Insights into Vapor-Mediated Interactions in a Nanocolloidal Droplet System: Evaporation Dynamics and Affects on Self-Assembly Topologies on Macro- to Microscales

Functional supraparticles produced by the evaporation of binary colloidal suspensions on superhydrophobic surfaces

Hierarchically structured supraparticles can be produced by drying droplets of colloidal suspensions. Using binary suspensions provides degrees of structural and functional control beyond those possible for single components, while remaining tractable for fundamental mechanistic studies. Here, we implement evaporative co-assembly of two distinct particle types - 'large' polystyrene microparticles and 'small' inorganic oxide nanoparticles (silica, titania, zirconia, or ceria) - dried on superhydrophobic surfaces to produce bowl-shaped supraparticles. We extend this method to raspberry colloid templating, in which the binary suspension consists of titania nanoparticles together with gold-decorated polystyrene colloids. Following removal of the polymer particles, we demonstrate catalytic oxidative coupling of methanol to methyl formate using the resulting mesoporous supraparticles, showcasing their practical application.

Analysis of Evaporation of Droplet Pairs by a Quasi-Steady-State Diffusion Model Coupled with the Evaporative Cooling Effect

Multidroplet evaporation is a common phase-change phenomenon not only in nature but also in many industrial applications, including inkjet printing and spray cooling. The evaporation behavior of these droplets is strongly affected by the distance between neighboring droplets, and in particular, evaporation suppression occurs as the distance decreases. However, further quantitative information, such as the temperature and local evaporation flux, is limited because the analytical models of multidroplet evaporation only treat vapor diffusion, and the effect of the latent heat transfer through the liquid-vapor phase change is ignored. Here, we perform a numerical analysis of evaporating droplet pairs that linked vapor diffusion from the droplet surface and evaporative cooling. Heat transfer through the liquid and gas phases is also considered because the saturation pressure depends on the temperature. The results show an increase in the vapor concentration in the region between the two droplets. Consequently, the local evaporation flux in the proximate region significantly decreases with decreasing separation distance. This means that the latent heat transfer through the phase change is diminished, and an asymmetrical temperature distribution occurs in the liquid and gas phases. These numerical results provide quantitative information about the temperature and local evaporation flux of evaporating droplet pairs, and they will guide further investigation of multiple droplet evaporation.

Two-dimensional printing of nanoparticles as a promising therapeutic method for personalized drug administration

The necessity for personalized patient treatment has drastically increased since the contribution of genes to the differences in physiological and metabolic state of individuals have been exposed. Different approaches have been considered so far in order to satisfy all of the diversities in patient needs, yet none of them have been fully implemented thus far. In this framework, various types of 2D printing technologies have been identified to offer some potential solutions for personalized medication, which development is increasing rapidly. Accurate drug-on-demand deposition, the possibility of consuming multiple drug substances in one product and adjusting individual drug concentration are just some of the few benefits over existing bulk pharmaceuticals manufacture, which printing technologies brings. With inclusion of nanotechnology by printing nanoparticles from its dispersions some further opportunities such as controlled and stimuli-responsive drug release or targeted and dose depending on drug delivery were highlighted. Yet, there are still some challenges to be solved before such products can reach the pharmaceutical market. In those terms mostly chemical, physical as well as microbiological stability concerns should be answered, with which 2D printing technology could meet the treatment needs of every individual and fulfill some existing drawbacks of large-scale batch production of pharmaceuticals we possess today.

M. T, Cho G, Lee J. Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications

The uneven deposition at the edges of an evaporating droplet, termed the coffee-ring effect, has been extensively studied during the past few decades to better understand the underlying cause, namely the flow dynamics, and the subsequent patterns formed after drying. The non-uniform evaporation rate across the colloidal droplet hampers the formation of a uniform and homogeneous film in printed electronics, rechargeable batteries, etc., and often causes device failures. This review aims to highlight the diverse range of techniques used to alleviate the coffee-ring effect, from classic methods such as adding chemical additives, applying external sources, and manipulating geometrical configurations to recently developed advancements, specifically using bubbles, humidity, confined systems, etc., which do not involve modification of surface, particle or liquid properties. Each of these methodologies mitigates the edge deposition via multi-body interactions, for example, particle-liquid, particle-particle, particle-solid interfaces and particle-flow interactions. The mechanisms behind each of these approaches help to find methods to inhibit the non-uniform film formation, and the corresponding applications have been discussed together with a critical comparison in detail. This review could pave the way for developing inks and processes to apply in functional coatings and printed electronic devices with improved efficiency and device yield.

