Motion Of Oil Droplets Research Articles

Molecular modelling of active oil droplet propulsion: Insights from dissipative particle dynamics simulation

This study employed dissipative particle dynamics (DPD) simulations to investigate the self-propelled motion of oil droplets in water–oil–surfactant systems. It is the first attempt to replicate self-propulsion models of oil droplets at the molecular level, contrasting previous simulations focused on Brownian motion and hydrodynamic behaviour of colloidal particles. The DPD model reproduced droplet propulsion and visualised internal Marangoni flow, showing that larger droplet radii and greater interfacial tension differences increase propulsion speeds. Additionally, surfactants with stronger oil–oil repulsion enhanced propulsion speed, suggesting that surfactant-induced local structures are crucial for the self-propulsion mechanism.

Observation of Brownian elastohydrodynamic forces acting on confined soft colloids

Confined motions in complex environments are ubiquitous in microbiology. These situations invariably involve the intricate coupling between fluid flow, soft boundaries, surface forces, and fluctuations. In the present study, such a coupling is investigated using a method combining holographic microscopy and advanced statistical inference. Specifically, the Brownian motion of soft micrometric oil droplets near rigid walls is quantitatively analyzed. All the key statistical observables are reconstructed with high precision, allowing for nanoscale resolution of local mobilities and femtonewton inference of conservative or nonconservative forces. Strikingly, the analysis reveals the existence of a novel, transient, but large, soft Brownian force. The latter might be of crucial importance for microbiological and nanophysical transport, target finding, or chemical reactions in crowded environments, and hence the whole life machinery.

Dynamics of a droplet on the surfactant-infested free surface of another liquid

The dynamics of liquid droplets surfing over the surfactant-infested free surface of another liquid have been explored experimentally. We analyze the motion of oil droplets that has been initiated through the creation of a surface tension gradient resulting from the deposition of a drop of surfactant at the water surface contained in the petri dish. The experiments reveal that the location of surfactant deposition with respect to the droplet position influences its motion. Due to the presence of a surface tension gradient, the footprint area of the droplet reduces and its shape changes. We have studied the temporal variation in the velocity (|vx|) of the droplets in relation to their proximity to a wall. Based on the evolution of droplet shape and change in droplet velocity, the drop dynamics can be experimentally divided into four distinct zones. Results indicate that in zone-1, |vx| grows with t as |vx|≈tn, where n is between 0.8 and 1.0. The scaling argument shows that in this zone, the surface tension force dominates the drag force, and thereby, |vx| of the droplets increases linearly with t expressed as |vx|∝t. The experimental investigation and the scaling law exhibit a reasonable agreement. In zone-2, |vx| remains more or less constant, as it is postulated that the surface tension force balances the drag force. In zone-3, a decrease in surface tension force results in a deceleration of the droplets. In zone-4, the deceleration becomes more prominent as the droplet approaches the petri dish wall.

Kinetic monitoring of molecular interactions during surfactant-driven self-propelled droplet motion by high spatial resolution waveguide sensing

Hypothesis: Self-driven actions, like motion, are fundamental characteristics of life. Today, intense research focuses on the kinetics of droplet motion. Quantifying macroscopic motion and exploring the underlying mechanisms are crucial in self-structuring and self-healing materials, advancements in soft robotics, innovations in self-cleaning environmental processes, and progress within the pharmaceutical industry. Usually, the driving forces inducing macroscopic motion act at the molecular scale, making their real-time and high-resolution investigation challenging. Label-free surface sensitive measurements with high lateral resolution could in situ measure both molecular-scale interactions and microscopic motion.Experiments: We employ surface-sensitive label-free sensors to investigate the kinetic changes in a self-assembled monolayer of the trimethyl(octadecyl)azanium chloride surfactant on a substrate surface during the self-propelled motion of nitrobenzene droplets. The adsorption–desorption of the surfactant at various concentrations, its removal due to the moving organic droplet, and rebuilding mechanisms at droplet-visited areas are all investigated with excellent time, spatial, and surface mass density resolution.Findings: We discovered concentration dependent velocity fluctuations, estimated the adsorbed amount of surfactant molecules, and revealed multilayer coverage at high concentrations. The desorption rate of surfactant (18.4 s−1) during the microscopic motion of oil droplets was determined by in situ differentiating between droplet visited and non-visited areas.

