Viscosity-Mediated Growth and Coalescence of Surface Nanodroplets

Solvent exchange is a simple method to produce surface nanodroplets on a substrate for a wide range of applications by displacing a solution of good solvent, poor solvent and oil (Solution A) by a poor solvent (Solution B). In this work, we show that the growth and coalescence of nanodroplets on a homogeneous surface is mediated by the viscosity of the solvent. We show that at high flow rates of viscous Solution B, the final droplet volume deviates from the scaling law that correlates final droplet volume to the flow rate of non-viscous Solution B, reported in previous work. We attribute this deviation to a two-regime growth in viscous Solution B, where transition from an initial, fast regime to a final slow regime influenced by the flow rate. Moreover, viscous solution B hinders the coalescence of growing droplets, leading to a distinct bimodal distribution of droplet size with stable nanodroplets, in contrast to a continuous size distribution of droplets in non-viscous case. We demonstrate that the group of small droplets produced in high viscosity environment may be applied for enhanced fluorescence detection with higher sensitivity and shorter response time. The finding of this work can potentially be applied for mediating the size distribution of surface nanodroplets on homogeneous surface without templates.Comment: This is an unedited author's version of the submitted work that has been peer-reviewed and accepted in the Journal of Physical Chemistry

Self-propelling Microdroplets Generated and Sustained by Liquid-liquid Phase Separation in Confined Spaces

Flow transport in confined spaces is ubiquitous in technological processes, ranging from separation and purification of pharmaceutical ingredients by microporous membranes and drug delivery in biomedical treatment to chemical and biomass conversion in catalyst-packed reactors and carbon dioxide sequestration. In this work, we suggest a distinct pathway for enhanced liquid transport in a confined space via self-propelling microdroplets. These microdroplets can form spontaneously from localized liquid-liquid phase separation as a ternary mixture is diluted by a diffusing poor solvent. High speed images reveal how the microdroplets grow, break up and propel rapidly along the solid surface, with a maximal velocity up to ~160 um/s, in response to a sharp concentration gradient resulting from phase separation. The microdroplet self-propulsion induces a replenishing flow between the walls of the confined space towards the location of phase separation, which in turn drives the mixture out of equilibrium and leads to a repeating cascade of events. Our findings on the complex and rich phenomena of self-propelling droplets suggest an effective approach to enhanced flow motion of multicomponent liquid mixtures within confined spaces for time effective separation and smart transport processes.Comment: This is the authors' submitted version of the manuscrip

Viscosity-Mediated Growth and Coalescence of Surface Nanodroplets

Solvent exchange is a simple method to produce surface nanodroplets on a substrate for a wide range of applications by displacing a solution of good solvent, poor solvent, and oil (solution A) by a poor solvent (solution B). In this work, we show that the growth and coalescence of nanodroplets on a homogeneous surface is mediated by the viscosity of the solvent. We show that at high flow rates of viscous solution B, the final droplet volume deviates from the scaling law that correlates the final droplet volume to the flow rate of nonviscous solution B, reported in previous work. We attribute this deviation to a two-regime growth in viscous solution B, where transition from an initial, fast regime to a final slow regime is influenced by the flow rate. Moreover, viscous solution B hinders the coalescence of growing droplets, leading to a distinct bimodal distribution of droplet size with stable nanodroplets in contrast to a continuous size distribution of droplets in nonviscous case. We demonstrate that the group of small droplets produced in a high-viscosity environment may be applied for enhanced fluorescence detection with higher sensitivity and shorter response time. The finding of this work can potentially be applied for mediating the size distribution of surface nanodroplets on a homogeneous surface without templates

Tuning Composition of Multicomponent Surface Nanodroplets in a Continuous Flow-In System

Droplets are excellent platforms for compartmentalization of many processes such as chemical reactions, liquid–liquid extraction, and biological or chemical analyses. Accurately controlling and optimizing the composition of these droplets is of high importance to maximize their functionality. In this work, the formation of multicomponent droplets with controllable composition by employing a continuous flow-in setup is demonstrated. Multiple streams of different oil solutions are introduced and mixed in a passive flow mixer and the outcoming mixture is subsequently fed into a flow chamber to form surface nanodroplets by solvent exchange. This method is time-effective, enabling programmable continuous processes for droplet formation and surface cleaning. The surface nanodroplets are formed within 2.5 min in one cycle, and the droplet formation is reliable with similar size distribution over multiple cycles. The composition of the resulting surface nanodroplet can be tuned at will simply by controlling the flow rate ratios of each stream of the oil solution. Using fluorescence imaging, it is shown that the composition of the binary surface nanodroplets agrees well with theoretical values predicted using the phase diagram

