Coffee Ring Research Articles

Suppression of the coffee-ring effect by controlling the solid particle density

Previous studies show that the coffee-ring effect can be suppressed by altering the droplet's evaporation rate, surface tension, surface properties, and shape of particles. This experiment used five types of particles with different densities to analyze their behavior during the droplet evaporation process. The results showed that when the particle density is close to the fluid density, the particles move within the droplet and accumulate at the edges, forming a pronounced coffee-ring effect. Conversely, with the higher difference between the particle density and the fluid density increases, they tended to deposit uniformly at the bottom of the droplet and were less likely to be pushed to the edges by capillary effects, effectively suppressing the coffee-ring effect. We also observed that the movement speed of high-density particles relatively slowed down through particle image velocimetry tracking technology. By analyzing the Peclet number and the timescale between particle sinking speed and particle movement speed due to capillary flow, we explained how particle density influences the critical factors of particle sinking and suspension, thereby inhibiting the formation of the coffee-ring effect.

Suppression of the coffee-ring effect using a contact-angle radial-band-patterned substrate: A lattice Boltzmann study

Suppression of the coffee-ring effect using a contact-angle radial-band-patterned substrate: A lattice Boltzmann study

Exploiting the Tunneling Coffee Ring Effect of Universal Colorimetric Nanomaterials for Ultrafast On-Site Microbial Monitoring.

The coffee-ring effect is an eye-catching circle originating from a material-suspended liquid droplet at a solid substrate after liquid evaporation, but the low speediness has restricted practical applications. When nanomaterial aqueous solutions are dropped onto porous nitrocellulose (NC), the liquid is immediately absorbed through the porous tunnels of paper fibers, and nanomaterials are rapidly enriched on the contact lines between droplets and membranes. We called this ultrafast variant of the coffee ring effect the "tunneling coffee ring" (TCR). When nanomaterial sizes are smaller than that of pores, a larger-diameter ring of nanomaterials quickly materializes. The real-time particle size-dependent TCRs and liquid diffusion rings exhibit a dual-ring pattern on the NC membrane. The tunneling speed of the capillary effect is so fast that the pattern appears within seconds. We apply the TCR effect as a size-surface affinity-particle/fluid separation sensor for bacteria. Dextran-modified Au and MoS2 nanostructures are proposed to be antibody-free microbe kits. Our TCR effect is used to distinguish between particles of different sizes and affinities, which are highly relevant in complicated systems without electricity and equipment in resource-poor settings.

Study on Enhancing the Quantitative Performance of LIBS for Cu Element in Liquids Based on the Chitosan-Parafilm Enrichment Method

Abstract To enhance the accuracy of laser-induced breakdown spectroscopy (LIBS) in detecting heavy metal elements in solutions, a Chitosan (CS)-Parafilm (PM) enrichment method is proposed. This method involves drying chitosan-heavy metal complexes on a Parafilm substrate. During the drying of droplets, electrostatic attraction and chelation by CS effectively concentrate analytes. Additionally, the hydrophobic effect of the PM substrate induces Marangoni flow, which drags analytes from the bottom edge to the central top of the droplet surface, effectively suppressing the coffee ring effect (CRE).LIBS spectra were collected in a uniform array on the surface of sediments, and spectral stability was assessed through cumulative excitation spectra measurements. The results showed that the spectral stability of emission lines Cu I 324.754 nm and Cu I 327.396 nm decreased to 3.85% and 3.78%, respectively, indicating that the CS-PM enrichment method allows for uniform deposition of analytes within samples, effectively enhancing detection repeatability.Quantitative analysis of Cu elements using the CS-PM enrichment method was conducted using PSO-SVM, PSO-BPNN, and RF algorithms. The RF algorithm demonstrated the best predictive performance with Rp² of 0.979, RMSEP of 4.60 mg/L, MAE of 3.19 mg/L, and RPD of 6.75. These results indicate that CS-PM effectively improves spectral stability and quantitative analysis accuracy, providing a technical reference for enhancing the stability and quantitative performance of element analysis in liquids using LIBS.

