Interactions between colliding oil drops coated with non-ionic surfactant determined using optical tweezers

Experimental study on the desorption effect of penetrant on gas-containing coal

Two non-ionic reagents, polyethylene glycol 4000 and Tween-80, two anionic reagents, sodium dodecyl benzenesulfonate and sodium lauryl sulfate, and a mixture of these non-ionic and anionic reagents were used as penetrants. The processes of replacement desorption and relief-pressure desorption of gas-containing coal were studied, the influence of the penetrant on the amount of gas replacement desorption and relief-pressure desorption was explored, and the change rule of the amounts of gas replacement desorption and relief-pressure desorption was analysed. The results show that the increase rate of the replacement desorption amount of the mixed penetrant is 11.81%-34.75%, and the decrease rate of the relief-pressure desorption amount is 51.68%-72.69%, which are higher values than those with a single penetrant. As the mass fraction of penetrant increases within the range of 0.5%~2%, the capacity of gas replacement desorption and hindering gas relief-pressure desorption will increase. At the same mass fraction, the effect of the mixed penetrant is better than that of the anionic penetrant, which in turn is better than that of the non-ionic penetrant.

Gemini and Bicephalous Surfactants: A Review on Their Synthesis, Micelle Formation, and Uses

The use of surfactants in polymerization reactions is particularly important, mainly in emulsion polymerizations. Further, micelles from biocompatible surfactants find use in pharmaceutical dosage forms. This paper reviews recent developments in the synthesis of novel gemini and bicephalous surfactants, micelle formation, and their applications in polymer and nanoparticle synthesis, oil recovery, catalysis, corrosion, protein binding, and biomedical area, particularly in drug delivery.

Exploring the effects of approach velocity on depletion force and coalescence in oil-in-water emulsions

An emulsion is a thermodynamically unstable system consisting of at least two immiscible liquid phases, one of which is dispersed in the other in the form of droplets of varying size. Most studies on emulsions have focused on the behaviour of emulsion droplets with diameter from ∼50 μm and upwards. However, the properties of smaller droplets may be highly relevant in order to understand the behaviour of emulsions, including their performance in numerous applications within the fields of food, industry, and medical science. The relatively long life-time and small size of these droplets compared to other emulsion droplets, make them suited for optical trapping and micromanipulation technologies. Optical tweezers have previously shown potential in the study of stabilized emulsions. Here we employ optical tweezers to examine unstable oil-in-water emulsions to determine the effects of system parameters on depletion force and coalescence times.

Laser Induced Aggregation of Light Absorbing Particles by Marangoni Convection

Laser induced Marangoni convection can be used to accumulate micro-particles. In this paper, a method is developed to control and accumulate the light absorbing particles dispersed in a thin solution layer. The particles are irradiated by a focused laser beam. Due to the photothermal effect of the particles, the laser heating generates a thermal gradient and induces a convective flow around the laser’s heating center. The convective flow drives the particles to accumulate and form a particle aggregate close to the laser’s heating center. The motion of particles is dominated by the Marangoni convection. When the laser power is high, the vapor bubbles generated by laser heating on particles strengthen the convection, which accelerates the particles’ aggregation.

Interactions between CO2-Responsive Switchable Emulsion Droplets Determined by Using Optical Tweezers

CO₂-responsive switchable emulsions have been of great interest in some industrial processes where the stability of the emulsion is only required temporarily, such as oil transport, drug delivery, and fossil fuel production. The good understanding of the stability and instability mechanism is vital to the switchable behavior between emulsification and demulsification. Herein, a novel approach was developed to determine the interactions between two switchable emulsion droplets directly by a dual-laser optical tweezers instrument. The repulsive force between a couple of tetradecane droplets occurs to increase progressively with the increasing concentration of switchable surfactant in solutions. However, the repulsive force appears to decrease progressively in turn when the switchable surfactant concentration is far higher than the critical micelle concentration (CMC). Moreover, the depletion effect starts to emerge in the higher surfactant concentration which is attributed to the switchable surfactant micelles generated in solutions. In addition, according to the measurements of interaction forces, a mechanism of the switchable behavior is well proposed, which is established by the principle of self-assembly/detachment of the switchable surfactant, resulting in the weakening and re-enhancing of the electrostatic double-layer (EDL) repulsive forces between tetradecane droplets, upon selective introduction and removal of CO₂. Based on this work, a novel perspective was provided to study the switchable emulsion, which can contribute instructive messages for the understanding of stability and instability mechanisms of switchable emulsions.

