 fuel cell. This study establishes a correlation between the ink structure, CL structure, and fuel cell performance, emphasizing the usefulness of the ultrasonic spray coating method in advancing such investigations.

Gravitational effects on coffee-ring formation during the evaporation of sessile droplets

We consider the role of gravity in solute transport when a thin droplet evaporates. Under the physically relevant assumptions that the contact line is pinned and the solutal Péclet number, ${Pe}$ , is large, we identify two asymptotic regimes that depend on the size of the Bond number, ${Bo}$ . When ${Bo} = O(1)$ as ${Pe}\rightarrow \infty$ , the asymptotic structure of solute transport follows directly from the surface-tension-dominated regime, whereby advection drives solute towards the contact line, only to be countered by local diffusive effects, leading to the formation of the famous ‘coffee ring.’ In the distinguished limit in which ${Bo} = O({Pe}^{4/3})$ as ${Pe}\rightarrow \infty$ , this interplay between advection and diffusion takes place alongside that between surface tension and gravity. In each regime, we perform a systematic asymptotic analysis of the solute transport and compare our predictions to numerical simulations. We identify the effect of gravity on the nascent coffee ring, providing quantitative predictions of the size, location and shape of the solute mass profile. In particular, for a fixed Péclet number, as the effect of gravity increases, the coffee ring is diminished in height and situated further from the contact line. Furthermore, for certain values of ${Bo}$ , ${Pe}$ and the evaporation time, a secondary peak may exist inside the classical coffee ring. The onset of this secondary peak is linked to the change in type of the critical point in the solute mass profile at the droplet centre. Both the onset and the peak characteristics are shown to be independent of ${Pe}$ .

Inclined colloidal drops: Evaporation kinetics and pattern formation

The drying of solute-laden drops is ubiquitous in everyday life, from paints and printers to the raindrops drying on our windows. Nonetheless, scientific interest has primarily focused on understanding the evaporation kinetics on flat surface, with the key parameter of substrate inclination only recently started being addressed. This work focuses on the influence of moderate substrate inclinations at 20° and 40° on the evaporation kinetics and associated deposit patterns of colloidal drops. Inclination altered the shape of the drops which formed a lower contact angle at the upper side of the drop (rear edge) and larger contact angle at the lower side (front edge). As evaporation rate is a function of contact angle, which in turn is a function of inclination, the evaporation lifetime was extended by 43% and 61% for 20° and 40°, respectively, compared to a flat drop. A theoretical approximation of the evaporative flux across the liquid-vapour interface of the drops showed the contribution of each edge to the evaporation kinetics. These differences in the evaporative fluxes altered the internal flows within the drop and in turn the coffee-ring formation mechanism. The particle deposit shape at the two edges for each drop was visualised which combined with the theoretical arguments allowed the proposition of the particle deposition mechanism in inclined drops: inclination added a gravitationally-driven velocity flow component within the drops, which is perpendicular and hence negligible in flat drops. This additional flow hindered or enhanced the number of particles arriving at the rear and front edges of the inclined drops, respectively, and hence influenced the dimensions of the coffee-ring patterns. Eventually, the particle deposits grew sufficiently tall to effectively stagnate the outward flow which resulted in enhanced particle accumulation at the interior of the drops as inclination increased.

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.

Printing Crack‐Free Microporous Structures by Combining Additive Manufacturing with Colloidal Assembly

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle-templates,how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"-strategy is introduced to print macroscale, crack-free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near-infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal-based printed structures.

Coffee-ring formation through the use of the multi-ring mechanism guided by the self-assembly of magnetic nanoparticles

The control of the mechanism leading to the appearance of the ring-shaped stains from the dried liquid colloidal droplets has been the subject of intense studies over the last 25 years. This stems from the immense significance of this effect for technological applications. One of the key open topics in this field is the emergence of a regular multi-ring deposit from the dried droplet. Here, we show that magnetic nanoparticles in a drying magnetic liquid droplet can self-assemble into a multi-ring deposit structure, and even more importantly, a magnetic field can be turned on to control the underlying processes. The magnetic liquid is prepared as an aqueous suspension of Fe_3O_4 magnetic nanoparticles stabilized with (3-Aminopropyl)triethoxysilane (APTES) and its droplets are placed on low-density polyethylene (LDPE) film. The results of this work are expected to be very promising in the case of multiple applications including ink-jet printing methods and 2D printed electronics.

The nascent coffee ring with arbitrary droplet contact set: an asymptotic analysis

We consider the effect of droplet geometry on the early-stages of coffee-ring formation during the evaporation of a thin droplet with an arbitrary simple, smooth, pinned contact line. We perform a systematic matched asymptotic analysis of the small capillary number, large solutal Péclet number limit for two different evaporative models: a kinetic model, in which the evaporative flux is effectively constant across the droplet, and a diffusive model, in which the flux is singular at the contact line. For both evaporative models, solute is transported to the contact line by a capillary flow in the droplet bulk, while local to the contact line, solute diffusion counters advection. The resulting interplay leads to the formation of the nascent coffee-ring profile. By exploiting a coordinate system embedded in the contact line, we solve explicitly the local leading-order problem, deriving a similarity profile (in the form of a gamma distribution) that describes the nascent coffee ring. Notably, for an arbitrary contact line geometry, the ring characteristics change due to the concomitant asymmetry in the shape of the droplet free surface, the evaporative flux (for diffusive evaporation) and the mass flux into the contact line. We utilize the asymptotic model to determine the effects of contact line geometry on the growth of the coffee ring for a droplet with an elliptical contact set. Our results offer mechanistic insight into the effect of contact line curvature on the development of the coffee ring from deposition up to jamming of the solute; moreover, our model predicts when finite concentration effects become relevant.