Analysis of deliquescent chloride salt by laser-induced breakdown spectroscopy with controlled uniform precipitation

BackgroundThe electrolytic efficiency in magnesium (Mg) production by molten salt electrolysis is mainly affected by the chloride content, which is determined by the metal content in the cooled molten salt. Laser-induced breakdown spectroscopy (LIBS) is a potential element detection method in cooled molten salt element detection. However, the cooled molten chloride salt is easily deliquescent, which greatly affects the LIBS detection results. ResultsTo solve the problem, a liquid-phase precipitation method based on the addition of a dispersant named as controlled uniform precipitation (CUP) method was proposed to pretreat the samples. Compared with the conventional powder compacting method and precipitation method, the CUP obtained the highest spectral stability and detection accuracy. The average relative standard deviation (ARSD) of Mg, Ca, and Na decreased from the 51.80%, 77.04%, and 44.01% to 6.77%, 6.27%, and 29.97%, the determination coefficient of the calibration curve (R2) increased from 0.044, 0.084, and -0.109 to 0.974, 0.954, and 0.802, and the average relative error (ARE) decreased from 58.49%, 52.46%, and 99.78% to 8.58%, 11.28%, and 29.38%, respectively, compared with the powder compacting method. Further investigation showed that the CUP method suppressed the sample deliquescence and the coffee ring effect, leading to the performance improvement of LIBS quantitative detection. SignificanceThe CUP provided an effective method of detecting metal elements for deliquescent samples and showed applied potential for other precipitable elements detection.

Role of motility and nutrient availability in drying patterns of algal droplets.

Sessile drying droplets in various bio-related systems attracted attention due to the complex interactions between convective flows, droplet pinning, mechanical stress, wettability, and the emergence of unique patterns. This study focuses on the drying dynamics of Chlamydomonas reinhardtii (chlamys), a versatile model algae used in molecular biology and biotechnology. The experimental findings shed light on how motility and nutrient availability influence morphological patterns- a fusion of macroscopic fluid dynamics and microbiology. This paper further discusses the interplay of two competing stressors during drying- nutrient scarcity (quantitative analysis) and mechanical stress (qualitative analysis), where the global mechanical stress does not induce cracks. Interestingly, motile chlamys form clusters under nutrient scarcity due to metabolic stress, indicating the onset of flocculation, a common feature observed in microbial systems. Moreover, non-motile chlamys exhibit an "anomalous coffee-ring effect" in the presence of nutrients, with an inward movement observed near the droplet edge despite sufficient water in the droplet. The quantitative image processing techniques provide fundamental insights into these behaviors in classifying the patterns into four categories (motile+with nutrients, motile+without nutrients, non-motile+with nutrients, and non-motile+without nutrients) across five distinct drying stages- Droplet Deposition, Capillary Flow, Dynamic Droplet Phase, Aggregation Phase, and Dried Morphology.

Rapid identification of milk powder adulteration based on surface-enhanced Raman spectroscopy

In recent years, milk powder adulteration has emerged as a matter of great concern. In this study, a rapid, accurate, and efficient detection method based on surface-enhanced Raman spectroscopy (SERS) combined with principal component analysis (PCA) was established to detect milk powder adulteration. The “coffee ring” effect-based gold nanoparticles (Au NPs) as the SERS-enhancing substrate were coupled with a portable Raman spectrometer, which enabled the differentiation of various brands of milk powder and the detection of melamine in milk powder. The substrate exhibited good SERS enhancement ability with an enhancement factor of 104. Furthermore, a strong linear correlation with a correlation coefficient (R2) of 0.9903 was observed between the melamine Raman intensity and concentration from 0.5 to 5.0 mg/kg. The calculated limit of detection of melamine (LOD) was 0.15 mg/kg, while the limit of quantitation (LOQ) was 0.5 mg/kg. In addition, when the method was applied to the detection of melamine in milk powder samples, this method achieved the recovery rates of melamine in milk powder samples ranged from 92.83% to 98.86% with relative standard deviations between 0.84% and 1.14%. In summary, the established method offers the advantages of cost-effectiveness, less sample requirement, and shorter detection time, meeting the needs for milk powder classification and rapid melamine detection.

