Oil Droplets In Water Research Articles

Run-and-halt motility of droplets in response to light

SummaryMicroscopic motility is a property that emerges from systems of interacting molecules. Unraveling the mechanisms underlying such motion requires coupling the chemistry of molecules with physical processes that operate at larger length scales. Here, we show that photoactive micelles composed of molecular switches gate the autonomous motion of oil droplets in water. These micelles switch from large trans-micelles to smaller cis-micelles in response to light, and only the trans-micelles are effective fuel for the motion. Ultimately, it is this light that controls the movement of the droplets via the photochemistry of the molecules composing the micelles used as fuel. Notably, the droplets evolve positive photokinetic movement, and in patchy light environments, they preferentially move toward peripheral areas as a result of the difference in illumination conditions at the periphery. Our findings demonstrate that engineering the interplay between molecular photo-chemistry and microscopic motility allows designing motile systems rationally.

Liquid-Liquid Phase Separation Bridges Physics, Chemistry, and Biology.

There is a fundamental gap between the inherent complexity of biology and the simplicity that physicists and chemists often seek. In this Perspective, we reason that liquid-liquid phase separation (LLPS) could be utilized to (partially) fill this gap and to bridge different disciplines because LLPS can produce condensed droplets with simplicity and complexity at the same time. Specifically, the droplets are often compositionally simple (made of, for example, proteins and polyelectrolytes) and structurally uniform (not so different from an oil droplet in water). Contrary to this simplicity is their functional complexity─the droplets can perform various physiological activities with subcellular precision. This spatiotemporal precision further stimulates an ongoing endeavor in the synthetic realm to develop regulatory strategies that may ultimately match or even surpass their biological counterparts. We envision the phase-separated droplets to open a window of simplicity for us to peek into the complexity of biology, and we foresee that joined forces across different disciplines would substantially advance our understanding of LLPS in biotic and abiotic contexts.

Investigation of the detachment of oil droplets driven by water shear field on solid surfaces with different wettabilities

The movement and detachment of tiny oil droplets in water on a substrate is the key technology to strengthen oil–water separation by using corrugated plate oil–water separation technology. Here, the detachment behaviors of oil droplets adhered to substrates with different wettabilities in a laminar flow field are studied experimentally and theoretically. The morphologic changes of mother and daughter oil droplets are obtained when the adhered oil droplets are continuously separated. The shedding mechanism of oil droplets is analyzed in detail, and the critical condition for the effective detachment of oil droplets is obtained, which can be used to guide the design of the surface wettability and water flow velocity in the actual production of oil–water separation.

Numerical assessment of ultrasound supported coalescence of water droplets in crude oil

In this study, a numerical assessment of the coalescence of binary water droplets in water-in-oil emulsion was conducted. The investigation addressed the effect of various parameters on the acoustic pressure and coalescence time of water droplets in oil phase. These include transducer material, initial droplet diameter (0.05–0.2 in), interfacial tension (0.012–0.082 N/m), dynamic viscosity (10.6–530 mPas), temperature (20–100 °C), US (ultra sound) frequency (26.04–43.53 kHz) and transducer power (2.5–40 W). The materials assessed are lead zirconate titanate (PZT), lithium niobate (LiNbO3), zinc oxide (ZnO), aluminum nitride (AlN), polyvinylidene fluoride (PVDF), and barium titanate (BaTiO3). The numerical simulation of the binary droplet coalescence showed good agreement with experimental data in the literature. The US implementation at a fixed frequency produced enhanced coalescence (t = 5.9–8.5 ms) as compared to gravitational settling (t = 9.8 ms). At different ultrasound (US) frequencies and transducer materials, variation in the acoustic pressure distribution was observed. Possible attenuation of the US waves, and the subsequent inhibitive coalescence effect under various US frequencies and viscosities, were discussed. Moreover, the results showed that the coalescence time reduced across the range of interfacial tensions which was considered. This reduction can be attributed to the fact that lower interfacial tension produces emulsions which are relatively more stable. Hence, at lower interface tension between the water and crude oil, there was more resistance to the coalescence of the water droplets due to their improved emulsion stability. The increment of the Weber number at higher droplet sizes leads to a delay in the recovery of the droplet to spherical forms after their starting deformation. These findings provide significant insights that could aid further developments in demulsification of crude oil emulsions under varying US and emulsion properties.