Experimental investigation of free-rising characteristics of spilled oil droplets in natural waters

This work focuses on investigating the dynamics of different types of oil droplets rising freely in water columns. Fifteen different types of oils were selected to cover most of the conventional oils. The particle size range of the study is 1.41∼13.58 mm, which can cover the largest oil droplet size that may occur in the real environment. Investigation results show that oil droplet characteristics greatly influence the dynamics process of oil droplet motion. The characteristics which play a dominant role will change as the oil droplet particle size increases. Meanwhile, a set of computational models applicable to the terminal velocity and drag coefficient of oil droplets in the range of 20 < Re < 1200 in a non-pure system are obtained by organizing the experimental data and comparing with the traditional correlation equation, respectively. The models involve dimensionless parameters such as Eo, Mo, and We numbers with fitting coefficients up to 0.984.

Measurement of a Three-Dimensional Rotating Flow Field and Analysis of the Internal Oil Droplet Migration

Investigating the motion of discrete oil droplets in a rotating flow field can provide a theoretical basis for optimizing the flow field and structural parameters of hydrocyclones and centrifugal separation equipment. In the present work, the particle image velocimetry (PIV) method was applied to study the velocity distribution of a three-dimensional axial-rotor-driven rotating flow field and the influence of the velocity distribution of different rotor speeds on the flow field. The radial migration of oil droplets with different particle sizes in the rotating flow field was visually analyzed using high-speed video (HSV). The results showed that the oil droplets with the same radial position had diameters of 2.677 and 4.391 mm, whereas the movement times to the axis were 0.902 and 0.752 s. The larger the oil droplet size, the shorter the time to move to the axial center of the rotating flow field. The radial velocities of oil droplets with diameters of 2.677 and 2.714 mm were 0.0221 and 0.02 m·s−1, respectively. In addition, a mathematical expression was established between the radial migration time and the oil droplet size in the rotating flow field. The accuracy of the proposed expression was verified using experiments.

Kinetic analysis of a freely rising droplet in water during collision with the horizontal wall

An in-depth analysis of the kinetics of the collision between freely rising oil droplets in water in the range of Re = 4.64–463.3 was carried out to understand the physical mechanism and detailed kinetics of the interaction between the oil droplets and wall. The results show that when oil droplets with Re ≥ 27.8 hit the wall vertically at terminal velocities, a “dimple-like” water film is formed near the wall, which significantly affects the pressure distribution within the water film, the oil–water interfacial shear force, and forces on oil droplet. A coupled model describing water film thickness and pressure accurately captures the kinetic behavior of water film drainage near the wall. The film-induced force based on lubrication theory can reasonably predict the motion trajectory of oil droplets near the wall and dominate the motion of oil droplets colliding with the wall. The motion phase diagram with (Re, We) as the control parameter was established to quickly identify the droplet motion rule under different liquid–liquid density ratios.

Photoinduced collective motion of oil droplets and concurrent pattern formation in surfactant solution

Photoinduced collective motion of oil droplets and concurrent pattern formation in surfactant solution

Droplet Navigation by Photothermal Pumping in an Optofluidic System.

Droplets with guided motion have potential applications as microreactors and delivery vehicles. Directing long-range migration powered solely by light is particularly advantageous since light can be applied remotely, patterned with a photomask, and readily translated to irradiate specified locations. Herein, we describe a universal platform that allows fast directional navigation and collective merging of droplets controlled by either ultraviolet or visible light. The guided motion of water and oil droplets follows density-driven convective flows arising from photothermal conversion at a light-absorbing amphiphobic substrate. Because of the relatively high photothermal efficiency, a low-intensity light beam can be employed. Further, we demonstrate that the moving droplets can function as carriers and on-demand reaction chambers.