Controlling Drying Conditions in Vacuum for Uniform Film Formation in Inkjet-Printed OLEDs.

The drying process of inkjet-printed organic light-emitting diodes (OLEDs) is influenced by both ink properties and external environmental factors, which ultimately affect the film profile. First, we conducted a detailed investigation of the drying time based on changes in the boiling point (BP) of mixed solvents and analyzed the correlation with the film profile. Under atmospheric drying conditions in a nitrogen (N2) atmosphere, the increased drying time under capillary-driven flow leads to greater particle movement toward the edges, significantly increasing the coffee-ring effect. Additionally, using a high-boiling-point solvent mixture of ethyl 4-methylbenzoate (EMB) and 2-ethylhexyl benzoate (EHB), we produced uniform thin films both between and within pixels (inter and intrapixel uniformity) through a vacuum drying process. In particular, we proposed a drying process model that divides the drying of inkjet pixels into a microfluidic phase and a gelation phase. Through five gelation phase-controlled vacuum drying experiments, the morphology within the pixels was precisely investigated. By sufficiently removing residual solvents after the microfluidic phase and then proceeding with heating, we produced uniform thin films. Furthermore, we fabricated OLED devices using this gelation phase-controlled vacuum drying process, achieving uniform pixel emission and improved device performance.

Space-Confined Synthesis of Thinner Ether-Functionalized Nanofiltration Membranes with Coffee Ring Structure for Li+/Mg2+ Separation.

Positively charged nanofiltration membranes have attracted much attention in the field of lithium extraction from salt lakes due to their excellent ability to separate mono- and multi-valent cations. However, the thicker selective layer and the lower affinity for Li+ result in lower separation efficiency of the membranes. Here, PEI-P membranes with highly efficient Li+/Mg2+ separation performance are prepared by introducing highly lithophilic 4,7,10-Trioxygen-1,13-tridecanediamine (DCA) on the surface of PEI-TMC membranes using a post-modification method. Characterization and experimental results show that the utilization of the DCA-TMC crosslinked structure as a space-confined layer to inhibit the diffusion of the monomer not only increases the positive charge density of the membrane but also reduces its thickness by ≈35% and presents a unique coffee-ring structure, which ensures excellent water permeability and rejection of Mg2+. The ion-dipole interaction of the ether chains with Li+ facilitates Li+ transport and improves the Li+/Mg2+ selectivity (SLi,Mg=23.3). In a three-stage nanofiltration process for treating simulated salt lake water, the PEI-P membrane can reduce the Mg2+/Li+ ratio of the salt lake by 400-fold and produce Li2CO3 with a purity of more than 99.5%, demonstrating its potential application in lithium extraction from salt lakes.

A Surface-Enhanced Raman Scattering Substrate with Tunable Localized Surface Plasmon Resonance Absorption Based on AgNPs.

In this paper, a three-layer structure of silver particle (AgNP)-dielectric-metal is proposed and constructed based on the characteristics of AgNPs that can excite LSPR (Localized Surface Plasmon Resonance) in free space. In order to overcome the problem of AgNPs easily oxidizing in the air, this paper synthesizes AgNPs using the improved Tollens method and effectively suppresses the coffee-ring effect by changing the solution evaporation conditions, so that the distribution of AgNPs in the deposition area is relatively uniform. The structure proposed in this paper takes advantage of the flexibility of nanoparticle application. The AgNPs deposited on the dielectric layer can effectively localize energy and regulate the LSPR of the device well. The structure can not only achieve precise regulation of the LSPR resonance peak of AgNPs but also can be used as a SERS substrate.