Quantifying Uniform Droplet Formation in Microfluidics Using Variational Mode Decomposition

Using variational mode decomposition, we analyze the signal from velocities at the center of the channel of a microfluidics drop-maker. We simulate the formation of water in oil droplets in a microfluidic device. To compare signals from different drop-makers, we choose the length of the water inlet in one drop-maker to be slightly shorter than the other. This small difference in length leads to the formation of satellite droplets and uncertainty in droplet uniformity in one of the drop-makers. By decomposing the velocity signal into only five intrinsic modes, we can fully separate the oscillatory and noisy parts of the velocity from an underlying average flow at the center of the channel. We show that the fifth intrinsic mode is solely sufficient to identify the uniform droplet formation while the other modes encompass the oscillations and noise. Mono-disperse droplets are formed consistently and as long as the fifth mode is a plateau with a local standard deviation of less than 0.02 for a normalized signal at the channel inlet. Spikes in the fifth mode appear, coinciding with fluctuations in the sizes of droplets. Interestingly, the spikes in the fifth mode indicate non-uniform droplet formation even for the velocities measured upstream in the water inlet in a region far before where droplets form. These results are not sensitive to the spatial resolution of the signal, as we decompose a velocity signal averaged over an area as wide as 40% of the channel width.

Mesoscale evaluation of oil submerging and floating processes during marine oil spill response: Effects of dispersant on submerging stability and the associated mechanism

The migration of oil spills in marine environment is still not clear, especially the key processes of submerging and floating, which is an important concern for effective disposal of oil spills. In mesoscale wave tank (32 m × 0.8 m × 2 m), this study has evaluated the characteristics of oil submergence based on oil concentration and oil droplet size. The concept of effective submergence is put forward for the first time, utilized to analyze the effects of dispersant on submerging stability and associated mechanisms. The results indicate dispersants increase submerged oil concentration and promote homogeneous distribution and vertical penetration. Of concern is that dispersants increase the proportion of small oil droplets (2.5–70 µm), prolonging the residence time of oil droplets in water by delaying the floating process. Dispersants sharply reduce oil droplets size (VMD<44 µm) thus decreasing the coalescence probability. These contribute to better submerging stability. By contrast, the submerged oil, formed as oil patches, oil streamers, and large oil droplets (VMD>170 µm) when without dispersant, will float and reattach to oil slicks more quickly due to their large volume. These findings help to clarify spilled oil behaviors and provide a new idea for the research on oil submergence.

A one-step preparation of superhydrophobic stainless steel mesh and application in raffinate cleaning technology as a coalescer for contaminated organic-water separation

Improving raffinate oil-water separation efficiency and simplifying the production process of coalescing material are both in great demand but remain difficult. In this study, a superhydrophobic stainless steel surface was created using a simple one-step electrodeposition process in an electrolyte solution comprising nickel chloride (NiCl2·6H2O), stearic acid, and ethanol. The surface shape, chemical content, and superhydrophobic property were studied using a scanning electron microscope (SEM), Fourier-transform infrared (FT-IR) spectrometer, energy dispersive spectroscopy (EDS), and contact angle measurement. The cotton-like microstructure, comprised of crystals of Ni metal and Ni(CH3(CH2)16COO)2, demonstrated outstanding acid and alkaline resistance with a maximum surface contact angle of 166.8°. The oil-water separation performance of raffinate was tested in a mixer-settler, and the separation efficiency ranged from 75% to 80% in a P507 + Kerosene/water system, based on Chemical Oxygen Demand (COD) indicating its potential applicability in the solvent extraction process. A high-speed camera was also used to observe the coalescing behavior of oil droplets in water.