Spatio-temporal modulation of self-assembled central aggregates of buoyant colloids in sessile droplets using vapor mediated interactions

A functional sessile droplet containing buoyant colloids (ubiquitous in applications like chemical sensors, drug delivery systems, and nanoreactors) forms self-assembled aggregates. The particles initially dispersed over the entire drop-flocculates at the center. We attribute the formation of such aggregates to the finite radius of curvature of the drop and the buoyant nature of particles. Initially, larger particles rise to the top of the droplet (due to higher buoyancy force), and later the smaller particles join the league, leading to the graded size distribution of the central aggregate. This can be used to segregate polydisperse hollow spheres based on size. The proposed scaling analysis unveils insights into the distinctive particle transport during evaporation. However, the formation of prominent aggregates can be detrimental in applications like spray painting, sprinkling of pesticides, washing, coating, lubrication, etc. One way to avoid the central aggregate is to spread the droplets completely (contact angle ~ 0⁰), thus theoretically creating an infinite radius of curvature leading to uniform deposition of buoyant particles. Practically, this requires a highly hydrophilic surface, and even a small inhomogeneity on the surface would pin the droplet giving it a finite radius of curvature. Here, we demonstrate using non-intrusive vapor mediated Marangoni convection (Velocity scale ~ O(10³) higher than the evaporation-driven convection) can be vital to an efficient and on-demand manipulation of the suspended micro-objects. The interplay of surface tension and buoyancy force results in the transformation of flow inside the droplet leads to spatiotemporal disbanding of agglomeration at the center of the droplet.

Optimization of electromagnetic hot spots in surface-enhanced Raman scattering substrates for an ultrasensitive drug assay of emergency department patients' plasma

Herein we report the programmable preparation of ultrasensitive surface-enhanced Raman scattering (SERS)-based nanoplasmonic superlattice substrates to assay fentanyl and cocaine (detection and quantification) from 10 μL aliquots of emergency department patient plasma without the need for purification steps. Highly homogeneous three-dimensional (3D) nanoplasmonic superlattices are generated through the droplet evaporation-based self-assembly process of chemically-synthesized, polyethylene glycol thiolate-coated gold triangular nanoprisms (Au TNPs). Close-packed, solid-state 3D superlattice substrates produce electromagnetic hot spots due to near-field plasmonic coupling of Au TNPs, which display unique localized surface plasmonic resonance properties. These uniquely prepared superlattice substrates enable strong SERS enhancement to achieve a parts-per-quadrillion limit of detection using the label-free SERS-based technique. Our reported limit of detection is at least 100-fold better than any known SERS substrates for the drug assay. Importantly, our density functional theory calculations show that a specific electronic interaction between the drug molecule and novel nanoplasmonic superlattice substrates plays a critical role that may trigger achieving this unprecedentedly high sensitivity. Additionally, we show high selectivity of the superlattice substrate in the SERS-based detection of analytes from different patient samples, which do and do not contain target analytes (i.e., fentanyl and/or cocaine). The demonstrated sensitivity and selectivity of 3D superlattice substrates for SERS-based drug analysis in real toxicological samples are expected to advance the field of measurement science, and forensic and clinical toxicology by obviating the need for complicated sample processing steps, long assay times, and the low sensitivity of existing "gold standard" analytical techniques including gas chromatography/mass spectrometry, liquid chromatography/mass spectrometry and enzyme-linked immunosorbent assays. Taken together, we believe that this entirely new and reproducible superlattice substrate for the SERS analysis will aid scientific, forensic, and healthcare communities to battle the drug overdose epidemic in the United States.

Modeling the role of respiratory droplets in Covid-19 type pandemics

In this paper, we develop a first principles model that connects respiratory droplet physics with the evolution of a pandemic such as the ongoing Covid-19. The model has two parts. First, we model the growth rate of the infected population based on a reaction mechanism. The advantage of modeling the pandemic using the reaction mechanism is that the rate constants have sound physical interpretation. The infection rate constant is derived using collision rate theory and shown to be a function of the respiratory droplet lifetime. In the second part, we have emulated the respiratory droplets responsible for disease transmission as salt solution droplets and computed their evaporation time, accounting for droplet cooling, heat and mass transfer, and finally, crystallization of the dissolved salt. The model output favourably compares with the experimentally obtained evaporation characteristics of levitated droplets of pure water and salt solution, respectively, ensuring fidelity of the model. The droplet evaporation/desiccation time is, indeed, dependent on ambient temperature and is also a strong function of relative humidity. The multi-scale model thus developed and the firm theoretical underpinning that connects the two scales-macro-scale pandemic dynamics and micro-scale droplet physics-thus could emerge as a powerful tool in elucidating the role of environmental factors on infection spread through respiratory droplets.