Enhanced Sensitivity and Homogeneity of SERS Signals on Plasmonic Substrate When Coupled to Paper Spray Ionization–Mass Spectrometry

This work reports on the development of an analyte sampling strategy on a plasmonic substrate to amplify the detection capability of a dual analytical system, paper spray ionization–mass spectrometry (PSI-MS) and surface-enhanced Raman spectroscopy (SERS). While simply applying only an analyte solution to the plasmonic paper results in a limited degree of SERS enhancement, the introduction of plasmonic gold nanoparticles (AuNPs) greatly improves the SERS signals without sacrificing PSI-MS sensitivity. It is initially revealed that the concentration of AuNPs and the type of analytes highly influence the SERS signals and their variations due to the “coffee ring effect” flow mechanism induced during sampling and the degree of the interfacial interactions (e.g., van der Waals, electrostatic, covalent) between the plasmonic substrate and analyte. Subsequent PSI treatment at high voltage conditions further impacts the overall SERS responses, where the signal sensitivity and homogeneity significantly increase throughout the entire substrate, suggesting the ready migration of adsorbed analytes regardless of their interfacial attractive forces. The PSI-induced notable SERS enhancements are presumably associated with creating unique conditions for local aggregation of the AuNPs to induce effective plasmonic couplings and hot spots (i.e., electromagnetic effect) and for repositioning analytes in close proximity to a plasmonic surface to increase polarizability (i.e., chemical effect). The optimized sampling and PSI conditions are also applicable to multi-analyte analysis by SERS and MS, with greatly enhanced detection capability and signal uniformity.

Profile improvement of blade coated circuits by the capillary force originating from the hydrophobic sidewalls

Abstract Restricting the diffusion of conductive inks plays a key role in printed electronics application. Micro-channels with different sidewall surface energies, which can be approximated as a capillary, are fabricated to restrict the blade-coated ink diffusion using both of the gravitational effect and the capillary force. The coffee ring effect of aqueous silver ink is inhibited by the capillary force when the hydrophobic sidewalls distance is no more than 50 μm in this paper. As a result, the conductive lines with improved cross-sectional profiles are obtained by this method, with the resistances about 1E8 times lower than the samples with hydrophilic sidewalls. The capillary force was also found to lose its effect when the width is larger enough, which needs surfactant addition to improve the silver film property. I-V curves and resistances of the original aqueous ink and the ink improved by traditional methods are compared, which confirms that the profile improvement by the hydrophobic sidewall can be used with other ink improving methods cooperatively. These studies open up the possibility of improving the printed conductive patterns by this method as an auxiliary tool used together with the traditional methods reported before.

High performance pixelated quantum dots array on Micro-LED by inkjet printing

Micro-LED display has risen to prominence in research and industry, distinguished by their numerous advantages. Quantum dots (QDs) film, serving as color conversion layers (CCLs) for Micro-LED, are considered a leading candidate for the next generation of displays, owing to their low power consumption and broad color gamut. Inkjet printing stands out as a feasible approach for crafting QDs CCLs. The essence of the process hinges on attaining a printed QDs film that boasts both uniformity and high-precision QDs array. However, the unstable printing of QDs ink significantly impacts the surface morphology of QDs arrays and the display performance of Micro-LEDs. Therefore, elucidating the mechanism behind the unstable printing of QDs in inkjet printing and fabricating QDs is of paramount importance. In this study, through the simulation of unstable printing with conventional QDs ink, we identified nozzle blockage and the rapid evaporation of low-boiling-point ink as the primary causes of unstable printing. This leads to aggregation and deposition of QDs, obstructing the nozzle and altering the fluid path. A novel binary organic composite ink was employed to print the QDs array on the substrate with ultra-stability, achieving a high yield and greatly reduced coffee-ring effect Micro-LED full-color display.

