Multicomponent Drops Research Articles

Dynamics of phase-separated microdroplets near the contact line of evaporating all-aqueous drops.

Evaporation of multicomponent drops can induce liquid-liquid phase separation and spatial reconfiguration of phases. Here, we unveil several novel dynamics near the contact line of evaporating multicomponent drops containing polyethylene glycol and dextran. The interplay between background Marangoni flow and self-migration of nucleated microdroplets creates both unstable and stable equilibrium points. This leads to either continuous migration or stepwise advancement of microdroplets, influenced by random coalescence events. Tiny dextran microdroplets nucleating at the contact line can migrate toward the bulk only by growing in size with coalescence events. Our findings offer new insights into the fundamental understanding of evaporating multicomponent drops and factors influencing the spatial segregation of phases in evaporative liquid-liquid phase separation with implications in prebiotic biomolecular reactions to industrial applications.

Modeling the evaporation of multicomponent drops of general shape on the basis of an analytical solution to the Stefan–Maxwell equations

This paper proposes an analytical approach to model the evaporation of multicomponent drops of general shape, which is based on the solution to Stefan–Maxwell equations. The model predicts the quasi-steady molar fractions and temperature distributions in the gas phase as well as the heat rate and the species evaporation rates. The model unifies previous approaches to this problem, namely, for spherical and spheroidal drops, under a unique model and proposes solutions for other shapes and geometries, such as sessile drops and drop pairs. To assess the model, a comparison with a numerical solution to the conservation equations is also reported for both different drop configurations and different compositions.

Microexplosions of multicomponent drops in spray flames

Experimental investigations of in-flight droplet microexplosions (μ-explosions) during flame spray pyrolysis and corresponding single drop experiments are carried out. High-speed shadowgraphy recordings with an iron nitrate-based precursor spray flame revealed the occurrence of distinct μ-explosions in high frequencies during the synthesis of iron oxide nanoparticles. This finding may lay the foundation for the investigation of the role and relevance of μ-explosions on the production and formulation of functional nanoparticle with distinct properties.

Combustion of Multi-Component Drops during Flame Spray Pyrolysis

Combustion of Multi-Component Drops during Flame Spray Pyrolysis

An analytical approach to model heating and evaporation of multicomponent ellipsoidal drops

An analytical model for heating and evaporation of non-spherical multicomponent drops is reported, based on the solution of energy and species conservation equations in orthogonal curvilinear coordinates, accounting also for the effect of convective conditions according to the film theory approach. The instantaneous heat and evaporation rates of multicomponent drops is calculated as function of increased surface area due to drop deformation. The model is applied to predict evaporation characteristics of spheroidal drops under quasi-steady and transient conditions, analysing the effect of drop size, composition and velocity on the temperature evolution, for a range of drop and gas temperature conditions

"Self-Shaping" of Multicomponent Drops.

In our recent study we showed that single-component emulsion drops, stabilized by proper surfactants, can spontaneously break symmetry and transform into various polygonal shapes during cooling [ Denkov Nature 2015 , 528 , 392 - 395 ]. This process involves the formation of a plastic rotator phase of self-assembled oil molecules beneath the drop surface. The plastic phase spontaneously forms a frame of plastic rods at the oil drop perimeter which supports the polygonal shapes. However, most of the common substances used in industry appear as mixtures of molecules rather than pure substances. Here we present a systematic study of the ability of multicomponent emulsion drops to deform upon cooling. The observed trends can be summarized as follows: (1) The general drop-shape evolution for multicomponent drops during cooling is the same as with single-component drops; however, some additional shapes are observed. (2) Preservation of the particle shape upon freezing is possible for alkane mixtures with chain length difference Δn ≤ 4; for greater Δn, phase separation within the droplet is observed. (3) Multicomponent particles prepared from alkanes with Δn ≤ 4 plastify upon cooling due to the formation of a bulk rotator phase within the particles. (4) If a compound, which cannot induce self-shaping when pure, is mixed with a certain amount of a compound which induces self-shaping, then drops prepared from this mixture can also self-shape upon cooling. (5) Self-emulsification phenomena are also observed for multicomponent drops. In addition to the three recently reported mechanisms of self-emulsification [ Tcholakova Nat. Commun. 2017 , ( 8 ), 15012 ], a new (fourth) mechanism is observed upon freezing for alkane mixtures with Δn > 4. It involves disintegration of the particles due to a phase separation of alkanes upon freezing.