Moses Effect: Splitting a Sessile Droplet Using a Vapor-Mediated Marangoni Effect Leading to Designer Surface Patterns

In this work, we showcase a mechanism of rapid and focused solvent depletion using vapor-mediated interaction that can nonintrusively cleave a sessile water droplet reminiscent of Moses parting the Red Sea. The Marangoni effect is induced by the differential adsorption of vapor from a nearby pendant droplet of ethanol, leading to an exponential increase in surface velocity inside the water droplet. The Marangoni convection leads to the drainage of liquid from the central section of the water droplet and consequently splits it. By encoding the position of the ethanol (vertical as well as horizontal) droplet, an array of liquid motion is observed (split, shift, and slosh) in the water droplet. This method is further extended to nanocolloidal systems, where the liquid motion can be exploited to generate a wide gamut of deposit patterns ranging from uniform precipitate to sporadic islands without resorting to the more traditional evaporation-driven capillary flows ("coffee stains") or custom engineering of the shape of the nanoparticles. We further provide a detailed exposition of the physical mechanisms responsible for the splitting of the liquid drop and consequent particle deposition. The concept can be extended to liquid actuation in open channel microfluidic chips and surface patterning as in medical diagnostics, optoelectronics, and thermal management.

Controlling self-assembly and buckling in nano fluid droplets through vapour mediated interaction of adjacent droplets

HYPOTHESIS

Sessile droplets of contrasting volatilities can communicate via long range (∼O (1) mm) vapour-mediated interactions which allow the remote control of the flow driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets.

EXPERIMENTS

A nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption induced Marangoni flow (∼O (1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet leaving the other side to develop a relatively weaker shell which buckles under the effect of evaporation driven capillary pressure.

FINDINGS

The inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates (∼1000 times) in droplet-based bioassays with minimal contamination.

Evaporation-Oscillation Driven Assembly: Microtailoring the Spatial Ordering of Particles in Sessile Droplets

This work explores the physical mechanism that can be used to control the final residual pattern of nanoparticles obtained from an evaporating-oscillating sessile droplet. To that end, the substrate is vibrated in the vertical direction with a constant amplitude, while the frequency of excitation is varied. It is found that evaporation progressively shifts the mode number of the oscillating droplet to lower values, while the oscillations enhance the rate of solvent loss, causing a reduction in the droplet lifetime. The coupling between evaporation and oscillation drives the internal flow through two distinct regimes. Initially, oscillation leads to inner flow recirculation, which delays the evaporation driven edge deposition of particles. Subsequently at lower modes, caused by solvent depletion, the effect of oscillation is weakened, which allows evaporation-driven flow to gain prominence and thus transport the dispersed particles to the contact line. We demonstrate here how this delay in particle migration can be controlled to engineer morphological changes in not just the resulting macroscopic aspect of the deposit but also its microstructure. We especially focus on the relatively unexplored microstructural pattern of deposits from evaporating-oscillating droplets. Using scanning electron micrograph and Voronoi tessellation of the final deposit, we show unique spatial variation in particle ordering at macro-micro length scales. Thus, droplet oscillation tunes the spatial extent of the particle ordering crucial in applications like photonic crystals and photonic glass.

Multiscale Dynamic Growth and Energy Transport of Droplets during Condensation

Condensation is an important physical process and has direct relevance for a range of engineering applications, including heat transfer, antifrosting, and self-cleaning. Understanding the mechanism of droplet growth during condensation is an important aspect, but past works have not typically considered the dynamics of the multiscale process. In this paper, we developed a dynamic growth model, which considers the continuous and multiscale nature of the droplet growth process from several nanometers to hundreds of microns. This model couples the transient phase change heat transfer and two-phase flow both inside and outside the droplet. Accordingly, the energy transport is distinct from the classical pure conduction model. We show that convection near the liquid-vapor interface and inside the droplets plays an increasingly important role as droplets grow and finally dominates the energy transport process. Driven by strong convection, the droplets mix well and the discrete layers of temperature observed in the pure conduction model disappear at the microscale. This model that considers convection can lead to over 4 times higher predicted overall heat transfer than that obtained with the pure conduction model. The interfacial mass flow through the liquid-vapor interface is the dominant factor responsible for the strong convection. We studied the critical radius where convection starts to have a significant influence on droplet growth under different subcooling temperatures and contact angles. Droplets have smaller critical radii under larger subcooling temperatures or larger contact angles, ranging from 0.5 to 20 μm. This work identifies the modes of energy transport in condensation at different scales, which not only enhances our fundamental understanding of individual droplet growth but provides design guidelines for various dropwise and jumping-droplet condensation research.