Surface nanodroplet-based nanoextraction from sub-milliliter volumes of dense suspensions

A greener analytical technique for quantifying compounds in dense suspensions is needed for wastewater and environmental analysis, chemical or bio-conversion process monitoring, biomedical diagnostics, and food quality control, among others. In this work, we introduce a green, fast, one-step method called nanoextraction for extraction and detection of target analytes from sub-milliliter dense suspensions using surface nanodroplets without toxic solvents and pre-removal of the solid contents. With nanoextraction, we achieve a limit of detection (LOD) of 10−9M for a fluorescent model analyte obtained from a particle suspension sample. The LOD is lower than that in water without particles (10−8M), potentially due to the interaction of particles and the analyte. The high particle concentration in the suspension sample, thus, does not reduce the extraction efficiency, although the extraction process was slowed down up to 5 min. As a proof of principle, we demonstrate the nanoextraction for the quantification of model compounds in wastewater slurry containing 30 wt% solids and oily components (i.e.heavy oils). The nanoextraction and detection technology developed in this work may be used in fast analytical technologies for complex slurry samples in the environment, industrial waste, or in biomedical diagnostics

Primary submicron particles from early stage asphaltene precipitation revealed in situ by total internal reflection fluorescence microscopy in a model oil system

Paraffinic froth treatment (PFT) is an essential step in oilsands extraction. The ability to quantitatively understand and control asphaltene precipitation induced by solvent dilution is key to technology innovation in PFT process. In this work, we investigate the early stage of asphaltene precipitation in a model oil system in response to diffusive solvent addition in quasi-2D confinement. Using total internal reflection fluorescence microscope, we provide direct visualization of the size distribution and structural characteristics of asphaltene precipitates with a spatial resolution of ~200 nm and temporal resolution of 250 ms. Our results show the correlation between the size and number of the asphaltene particles and the concentration of the paraffinic solvent in the diluent. Notably the aggregates were found to consist of primary submicron particles with a similar size from 200 nm to 400 nm in radius. These particles may be the primary elementary units that aggregate and form bigger particles via aggregation. The growth time of asphaltene particles decreases with increase n-pentane concentration in the observation area. The findings from this work provide new insight into the effects of solvent mixing on the size distribution and morphological characteristics of asphaltene precipitates that are important for association with water and solids and separation properties

Enhanced displacement of phase separating liquid mixtures in 2D confined spaces

Displacing liquids in a confined space is important for technological processes ranging from porous membrane separation to CO2 sequestration. The liquid to be displaced usually consists of multiple components with different solubilities in the displacing liquid. Phase separation and chemical composition gradients in the liquids can influence the displacement rate. In this work, we investigate the effects of liquid composition on the displacement process of ternary liquid mixtures in a quasi-2D microchannel where liquid−liquid phase separation occurs concurrently. We focused on model ternary mixtures containing 1-octanol (a model oil), ethanol (a good solvent), and water (a poor solvent). These mixtures are displaced with water or with an ethanol aqueous solution. As a comparison, for some experiments, water was displaced by ternary mixtures. The bright-field and fluorescence imaging measurements reveal distinct phase separation behaviors. The spatial distribution of subphases arising from phase separation and the displacement rates of the solution are impacted by the initial ternary solution composition. The boundary between the solution and displacing liquid changes from a defined interface to a diffusive interface as the initial 1-octanol composition in the solution is reduced. The displacement rate also varies non-linearly with the initial 1-octanol composition. The slowest displacement rate arises at intermediate 1-octanol concentration, where a stable three-zone configuration forms at the boundary. At very low 1-octanol concentration, the displacement rate is fast, associated with droplet formation and motion driven by the chemical concentration gradients formed during phase separation. The excess energy provided from phase separation may contribute to the enhanced displacement at intermediate to high 1-octanol concentrations but not at low 1-octanol concentration with enhancement from induced flow in confinement. The knowledge gained from this study highlights the importance of manipulating phase separation to enhance mass transport in confinement for a wide range of separation processes