Evaluating oil removal by amphiphilic MoS2/cellulose acetate fibrous sponge in a flow-through reactor and by artificial neural network

The amphiphilic Janus-like molybdenum disulfide/cellulose acetate (MoS2/CA) fibrous sponges were synthesized and employed to remove oil from wastewater. The efficiency of treating 100 L of emulsion containing 10 mg/L of oil by MoS2/CA fibrous sponge in a one-pass flow-through reactor was examined under different flow rates, pH, and temperature conditions. The oil concentrations in the treated effluent were all below the industrial discharge requirement of 5 mg/L. The spent sponges were successfully regenerated and reused in multiple cycles. It was found that the adsorption of oil followed the Langmuir adsorption isotherm (R2 = 0.988) and pseudo-second-order kinetic (R2 = 0.995). The hydrophobic/oleophilic MoS2 component of the MoS2/CA sponges was responsible for oil removal while its hydrophilic CA component allowed the material to be stable in the water. Artificial neural networking (ANN) was developed to optimize oil removal. The trained ANN was used to predict the oil removal of two new conditions. This study proved that our Janus-like MoS2/CA sponge was both surfaces active and amphiphilic so can reach the targeted oil droplets in water. These findings provided the basis for the development of an efficient material that can treat large volumes of dilute oil-containing wastewater for water reuse.

Directed Discovery of Tetrapeptide Emulsifiers

Oil in water emulsions are an important class of soft material that are used in the food, cosmetic, and biomedical industries. These materials are formed through the use of emulsifiers that are able to stabilize oil droplets in water. Historically emulsifiers have been developed from lipids or from large biomolecules such as proteins. However, the ability to use short peptides, which have favorable degradability and toxicity profiles is seen as an attractive alternative. In this work, we demonstrate that it is possible to design emulsifiers from short (tetra) peptides that have tunability (i.e., the surface activity of the emulsion can be tuned according to the peptide primary sequence). This design process is achieved by applying coarse grain molecular dynamics simulation to consecutively reduce the molecular search space from the 83,521 candidates initially considered in the screen to four top ranking candidates that were then studied experimentally. The results of the experimental study correspond well to the predicted results from the computational screening verifying the potential of this screening methodology to be applied to a range of different molecular systems.

Influence of the Dissolved Gas on the Interfacial Properties of Decane Surface Nanodroplets.

Surface nanodroplets have received extensive attention recently due to their potential in the fabrication of functional materials with nanostructures and chemical reactions at micro- and nanoscales. Although the effect of dissolved gas in water has been realized in some important processes such as spontaneous emulsification of oil droplets in water, its roles in the wetting behavior of surface nanodroplets at the hydrophobic interface have been largely neglected. Here, we focused on the influence of dissolved gas on the interfacial properties of surface nanodroplets and characterized their morphological evolution when exposed to different air-saturated water samples. Results indicated that the morphology of surface nanodroplets barely changed in air-oversaturated cold water. However, their contact angle first decreases gradually in deionized water, increases immediately after replacement with degassed water, and eventually decreases gradually with time. Furthermore, the surface tension of nanodroplets would change similarly after the injection of degassed water. We considered these changes to be caused by the removal or reduction of the enriched gas at the substrate interface, in which the surface hydrophobicity was changed. Our findings could shed some light on the wetting behavior of nanodroplets at the hydrophobic surface in different air-saturated water samples and inspire the microscale manipulation and reaction of surface nanodroplets.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Development of a microfluidic cell culture system for the adaptive laboratory evolution

We construct a microfluidics-based long-term cell culture system that can perform multiple adaptive laboratory evolution (ALE) experiments in parallel under different conditions. Single E. coli cells were isolated into water droplets in oil (W/O emulsion) containing agarose gel. Micrometric gel balls (GBs) were then extracted from the oil phase and placed in a microfluidic perfusion culture device, which can generate a substrate concentration gradient. The growth of the clonal colonies was monitored under each condition. We tested this GB-based culture device to expand the application of ALE after the gene knockout based on the flux balance analysis (FBA) performed in our previous studies.

Study on the Collision Dynamics of a Freely Rising Oil Droplet in Water Near the Horizontal Wall

Study on the Collision Dynamics of a Freely Rising Oil Droplet in Water Near the Horizontal Wall

Tubulation and dispersion of oil by bacterial growth on droplets.

Bacteria on surfaces exhibit collective behaviors, such as active turbulence and active stresses, which result from their motility, growth, and interactions with their local surroundings. However, interfacial deformations on soft surfaces and liquid interfaces caused by active growth, particularly over long time scales, are not well understood. Here, we describe experimental observations on the emergence of tubular structures arising from the growth of rod-shaped bacteria at the interface of oil droplets in water. Using microfluidics and timelapse microscopy, the dimensions and extension rates of individual tubular structures as well as bulk bio-aggregate formation are quantified for hundreds of droplets over 72 hours. Tubular structures are comparable in length to the initial droplet radius and are composed of an outer shell of bacteria that stabilize an inner filament of oil. The oil filament breaks up into smaller microdroplets dispersed within the bacterial shell. This work provides insight into active stresses at deformable interfaces and improves our understanding of microbial oil biodegradation and its potential influence on the transport of droplets in the ocean water column.