Dynamics and universal scaling law in geometrically-controlled sessile drop evaporation

The evaporation of a liquid drop on a solid substrate is a remarkably common phenomenon. Yet, the complexity of the underlying mechanisms has constrained previous studies to spherically symmetric configurations. Here we investigate well-defined, non-spherical evaporating drops of pure liquids and binary mixtures. We deduce a universal scaling law for the evaporation rate valid for any shape and demonstrate that more curved regions lead to preferential localized depositions in particle-laden drops. Furthermore, geometry induces well-defined flow structures within the drop that change according to the driving mechanism. In the case of binary mixtures, geometry dictates the spatial segregation of the more volatile component as it is depleted. Our results suggest that the drop geometry can be exploited to prescribe the particle deposition and evaporative dynamics of pure drops and the mixing characteristics of multicomponent drops, which may be of interest to a wide range of industrial and scientific applications.

Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface.

Evaporation of multi-component drops is crucial to various technologies and has numerous potential applications because of its ubiquity in nature. Superamphiphobic surfaces, which are both superhydrophobic and superoleophobic, can give a low wettability not only for water drops but also for oil drops. In this paper, we experimentally, numerically and theoretically investigate the evaporation process of millimetric sessile ouzo drops (a transparent mixture of water, ethanol, and trans-anethole) with low wettability on a superamphiphobic surface. The evaporation-triggered ouzo effect, i.e. the spontaneous emulsification of oil microdroplets below a specific ethanol concentration, preferentially occurs at the apex of the drop due to the evaporation flux distribution and volatility difference between water and ethanol. This observation is also reproduced by numerical simulations. The volume decrease of the ouzo drop is characterized by two distinct slopes. The initial steep slope is dominantly caused by the evaporation of ethanol, followed by the slower evaporation of water. At later stages, thanks to Marangoni forces the oil wraps around the drop and an oil shell forms. We propose an approximate diffusion model for the drying characteristics, which predicts the evaporation of the drops in agreement with experiment and numerical simulation results. This work provides an advanced understanding of the evaporation process of ouzo (multi-component) drops.

Evaporation of Multicomponent Droplets

ABSTRACT The evaporation of multicomponent drops having diameters on the order 10 /am was investigated using electrodynamic levitation and light-scattering measurements. Multicomponent drops representative of diesel fuels and binary drops were studied. The binary system consisted of the halogenated compounds 1-iododocane and 1-bromotetradecane, and the model fuel drops were composed of hexadecane, 1-methyl naphthalene, and cis- and trans-decahydronaphthalene (decalin). The effect of an additive, tertiary butyl peroxide, on the evaporation of the multicomponent system was explored, and some data are reported for the additive ethyl hexyl nitrate. The additives were found to have negligible effect on the evaporation rate. The measured evaporation rates are compared with a theoretical analysis based upon diffusion-controlled evaporation using the UNIFAC (Reid et al., 1987) model for liquid phase activity coefficients. The model was found to be in good agreement with experiment.

Evaporation of Multicomponent Drop Arrays

The conventional fuels that are used in the field of transportation are primarily composed of two or more components. Each component evaporates, mixes with hot oxidant gases, ignites, and burns. Since evaporation is the precursor of the sequence of events leading to combustion, the evaporation studies on the multi-component drops are essential for determining the governing parameters of spray evaporation. While single-component drop studies have been carried out extensively in the past, very limited literature exists on the multicomponent array evaporation. The present paper deals with the evaporation of multicomponent fuel droplets in an array using the recently developed point source method (PSM). First, the quasi-steady (QS) evaporation of an isolated, multicomponent droplet is briefly analyzed. The resultant governing equations, along with Raoult’s law and the Cox-Antoine relation, constitute the set of equations needed to arrive at the solutions for: (1) the droplet surface temperature, (2) the evaporation rate of each species, and (3) the vapor mass fraction of each species at the droplet surface. The PSM, which treats the droplet as a point mass source and heat sink, is then adopted to obtain an analytic expression for the evaporation rate of a multicomponent droplet in an array of liquid droplets. Defining the correction factor (η) as a ratio of the evaporation of a drop in an array to the evaporation rate of a similar isolated multi-component drop, an expression for the correction factor is obtained. The results of the point source method (PSM) are then compared with those obtained elsewhere for a three-drop array that uses the method of images (MOI). Excellent agreement is obtained. The treatment is then extended to a binary drop array to study the effect of interdrop spacing on vaporization. When the drops are close to each other, the evaporation rate of the droplet in the array containing the larger percentage of volatiles is higher than the rate under isolated conditions (η>1). The results qualitatively confirm the experimental data reported elsewhere. Parametric results were obtained for the effect of changing the composition on the correction factor and finally critical drop compositions in the binary array are given for which η>1. Even though the results for the average correction factor of the whole array of 2 to 9 drops obtained using PSM are almost the same as the results from MOI, the correction factor of the center drop under severe interaction may deviate from those results obtained with MOI.