Microfluidic device coupled with total internal reflection microscopy for in situ observation of precipitation

In situ observation of precipitation or phase separation induced by solvent addition is important in studying its dynamics. Combined with optical and fluorescence microscopy, microfluidic devices have been leveraged in studying the phase separation in various materials including biominerals, nanoparticles, and inorganic crystals. However, strong scattering from the subphases in the mixture is problematic for in situ study of phase separation with high temporal and spatial resolution. In this work, we present a quasi-2D microfluidic device combined with total internal reflection microscopy as an approach for in situ observation of phase separation. The quasi-2D microfluidic device comprises of a shallow main channel and a deep side channel. Mixing between a solution in the main channel (solution A) and another solution (solution B) in the side channel is predominantly driven by diffusion due to high fluid resistance from the shallow height of the main channel, which is confirmed using fluorescence microscopy. Moreover, relying on diffusive mixing, we can control the composition of the mixture in the main channel by tuning the composition of solution B. We demonstrate the application of our method for in situ observation of asphaltene precipitation and $$\beta$$-alanine crystallization

Surface Nanodroplet-Based Extraction Combined with Offline Analytic Techniques for Chemical Detection and Quantification

Liquid-liquid extraction based on surface nanodroplets can be a green and sustainable technique to extract and concentrate analytes from a sample flow. However, because of the extremely small volume of each droplet (<10 fL, tens of micrometers in base radius and a few or less than 1 μm in height), only a few in situ analytical techniques, such as surface-enhanced Raman spectroscopy, were applicable for the online detection and analysis based on nanodroplet extraction. To demonstrate the versatility of surface nanodroplet-based extraction, in this work, the formation of octanol surface nanodroplets and extraction were performed inside a 3 m Teflon capillary tube. After extraction, surface nanodroplets were collected by injecting air into the tube, by which the contact line of surface droplets was collected by the capillary force. As the capillary allows for the formation of ∼10 12surface nanodroplets on the capillary wall, ≥2 mL of octanol can be collected after extraction. The volume of the collected octanol was enough for the analysis of offline analytical techniques such as UV-vis, GC-MS, and others. Coupled with UV-vis, reliable extraction and detection of two common water pollutants, triclosan and chlorpyrifos, was shown by a linear relationship between the analyte concentration in the sample solution and UV-vis absorbance. Moreover, the limit of detection (LOD) as low as 2 × 10 -9M for triclosan (∼0.58 μg/L) and 3 × 10 -9M for chlorpyrifos (∼1.05 μg/L) could be achieved. The collected surface droplets were also analyzed via gas chromatography (GC) and fluorescence microscopy. Our work shows that surface nanodroplet extraction may potentially streamline the process in sample pretreatment for sensitive chemical detection and quantification by using common analytic tools

Water-mediated adhesion of oil sands on solid surfaces at low temperature

Adhesion of frozen granular materials on solid surfaces creates various problems for surface cleaning, reduces the carrying capacity of vehicles, and increases energy consumption for in-land transportation. Here we report that water content determines the adhesion strength of oil sands on solid surfaces at temperature of -2.5 ◦C to -20 ◦C. Our measurements by X-ray micro-computed tomography revealed that water forms capillary bridges between the sand particles and the solid substrate and more air gaps at the interface between oil sands and the substrate are filled with interstitial water at a higher content. We experimentally measured the minimal force required to push the frozen oil sands off the substrate and identified that the adhesion strength increased linearly with water content from 4% to 14% on both rubber and steel substrate. For short freezing time at a fixed water content, lowering the temperature increased the adhesion strength on the steel substrate. Fouling from a layer of bitumen or asphaltenes aggravated the adhesion of oil sands on steel. A theoretical model was proposed to rationalize the linear relationship between water content and the adhesion strength, based on the contact area between ice and the substrate. We also found an effective method to reduce the adhesion of oil sands by spraying a little amount of anti-freezing liquid on the substrate. Our approach may reduce the energy consumption in transport and processing of wet granular materials, and potentially save manpower and the cost from cleaning in industrial operations. The insight from our work may have wide applicability to many natural/industrial processes, such as soil formation, food processing, and porous structures in ice crystal-templating nanomaterials synthesis by freezing-drying