7-Aminocoumarin-4-acetic Acid as a Fluorescent Probe for Detecting Bacterial Dipeptidyl Peptidase Activities in Water-in-Oil Droplets and in Bulk.

Droplet-based microfluidic systems are a powerful tool for biological assays with high throughput. Water-in-oil droplets (WODLs) are typically used in droplet-based microfluidic systems to culture microorganisms and perform enzyme assays. However, because of the oil surrounding the nanoliter and picoliter volumes of WODLs, availability of suitable substrates is limited. For instance, although 7-amino-4-methylcoumarin (AMC) is commonly used as a fluorescent probe of the substrate to detect peptidase activity, AMC leaks from WODLs to the oil phase due to its high hydrophobicity. Thus, AMC substrates cannot be used in droplet-based microfluidic systems with WODLs. In this study, we developed a peptidase substrate consisting of a dipeptide and 7-aminocoumarin-4-acetic acid (ACA), an AMC-derived fluorogenic compound. ACA was retained in the WODL for more than 7 days, and the dipeptidyl ACA substrate detected dipeptidyl peptidase (DPP) activity in the WODL. Compared to AMC substrates, the substrate specificity constants of DPPs for ACA substrates increased up to 4.7-fold. Fluorescence-activated droplet sorting made high-throughput screening of microorganisms based on DPP activity using the dipeptidyl ACA substrate possible. Since ACA could be applied to various substrates as a fluorescent probe, detectable microbial enzyme activities for droplet-based microfluidic systems can be largely expanded.

Facile and scalable surface functionalization approach with small silane molecules for oil/water separation and demulsification of surfactant/asphaltenes-stabilized emulsions

Oil contaminations have been challenging issues in a wide range of engineering and environmental fields. Herein, we employ a small silane molecule, γ-mercaptopropyldi(trimethylsiloxy)methylsilane (MD(SH)M), to functionalize fiber-based substrates (i.e., stainless steel mesh, cotton fiber) for oil decontamination processes including oil–water separation, oil absorption and demulsification. The functionalized stainless-steel mesh (SSM@PM) can completely separate water and oils of high/low density without prewetting by oil. The functionalized cotton fibers (CF@PM) can absorb different oils from water without being wetted by water and maintain excellent oil absorption performance even after 10 cyclic reuses. Besides, CF@PM shows good demulsification performance to water-in-oil emulsions stabilized by span 80 and asphaltenes. The underlying interaction mechanisms are investigated using force measurements. The CF@PM shows much stronger repulsion to water droplets in air as compared to unmodified cotton fiber, and SSM@PM has strong attraction to oil droplets under water but almost no attraction to water droplets in oil, accounting for the effective oil/water separation capacity. Such strong attraction between oil and MD(SH)H-modified surfaces can be attributed to the hydrophobic interaction as confirmed by surface force measurements. This work provides a facile surface functionalization approach with great potential applications in oil/water separation and demulsification of surfactant- or asphaltenes-stabilized emulsions.

Charge transfer across C-H⋅⋅⋅O hydrogen bonds stabilizes oil droplets in water.

The hydrophobic–water interface plays a key role in biological interactions. However, both the hydrophobic–water interfacial molecular structure and the origin of the negative zeta potential of hydrophobic droplets in water are heavily contested. We report polarimetric vibrational sum-frequency scattering of the O–D and C-H stretch modes of 200-nanometer hexadecane oil droplets dispersed in water. An unusually broad spectral distribution (2550 to 2750 per centimeter) of interfacial water molecules that were not hydrogen bonded to other water molecules was observed, as well as a blue shift in the vibrational frequency of the interfacial hexadecane C-H stretch modes. Oil and water frequency shifts correlated with the negative electrostatic charge. Molecular dynamics simulations demonstrated that the unexpected strong charge-transfer interactions arose from interfacial C–H∙∙∙O hydrogen bonds.