Enhanced current path by circularly and periodically-aligned semiconducting single-walled carbon nanotubes for logic circuit device

Improving the performance of solution-processed single-walled carbon nanotube thin film transistors (SWCNT TFTs) is essential to their wide usage in next generation large-area electronic devices. However, uncontrollable tube-tube junction and random network formation from conventional solution processes of SWCNTs has limited mobility and on-current level of SWCNT TFTs. Herein, we demonstrate a facile method by switching idea of reducing coffee-ring of the conventionally solution-processed or inkjet-printed thin films. Spontaneous coffee-ring formation of the inkjet-printed droplets is found to enhance directional alignment of SWCNTs in the outer rim of the coffee-rings. The evaporation-driven capillary flow toward the rim inside induces migration of SWCNT and thus forms densely aligned SWCNT rings. Periodic connection of such rings can provide high-current path at a given voltage. Therefore, by additionally forming the periodically connected rings on a pre-established random network of SWCNT in channel area of TFTs, we significantly improved the mobility and I on/I off ratio of SWCNT TFTs without degradations in other electrical parameters such as threshold voltage and subthreshold swing. We also demonstrated all-solution-processed inverters with higher voltage-gain in comparison with conventional ones.

Numerical coffee-ring patterns with new interfacial schemes in 3D hybrid LB-LE model

Coffee-ring patterns are typical phenomena in evaporation-driven nanoparticles suspended (NPs-laden) droplet. However, NPs motion in droplet fails to exhibit characteristic patterns such as coffee-ring formation in lattice Boltzmann (LB) framework. With features of density diffuse interface, new interfacial schemes composed of density gradient force and interfacial force are proposed and coupled in LB-LE model. They offer a solution to well modulate the NPs dynamics in liquid-vapor interface as droplet deforms and evaporates. In the following simulations of one droplet and multi-droplets evaporation, typical coffee-ring pattern is obtained for one droplet evaporation with hysteresis window 5°< θ < 75°. Whereas for case with 5°< θ < 35°, NPs deposition generates weak ring pattern on droplet perimeter and plateau pattern on the interior region. For multi-droplets evaporation, more NPs are deposited on length sides with strong hysteresis window 5°< θ < 75° and most NPs deposit on the interior region with weak hysteresis window 5°< θ < 35°. The results show that the proposed interfacial schemes possess the strong capacities for modulating NPs in droplet liquid-vapor interface and are prone to be implemented in LB-LE model.

Tailoring Disorder and Quality of Photonic Glass Templates for Structural Coloration by Particle Charge Interactions

To obtain high-quality homogeneous photonic glass-based structural color films over large areas, it is essential to precisely control the degree of disorder of the spherical particles used and reduce the crack density within the films as much as possible. To tailor the disorder and quality of photonic glasses, a heteroaggregation-based process was developed by employing two oppositely charged equal-sized polystyrene (PS) particle types. The influence of the particle size ratio on the extent of heteroaggregation in the suspension mixes is investigated and correlated with both the morphology and the resultant optical properties of the films. The results show that the oppositely charged particle size ratio within the mix greatly influences the assembled structure in the films, affecting their roughness, crack density, and the coffee-ring formation. To better differentiate the morphology of the films, scanning electron microscopy images of the microstructures were classified by a supervised training of a deep convolutional neural network model to find distinctions that are inaccessible by conventional image analysis methods. Selected compositions were then infiltrated with TiO2 via atomic layer deposition, and after removal of the PS spheres, surface-templated inverse photonic glasses were obtained. Different color impressions and optical properties were obtained depending on the heteroaggregation level and thus the quality of the resultant films. The best results regarding the stability of the films and suppression of coffee-ring formation are obtained with a 35 wt % positively charged over negatively charged particle mix, which yielded enhanced structural coloration associated with improved film quality, tailored by the heteroaggregation fabrication process.

A green approach to water‐based graphene ink with reverse coffee ring effect

In printing electronics, graphene is used as a functional material due to its excellent physical properties. Therefore, to prepare a conductive graphene ink that contains non-toxic and environment-friendly solvent is very important. Direct preparation of graphene from graphite using liquid-phase exfoliation process is beneficial since it is simple and easy to use. Here in this work, we have synthesized the graphene dispersion using ethanol and water by liquid-phase exfoliation method and prepared a conductive graphene ink that shows an absence of coffee ring formation during the evaporation of solvents. A film has been prepared using the prepared ink which shows the electrical resistivity 1.06 Ω-cm at temperature 300 °C.