Enhanced Marangoni flow to fabricate uniform and porous SiO2 films: Toward superior high transparency, antifogging, and self- cleaning properties

Fogging on transparent substrate greatly impacts transmittance, causing inconvenience and potential hazards. Increasing surface roughness to enhance hydrophilicity often reduces transmittance. Here, by enhancing the Marangoni flow to inhibit the coffee-ring effect during SiO2 droplet drying, we propose for the first time to manufacture superhydrophilic and high-transmittance coatings with porous structures on various substrates, including glass and resin. This approach relies on a simple, potentially large-scale ultrasonic spray method to enhance the Marangoni flow of SiO2 dispersion with different particle sizes by adding multiple surfactants to reduce the surface tension of the dispersion. The water contact angle of the SiO2 film decreases to below 5° within 0.5 s, imparting anti-fogging properties to the substrate. The film exhibits a transmittance of 93.1 % at 580 nm, surpassing the inherent 90.8 % transmittance of the bare glass. Notably, the film shows remarkable self-cleaning properties due to potent hydration from eco-friendly SiO2 nanoparticles and the porous structure. Owing to the hydrogen bonding between tightly packed small-sized SiO2 particles as well as between the particles and substrate, the surface morphology of the film on glass remains intact even after washing, and soaking. The porous SiO2 film, with a contact angle dropping to below 5° within 0.5 s, maintains its superhydrophilicity even after being exposed to high temperatures, UV irradiation, and humidity conditions. The simple production process of the porous SiO2 films offers a promising method for preparing high-transmittance anti-fogging films with significant industrial potential.

Plasmonic array at liquid-liquid interface for trace microplastics detection

Microplastics (MPs) pollution has become a focal point of global concern, as the widespread distribution of MPs in the environment poses a potential threat to ecosystems and human health. Despite surface-enhanced Raman scattering (SERS) being one of the most promising detection techniques, the “coffee ring effect” of MPs and their unique size scale make effective contact with SERS substrates challenging. Here, we propose a novel strategy for MPs detection by constructing a plasmonic metasurface at the liquid-liquid interface (LLI). The fluidic properties of LLI provide more flexibility for the uniform dispersion of MPs and the assembly of plasmonic arrays. MPs are captured and assembled on an atomic-level metasurface in a core-shell structure (AuNPs@MPs), providing abundant “hot spots” and additional volume-enhanced Raman effects. This results in enhanced electromagnetic field around MPs and increased enhancement factors, demonstrating high SERS sensitivity and uniformity. This sensing platform provides a new approach for trace detection of MPs and has been validated in practical matrix.

Gold nano densities: Relationship with drying parameters

Gold nanorod stain deposits have been studied using helium ion microscopy (HIM). The multilayer thick deposits can become well-organized monolayers when the density drops, as demonstrated by the density fluctuation of nanosuspension. However, once the density of nanoparticles approaches certain extremely low values, this tendency has disappeared. The difference in density associated with the liquid crystalline (LC) phases that have developed on the surface. There is a correlation between the density of nanoparticles in suspension and the relationship between droplet volume and UV-Vis absorbance. The physical characteristics that affect and generate such LC phases are contrasted and examined with the assembly method. To highlight noteworthy aspects, a quantitative comparison between the HIM analysis and the SEM is also made. Analyzing the correlation between the coffee stain range, nanoparticle density, and droplet contact angle is essential. It is possible to quantify the difference in the ring stains from the outside to the midle and from the midle to the inner boundary. HIM is typically useful when attempting to determine the true composition of surface deposits. The aim of this work is to investigate the effects of particle density on liquid crystalline superstructures and the coffee stain effect.

Coffee-Ring Mediated Thinning and Thickness-Dependent Dewetting Modes in Printed Polymer Droplets Coupled with Assembly of Quantum Dots for Anti-Counterfeiting.