Engineering Interfacial Processes at Mini-Micro-Nano Scales Using Sessile Droplet Architecture

Evaporating sessile functional droplets act as the fundamental building block that controls the cumulative outcome of many industrial and biological applications such as surface patterning, 3D printing, photonic crystals, and DNA sequencing, to name a few. Additionally, a drying single sessile droplet forms a high-throughput processing technique using low material volume which is especially suitable for medical diagnosis. A sessile droplet also provides an elementary platform to study and analyze fundamental interfacial processes at various length scales ranging from macroscopically observable wetting and evaporation to microfluidic transport to interparticle forces operating at a nanometric length scale. As an example, to ascertain the quality of 3D printing we must understand the fundamental interfacial processes at the droplet scale. In this article, we review the coupled physics of evaporation flow-contact-line-driven particle transport in sessile colloidal droplets and provide methodologies to control the same. Through natural alterations in droplet vaporization, one can change the evaporative pattern and contact line dynamics leading to internal flow which will modulate the final particle assembly in a nontrivial fashion. We further show that control over particle transport can also be exerted by external stimuli which can be thermal, mechanical oscillations, vapor confinement (walled or a fellow droplet), or chemical (surfactant-induced) in nature. For example, significant augmentation of an otherwise evaporation-driven particle transport in sessile droplets can be brought about simply through controlled interfacial oscillations. The ability to control the final morphologies by manipulating the governing interfacial mechanisms in the precursor stages of droplet drying makes it perfectly suitable for fabrication-, mixing-, and diagnostic-based applications.

Hydrodynamics of Two Interacting Liquid Droplets of Aqueous Solution inside a Microchannel

We experimentally investigated the effect of a neighboring liquid droplet on fluid convection inside a liquid droplet of aqueous solution present inside a microchannel using the microscale particle image velocimetry technique. There is no physical contact between the two droplets, and the solute concentrations of the two droplets are set at different values. Vapor concentration near the interface of the two droplets is different due to the difference in solute concentration. Water vapor evaporates from the low-concentration droplet having higher vapor pressure and condenses on the high-concentration droplet having lower vapor pressure. Evaporation and condensation induce Rayleigh convection inside the two droplets. Flow pattern shows circulating loops inside both liquid droplets. The circulations at the interacting adjacent interface of the two droplets are opposite to each other. The strength of flow inside the liquid droplets decreases with time due to decrease in the difference of solute concentration between the two droplets. The flow strength inside the two interacting droplets is also a function of separation distance between the droplets. The flow strength inside the droplets decreases with increase in separation distance.

Evaporation Dynamics of Mixed-Nanocolloidal Sessile Droplets

Evaporation dynamics of a particle-laden droplet has been a topic of interest in recent times owing to its widespread applications, ranging from surface patterning to drug delivery systems. The interplay of evaporation-induced internal flow dynamics, contact line dynamics, and nanoparticle self-assembly govern the morphologies of the residual structures. Fine-tuning of these residual structures is thus possible by controlling the governing parameters. A nanoparticle-laden sessile droplet placed on a hydrophobic substrate undergoes buckling phenomenon that results in a domelike structure with cavity on the surface. In the present work, it is shown that the addition of sodium dodecyl sulfate (SDS) surfactant in minute concentrations (0.005-0.02 wt %) can affect the contact line dynamics and subsequent buckling dynamics of a nanoparticle-laden droplet evaporating on a hydrophobic substrate. With increase in the initial SDS concentration, the morphologies of the residual structures show transition from a buckled dome structure to a flat flowerlike shape. Moreover, a critical SDS concentration (>0.0075 wt % in 20 wt % silica) is identified for the complete suppression of buckling instabilities. Last, the effects of droplet spreading on the surface crack dynamics are discussed.