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.

Directional and Adaptive Oil Self‐Transport on a Multi‐Bioinspired Grooved Conical Spine

AbstractManipulating oil droplets in an aqueous solution offers great opportunities to develop advanced devices in the fields of water remediation, chemical micro‐reactor, etc. Although conical structures can achieve a directional oil droplet motion, the continuous and adaptive fluid self‐transport in complicated environments is still a challenge. Inspired by the distinctive oil transport capability of fishbone and the anisotropic grooved structure of rice leaves, this work presents a multi‐bioinspired grooved conical spine (BGCS) for improved oil manipulation. Benefiting from the cooperative effect of the asymmetric Laplace pressure and the surface capillary force, the BGCS possesses the reliable functions of ultrafast and continuous oil transport under water, in air, and even across the air–water interface. Compared with the original cone, the BGCS demonstrates a nine times faster transporting velocity and a five times larger capacity. It is envisioned that this updated oil‐collecting cone with its cooperative structure should find real‐world applications.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Analysis of different self-propulsion types of oil droplets based on electrostatic interaction effects.

This study aims to investigate electrical effects in self-propelled oil droplets and analyze the motion caused by electrical interactions between two droplets. We examine the motion of self-propelled oil droplets in terms of electrical effects; no similar analyses have been reported to date. When an oleic acid droplet is introduced into a surfactant solution, oleic acid is adsorbed at the droplet surface; as a result, the surface becomes negatively charged. However, the electrostatic interaction between oil droplets has not been studied so far. We focus on the chargeability of self-propelled oil droplets, and propose that their motion can be controlled by changing the charge state at the oil/water interface. This is realized by controlling the pH of the surfactant solution. We prepared several aqueous solutions with slightly different pH values, and introduced two oil droplets into each solution to investigate the relationship between the pH value and the correlated motion of the oil droplets. In an aqueous solution at pH 12.00, the two oil droplets showed two types of motion (follow-up and parallel motion), while in an aqueous solution at pH 12.10 they exhibited only one type of motion, i.e., repulsive motion. In order to analyze the three types of motion, we investigated electrical effects on the surface of the oil droplets. First, we measured the zeta potentials of aqueous solutions at pH 12.00 and 12.10, and analyzed the relationship between pH and charge. Furthermore, we calculated the amount of charge and the repulsive force generated on the surface of the oil droplets. For the pH 12.10 solution, the charge of one oil droplet and the repulsive force generated by the charge were found to be −30.79 pC and 282 nN, respectively. Taking into account the driving forces controlling the motion of oil droplets reported in previous studies, this repulsive force could be the driving force of the observed repulsive motion. Based on the experimental results, we inferred that the three types of self-propulsive motion were controlled by electrical effects at the oil/water interface. To further examine these motions, we analyzed the time dependences of the total distance traveled by a droplet and of the distance between oil droplets, as well as the droplet velocities and images of internal convection of oil droplets.

PH-Sensitive Controlled Motion of Micrometer-sized Oil Droplets in a Solution of Surfactants Containing Fumaric Acid Derivatives.

Self-propelled droplets are of considerable interest as an appropriate model for understanding the self-propulsion of objects in the fields of nonequilibrium physics and nonlinear science. Several research groups have reported the monodirectional motion of droplets, that is, chemotaxis, using stimuli-responsive materials. However, the precise control of chemotaxis remains challenging from the perspective of synthetic chemistry because chemotactic motion is primarily induced by the consumption of reactive oil or surfactants. Herein, we report a chemical system containing pH-responsive fumaric acid derivatives, in which the oil droplet exhibited positive chemotaxis over a wide pH range-from basic to acidic conditions. From the measurements of the interfacial tension between the oil and aqueous phases, it was deduced that the positive chemotaxis was due to heterogeneity in the interfacial tension of the droplet surface, which was accompanied by the production of surface-active compounds in the pH gradient in a linear-type channel.

Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Photoinduced Collective Motion of Oil Droplets and Concurrent Pattern Formation in Surfactant Solution

Oscillating Motion of Oil Droplets in the Emulsion Near the Air-Water Interface.

Numerous living organisms as well as artificially created self-propelled objects can form dissipative structures due to the nonlinear effects and nonequilibrium of the system. Here we present an active oil-in-water emulsion in which the oil droplets take part in the reciprocating motion under the action of Marangoni flow near the air-water interface. The droplet dynamics in the emulsion is governed by the chemical reaction proceeding between quiescent copper particles and ammonia and by the convective mixing of a surfactant. We established that the reciprocating motion of droplets in the emulsion arises as a result of a periodic change in the Marangoni flow direction at the air-water interface. The feature of the considered system is that the reciprocating motion of droplets is realized only when the surface area fraction of droplets in the emulsion is close to the density of a two-dimensional colloid crystal. Oscillations degenerate under the reduction in surface area fraction to the critical value of ∼50% since the existence of oscillations in the emulsion requires a suppression of the surfactant convective mixing between the inner layers of liquid film and the air-water interface.

Combination of electric field and medium coalescence for enhanced demulsification of oil-in-water emulsion

Currently, the green and efficient demulsification of oil-in-water (O/W) emulsion is a challenging problem in the treatment of oily wastewater. This work describes a nonchemical method for recovering oil from emulsions. A novel attempt to enhance the demulsification of O/W emulsions using the combination of an electric field and medium coalescence was carried out, and the combined treatment significantly improved the separation performance compared with medium coalescence or electrical demulsification alone. The electrohydrodynamic (EHD) effects could drive fine droplets to migrate and collide with the medium. The demulsification effects under various electric field parameters and medium properties were investigated. As the voltage increased, the EHD motion of oil droplets increased, and the demulsification effect was significantly improved. Under the same voltage, the demulsification effect of the DC electric field (DCEF) was better than that of pulsed electric field, but the current of bidirectional pulsed electric field (BPEF) is smaller. BPEF was more efficient at low frequencies less than 25 Hz. As the frequency increased, EHD motion was inhibited, especially when the frequency exceeded 1000 Hz, and the demulsification effect was poor. The particle size of the coalescence medium was also a key factor. The bed packed with small-sized particles showed a better demulsification effect whereas the wettability of the particles showed relatively little effect.

Active motion of multiphase oil droplets: emergent dynamics of squirmers with evolving internal structure.

Synthetic soft matter systems, when driven beyond equilibrium by active processes, offer the potential to achieve dynamical states and functions of a complexity found in living matter. Emulsions offer the basis of a simple yet versatile system for identification of the physicochemical principles underlying active soft matter, but how multiple internal phases within emulsion droplets (e.g., Janus morphologies) organize to impact emergent dynamics is not understood. Here, we create multiphase oil droplets with ultralow interfacial tensions but distinct viscosities, and drive them into motion in aqueous micellar solutions. Preferential solubilization of select components of the oil both drives the droplet motion and yields a progression of internal phase morphological states with distinct symmetries. We find the active droplets to exhibit five dynamical states during morphogenesis. By quantifying microscopic flow fields, we show that it is possible to map the diverse droplet behaviors to squirmer models of spherical microswimmers in Stokes flow, thus showing that multiphase droplets offer the basis of a versatile platform with which to study and engineer the hydrodynamics of microswimmers.

Control of motion of oil droplets in O/W emulsion by electric fields

Control of motion of oil droplets in O/W emulsion by electric fields

Mobilisation and stabilisation flow of multiple oil slugs in a capillary model

This research aims to investigate the mobilisation and flow of multi-oil slugs in a capillary model saturated with water, for simulating the motion of oil droplets trapped inside the porous medium ...