Development and characterization of structured water-in-oil emulsions with ethyl cellulose oleogels

The food industry confronts an enormous challenge to develop stable margarine-type water-in-vegetable oil (W/O) emulsion-based table spreads with reduced concentration of saturated fat and without trans fats. In the present work, we developed a gelled W/O emulsion (Gelled-W/O-E) containing 20% of water using a mixture of a conventional W/O emulsion (W/O-E) stabilized with glycerol monostearate (GMS), and an ethyl cellulose (EC) oleogel. The mechanical, microstructure and stability of the resulting gelled emulsion (Gelled-W/O-E) was compared with control systems consisting of conventional W/O emulsions (W/O-E) and EC-GMS oleogels (EC-GMS-O; no water added) formulated using the same GMS (0.5% and 1.0%) and EC (7%) concentration as in the Gelled-W/O-E. The Gelled-W/O-E showed higher elasticity and emulsion stability in comparison with the control systems. This in spite the EC and GMS concentrations used were below the minimal concentration required to develop a gel, and the tentatively lower solid content in the Gelled-W/O-E than in the EC-GMS-O because the presence of water. We observed that by increasing the GMS concentration in the Gelled-W/O-E, the water droplet size decreased as gel elasticity and W/O emulsion stability significantly increased. We associated this behavior to a synergistic GMS-EC interaction that kept the GMS at the water–oil droplet interface. These results showed the role of water droplets as active fillers in determining the rheological properties of the Gelled-W/O-E, and that the GMS efficiency as emulsifier increased in the presence of EC in the oil phase. After comparing the microstructural properties of commercial margarine spreads with those of the Gelled-W/O-E, we concluded that the structured W/O emulsion is a novel way to achieve similar functionality to margarine spreads, without the use of saturated and trans-fats.

Biomass-derived oriented neurovascular network-like superhydrophobic aerogel as robust and recyclable oil droplets captor for versatile oil/water separation

Using tubular kapok fibers (KF) and sodium alginate (SA) as the natural building block, we put forward a novel oriented neurovascular network-like superhydrophobic aerogel with robust dry and wet compression resilience by directional freeze-drying and chemical vapor deposition. In the block, SA forms aligned channel structures providing space for rapid oil transmission, while KF serves as vascular-like capillaries acting as instant “tentacle” to capture the tiny oil droplets in water, facilitating fascinating oil capture efficiency for versatile oil/water separation, The aerogel after dry and wet compression (under a strain of 60%) can recover 96.0% and 97.3% its original, respectively, facilitating stable oil recovery (81.1–89.8%) by squeezing, high separation efficiency (99.04–99.64%) and permeation flux separating oil contaminants from water. A pump-supported experiment shows the aerogel efficiently collecting oil contaminants from the water’s surface and bottom by 11503–25611 L·m−2·h−1. Particularly, the aerogel as robust oil droplets captor facilely achieves isolation of 99.39–99.68% emulsified oils from oil/water emulsions by novel oil trapping mechanism which simply involves exerting kinetic energy on emulsified oils through repeated oscillation, potentially indicating a simple and efficient alternative to membrane-based oily wastewater remediation via filtration.

Direct Calculation of Entropic Components in Cohesive Interaction Free Energies.

Cohesive interaction free energies entail an entropic component related to fluctuations of the energy associated with the attractive portion of the solute-solvent potential. The corresponding "fluctuation entropy" is fundamental in the solvation thermodynamics of macromolecular solutes and is linked to interfacial solvent density fluctuations and hydrophobic effects. Since the direct calculation of fluctuation entropy in molecular simulations is hampered by the poor sampling of high-energy tails in the solute-solvent energy distribution, indirect, and often approximate, routes for the calculation of fluctuation entropy are usually required, involving the modeling of geometrically frozen repulsive solute cavities in thermodynamic integration approaches. Herein, we propose a method to directly compute the fluctuation entropy by employing indirect umbrella sampling (INDUS). To validate the method, we consider model systems consisting of subnanometer oil droplets in water for which the fluctuation entropy can be computed exactly using indirect methods. The fluctuation entropy calculated with the newly proposed direct method agrees with the indirect reference calculations. We also observe that the solvation free energy and the contribution of the fluctuation entropy to it are of comparable magnitudes, particularly for larger oil droplets (∼1 nm). The proposed method can readily be employed for flexible macromolecular solutes and systems with extended hydrophobic surfaces or in the vicinity of a dewetting transition.