Predicting coffee ring formation upon drying in droplets of particle suspensions

Pattern formation is a common occurrence in drying colloidal systems. The most common in droplets, is a ring distribution where the constituents have relocated to the edge, which is referred to as a coffee ring. This deposit is unfavourable in many manufacturing processes and is of fundamental interest. In this study, we present a model capable of predicting when a coffee ring will be observed in hard spherical particle systems. Ring profiles are found to be formed at low contact angles with the specific angle predicated upon the initial concentration of the suspension. Modelling results are in agreement with experiments using latex suspensions.

Tunable coffee-ring formation of halloysite nanotubes by evaporating sessile drops.

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials with a length of 200-1000 nm and a diameter of ∼50 nm. Understanding the self-assembly behavior of such unique nanoparticles is important to develop their applications in functional devices. In this study, the "coffee-ring" patterns of HNTs are investigated which are formed by evaporation of the sessile droplets of HNT aqueous dispersion on different substrates. The coffee-ring pattern with various dimensions was characterized using a polarizing microscope (POM), a scanning electron microscope (SEM), and a 3D optical profilometer. The diameter, height, and area of the coffee-ring patterns depend on the concentration of HNT dispersion, the droplet volume, and surface wettability. POM and SEM results suggested that the nanotubes were highly ordered in the edge and the middle of the coffee-ring. The coffee-ring effect of HNTs could be suppressed by increasing the evaporation temperature of substrates or adding polymer additives. In addition, multiple-ring patterns consistent with protein rings surrounding HNT rings were formed, which can be utilized to detect the presence of proteins in biological samples. This work illustrated the relationship between the formation of coffee-ring patterns and the experimental conditions, which provided an additional research chance and allowed application development for HNTs using the liquid droplet self-assembly.

Suppression of coffee ring effect in high molecular weight polyacrylamide droplets evaporating on hydrophobic surfaces

In this work, we experimentally report the concentration and molecular weight dependence of the deposit patterns of polyacrylamide (PAM) droplets on hydrophobic surfaces. We find that with an increase in non-dimensional concentrations c/c* ranging from 0.16 (dilute) to 66.66 (semi-dilute entangled) there is a gradual transition from ring to non-ring patterns. However, with a decrease in the molecular weight of the polymer by one order, the coffee ring formation was not suppressed for the reported range of concentration. We attribute these results to the role played by the critical overlap concentration (c*) and diffusion coefficient of polymer along with the evaporation modes. The present study focuses on the drying of pure polymer droplets instead of the widely studied coffee ring effect in drying of droplets with rigid solid phase colloidal particles. We believe that these experimental findings would be of immense insight in applications such as inkjet printing, surface coatings, paints and detection assays in biotechnology for tuning polymer deposit patterns as a function of polymer molecular weight and concentration.

Patterning of colloidal droplet deposits on soft materials

Abstract

Effect of Immiscible Secondary Fluid on Particle Dynamics and Coffee Ring Characteristics during Suspension Drying.

Particle motion and coffee ring patterns in water-borne suspensions of polystyrene (PS) particle added with small amounts of secondary hydrophobic decalin are investigated during the drying of the suspension droplets, mainly employing light scattering methods. Very tiny secondary fluid insertions via high-speed agitation effectively link the particles through hydrophobic dissolution leading to the formation of multimodal particulate clusters, with resistance to the outward capillary flow and suppression of coffee ring formation after drying. The impact of decalin on particles is corroborated by actual images acquired from an optical profiler and a scanning electron microscope (SEM). The average particle motion inside the suspension changed by decalin was expressed in terms of mean square displacement (MSD) based on diffusing wave spectroscopy (DWS). Employing multispeckle diffusing wave spectroscopy (MSDWS), the rapid motion or β-relaxation of particles in various suspensions with and without decalin is quantified in early lag time during the drying of droplets. The change in particle dynamics during suspension drop drying, when adding a small secondary fluid, plays a key role in tuning coffee ring patterns.

Investigation of Droplet Evaporation on Copper Substrate with Different Roughness

In the present study, we investigated the evaporation process and deposition pattern of saline droplet on a copper substrate with different roughness under 40 °C ambient temperature. These four substrates are classified as smooth surface and rough surface based on their droplet contact angles. It has been found in this study that the evaporation pattern of droplets has a strong relationship to substrate roughness. The thickness boundary of the evaporation pattern on a smooth surface is larger than that on a rough surface and the particles are closer to boundary and the tendency is more obvious on a smooth surface. The below factors contribute to the result. On the smooth surface, the contact angle of droplet increases as the roughness decreases. On the rough surface, the contact angle increases as the roughness increases. With contact angle decreasing, the evaporation rate at the boundary increases leading to the particles at the boundary more easily sedimentate. Moreover, the capillary flow is hindered by increasing the substrate roughness, while the Marangoni flow remains constant, resulting in more particles remain in the center of the droplet on the rough surface. To sum up, the coffee-ring formation is suppressed by increasing the substrate roughness on a copper substrate under 40 °C temperature.