Molecular transport processes in printed polymer droplets hold enormous importance for understanding wetting phenomena and designing systems in applications such as encoding, electronics, photonics, and sensing. This paper studies thickness-dependent dewetting modes that are activated by thermal annealing and driven by interfacial interactions within microscopically confined polymeric features. The printing of poly(2-vinylpyridine) is performed in a regime where coffee-ring effects lead to strong thinning of the central region of the deposit. Thermal annealing leads to two different modes of dewetting that depend on the thickness of the central region. Mode I refers to the formation of randomly positioned small features surrounded by large hemispherical ones located along the periphery of the printed features and occurs when the central regions are thin. Observed at large central thicknesses, Mode II mediates significant molecular transport from edges toward the center of the printed droplet with thermal annealing and forms a hemispherical feature from the initial ring-like deposit. The selective adsorption of red, green, and blue emitting quantum dots over the poly(2-vinylpyridine) results in photoluminescent patterns. The selective assembly of photoluminescent quantum dots over patterned surfaces leads to deterministic and stochastic features beneficial to creating security labels for anti-counterfeiting applications.

Using a Flexible Fountain Pen to Directly Write Organic Semiconductor Patterns with Crystallization Regulated by the Precursor Film.

Organic semiconductor (OSC) films fabricated by meniscus-guided coating (MGC) methods are suitable for cost-effective and flexible electronics. However, achieving crystalline thin films by MGC methods is still challenging because the nucleation and crystal growth processes are influenced by the intertwined interactions among solvent evaporation, stochastic nucleation, and the fluid flow instabilities. Herein, a novel flexible fountain pen with active ink supply is designed and used to print OSCs. This direct-write method allows the flexible pen tip to contact the substrate, maintaining a robust meniscus by eliminating the gap found in conventional MGCs. An in situ optical microscopy observation system shows that the precursor film plays a critical role on the crystallization and the formation of coffee rings and dendrites. The computational fluid dynamics simulations demonstrate that the microstructure of the pen promotes extensional flows, facilitating mass transport and crystal alignment. Highly-aligned ribbon-shaped crystals of a small organic molecule (TIPS-pentacene), as well as a semiconducting polymer (N2200) with highly-ordered orientations, have been successfully printed by the flexible fountain pen. Organic field-effect transistors based on the flexible pen printed OSCs exhibit high performances and strong anisotropic mobility. In addition, the flexible fountain pen is expandable for printing multiple lines or large-area films.

Ring formation of transition metal trichalcogenide TaSe3 using vapor liquid process.

The synthesis of TaSe3 ring-shaped crystals displaying the coffee ring effect is investigated. By recrystallizing TaSe3 microcrystals dissolved in droplets of condensed Se gas, ring-shaped crystals were successfully grown. This novel method for ring formation effectively addressed the issue of connecting the edges of the crystal. Consequently, the synthesis method has the capability to grow MX3 ring-shaped crystals in any location where droplets can condense, can now be grown in specific locations, thus creating opportunities for advancements in electronic component development.

Nanopipette dynamic microscopy unveils nano coffee ring

Liquid-phase electron microscopy (LP-EM) imaging has revolutionized our understanding of nanosynthesis and assembly. However, the current closed geometry limits its application for open systems. The ubiquitous physical process of the coffee-ring phenomenon that underpins materials and engineering science remains elusive at the nanoscale due to the lack of experimental tools. We introduce a quartz nanopipette liquid cell with a tunable dimension that requires only standard microscopes. Depending on the imaging condition, the open geometry of the nanopipette allows the imaging of evaporation-induced pattern formation, but it can also function as an ordinary closed-geometry liquid cell where evaporation is negligible despite the nano opening. The nano coffee-ring phenomenon was observed by tracking individual nanoparticles in an evaporating nanodroplet created from a thin liquid film by interfacial instability. Nanoflows drive the assembly and disruption of a ring pattern with the absence of particle-particle correlations. With surface effects, nanoflows override thermal fluctuations at tens of nanometers, in which nanoparticles displayed a "drunken man trajectory" and performed work at a value much smaller than kBT.