In Situ Measurements of Interactions between Switchable Surface-Active Colloid Particles Using Optical Tweezers

Switchable surface-active colloid particles are critical to the preparation of switchable Pickering emulsions, which are widely involved in multitudinous fundamental and practical fields, such as biomedical, food products, and spinning cosmetics. The stability of switchable surface-active particles relies on the full understanding of interaction forces between individual colloid particles quantitatively. In this work, a dual-laser optical tweezers instrument was applied to measure the interaction forces between silica particles coated with a common cationic surfactant (cetyltrimethylammonium bromide, CTAB) in water, and all of the measured forces can be well fitted with the theoretical model derived from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. It was revealed that the minimum surface distance to engender the interaction forces between silica particles was closer progressively with the increase of CTAB concentrations, suggesting that the introduction of CTAB molecules in the solution thinned the electric double layer. In addition, the minimum surface distance between surface-inactive silica particles further decreased compared to surface-active states, although the ζ-potential has returned to the initial value of bare silica in pure water when the molecular ratio of 1:1 anionic surfactant (sodium dodecyl sulfate, SDS) was added into the solution to switch the surface-active silica particles to surface-inactive states. Our results provide a considerate methodology for quantifying the interaction forces and investigating the switchable behaviors of CTAB molecules from the adsorption to desorption at the particle-water interfaces, which provide vital foresights into the stabilization mechanism of switchable surface-active colloid particles and the further development of switchable Pickering emulsions.

CO2/N2-responsive oil-in-water emulsions using a novel switchable surfactant

HYPOTHESIS

Recently, switchable or stimuli-responsive emulsions have attracted much research interest in many industrial fields. In this work, a novel CO₂/N₂-responsive surfactant was designed and developed to facilitate the formation of switchable oil-in-water (O/W) emulsions with fast switching characteristics between a stable emulsion and separate phases upon alternatively bubbling CO₂ and N₂.

EXPERIMENTS

The novel CO₂/N₂-responsive surfactant was facilely prepared by mixing an anionic fatty acid (oleic acid) and a cationic amine (1,3-Bis (aminopropyl) tetramethyldisiloxane) at a 1:1 molecular ratio, which was assembled based on electrostatic interactions. The structure and properties of the novel CO₂/N₂-responsive switchable surfactant were investigated by Fourier-transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR) spectroscopy, and interfacial tensions.

FINDINGS

The developed surfactant shows an excellent interfacial activity at the oil/water interface, which can significantly reduce the dosage of the switchable surfactant compared with previous CO₂/N₂-responsive surfactants. The dynamic interfacial tension of n-decane and aqueous phase decreased from 45 mN m^-1 to 5 mN m^-1 within 100 s with the addition of 0.2 mM surfactant. In this work, a low concentration of the novel switchable surfactant (e.g., 20.0 mM) can realize reversible emulsification and demulsification in an emulsion system as compared with the high dosage (e.g., ~150 mM) in previous reports, which will bring huge economic benefits in industrial applications in the future. Moreover, this work expands the family of ion-pair surfactants to small amino-functionalized molecules beyond Jeffamine D-230, which promotes the development of simple and switchable ion-pair surfactant. It is found that the O/W emulsions stabilized by the switchable surfactant show excellent stability, which can be stored for over 60 days at room temperature without any obvious change. Interestingly, the stable O/W emulsion is completely demulsified upon bubbling CO₂ for 30 s and can be easily re-emulsified to the initial state after purging N₂ at 60 °C within 10 min, which demonstrates a rapid and highly efficient switching behavior. The reversible emulsification and demulsification process is ascribed to the reversible assembly and disassembly of the switchable surfactant, which is induced by the removal and purge of CO₂.

Aggregation and coalescence of partially crystalline emulsion drops investigated using optical tweezers

The solid content of viscoelastic emulsion drops is known to affect their propensity for aggregation and their subsequent coalescence behaviour, where the balance between the drive to reduce surface tension and the straining of an internal viscoelastic network is able to create a plethora of stable partially-coalesced states. The latter has previously been elegantly demonstrated in synthetic systems, generated using oil containing different phase volumes of added solids, with micro-pipette experiments carried out on emulsion drops of several tens of microns in size. Herein we carry out experiments in the same spirit but aided by optical tweezers (OT) and using smaller micron-sized emulsion drops generated from milk fat. Given the size dependence of Brownian fluctuations and Laplace pressure the experimental investigation of these smaller drops is not necessarily a trivial extension of the previous work. The solid content of initially separated drops is controlled using a temperature-cycling regime in the sample preparation protocol, and subsequently the propensity for drops to remain joined or not after being brought into contact was examined. Aggregated pairs of drops were then subjected to an increase in temperature, either locally using a high-powered laser, or more globally using a custom-made Peltier temperature-controller. By heating to different degrees, the amount of fat crystals in the drops was able to be controlled, with progressively more compact partially-coalesced states, and eventually complete coalescence generated as the solid content was reduced. While in contrast to previous studies, the emulsion studied here was quite different in size and nature, and the solid content was controlled using temperature, the same underlying physics was nevertheless observed.