Measurements of Static and Dynamic Bubble Surface Tension Using a Deformation-Based Microfluidic Tensiometer

Real-time probing of fast chemical reactions at the oil/water interface during microdroplet generation

HYPOTHESIS

Probing the interfacial reactions during microdroplet generation is experimentally challenging, limiting the understanding of the reaction dynamics at an early stage. Real-time monitoring of the temporal evolution of interfacial tension offers a quantitative approach for probing interfacial reactions and reveals the reaction rate-control mechanisms governing the interfacial reaction process.

EXPERIMENTS

An automated microfluidic platform is developed to monitor the dynamic interfacial tension during microdroplet generation within a second, using oleic acid (HOA) and sodium hydroxide (NaOH) as a representative reaction system. The evolution of interfacial concentration of the reaction product is quantified by experiments using various reactant concentrations and flow rates, allowing for identifying the rate-control mechanisms.

FINDINGS

The reactions are identified to be mass transfer-controlled or adsorption-controlled at different HOA concentrations with high NaOH/HOA concentration ratios, enabling the determinations of mass transfer boundary layer thickness and adsorption rate constant of HOA. A modified Damköhler number is introduced to characterize the transition between the rate-control mechanisms. Up to one-third of HOA in microdroplets is found to be consumed during the generation stage within one second, and smaller droplets exhibit higher consumption proportions.

Curvature- and temperature-dependent transport of soluble surfactant mixtures to the air-aqueous surface with applications in fluorine-free firefighting foams

Understanding surfactant adsorption to air-water interfaces is crucial to eliminating toxic fluorocarbon-based surfactants while retaining firefighting performance. The adsorption of commercial siloxane and Glucopon surfactants is investigated by measuring dynamic surface tension at different length scales using a pendant drop tensiometer and capillary pressure microtensiometer (CPM) for millimeter and micrometer sized bubbles at 23 °C and 60 °C. Higher surfactant concentration and higher curvature favor surfactant adsorption. The effect of interfacial curvature can be rationalized by rescaling respective times scales for diffusion-limited adsorption. For constant area adsorption in the capillary pressure microtensiometer, surfactants relevant to firefighting foams show stepwise adsorption. Model mixtures of ethoxylated surfactants of different chain lengths also show this stepwise adsorption, suggesting heterogeneity in tail lengths in the commercial surfactants. Surfactant adsorption is modeled by treating the mixtures as a quasi-single component using the Ward-Tordai equation and the Langmuir adsorption isotherm to characterize the temperature-dependent surfactant properties. While temperature increases the diffusivity of both Dow 502W and Triton X100, Dow 502W demonstrates key differences in surfactant adsorption compared to Triton X100 and the Glucopon surfactants at elevated temperatures. A deeper understanding of how different head and tail group lengths and temperature affect surfactant adsorption will help optimize new surfactant replacements for enhanced firefighting performance.

Vertical concentration gradients of soluble surfactants in the rupture of thin liquid films

HYPOTHESIS

Surfactant-laden thin liquid films can rupture due to van der Waals forces, and being able to accurately predict the rupture time is important for applications involving coatings, foams, and emulsions. A common simplification in modeling film rupture is to assume that diffusion along the film thickness is so rapid that the surfactant concentration can be replaced by an averaged value. However, we hypothesize that vertical concentration gradients can develop as a result of surfactant adsorption at the interface, potentially rendering the vertical-averaging (VA) approximation inaccurate under certain conditions. Simulations: We assess the accuracy and limitations of this approximation by performing calculations with a lubrication-theory-based model that explicitly accounts for surfactant concentration gradients along the film thickness for a film on a horizontal solid substrate. Linear stability analysis and nonlinear simulations are performed to understand the role of vertical concentration gradients on film rupture.

FINDINGS

Results show that when surfactant diffusion is slow relative to advection and adsorption, substantial surfactant vertical concentration gradients can emerge. These gradients slow down adsorption and increase stabilizing Marangoni stresses, leading the VA approximation to underestimate the rupture time. Significant deviations in predicted rupture time are also observed when the initial bulk and surfactant concentrations are not in equilibrium, which is common in industrial applications.

Surfactant Partitioning Dynamics in Freshly Generated Aerosol Droplets

Aerosol droplets are unique microcompartments with relevance to areas as diverse as materials and chemical synthesis, atmospheric chemistry, and cloud formation. Observations of highly accelerated and unusual chemistry taking place in such droplets have challenged our understanding of chemical kinetics in these microscopic systems. Due to their large surface-area-to-volume ratios, interfacial processes can play a dominant role in governing chemical reactivity and other processes in droplets. Quantitative knowledge about droplet surface properties is required to explain reaction mechanisms and product yields. However, our understanding of the compositions and properties of these dynamic, microscopic interfaces is poor compared to our understanding of bulk processes. Here, we measure the dynamic surface tensions of 14-25 μm radius (11-65 pL) droplets containing a strong surfactant (either sodium dodecyl sulfate or octyl-β-D-thioglucopyranoside) using a stroboscopic imaging approach, enabling observation of the dynamics of surfactant partitioning to the droplet-air interface on time scales of 10s to 100s of microseconds after droplet generation. The experimental results are interpreted with a state-of-the-art kinetic model accounting for the unique high surface-area-to-volume ratio inherent to aerosol droplets, providing insights into both the surfactant diffusion and adsorption kinetics as well as the time-dependence of the interfacial surfactant concentration. This study demonstrates that microscopic droplet interfaces can take up to many milliseconds to reach equilibrium. Such time scales should be considered when attempting to explain observations of accelerated chemistry in microcompartments.

Capillary pressure-based measurement of dynamic interfacial tension in a spontaneous microfluidic sensor

The size of droplets and bubbles, and the properties of emulsions and foams strongly depend on dynamic interfacial tension (γ_d) - a parameter that is often inaccessible due to the very short time scales for droplet and bubble formation, and the inaccessibility of (e.g., food) production lines. To solve this challenge, we developed a microfluidic tensiometer that can measure γ_d by monitoring the formation time of both droplets and bubbles. Our tensiometer is a pressure-driven microfluidic device that operates based on the principle of a pressure balance: the formation of a droplet (or a bubble) is initialized when the Laplace pressure of the interface is decreased below the externally applied pressure, and this decrease is caused by a reduction in γ_d that can be calculated from the applied pressure and the Young-Laplace equation. The decay of γ_d due to surfactant adsorption can be followed at the characteristic time scale, which is dependent on surfactant type and concentration. For 0.05-1% wt sodium dodecyl sulfate (SDS), we were able to measure γ_d at time scales down to 1 ms and 0.1 ms for droplet and bubble interfaces, respectively, at increasing applied pressures and SDS concentrations. Our tensiometer proves to be a simple, robust method that inherently allows access to nearly the full range of dynamic interfacial tension at relevant time scales.

Investigating the effect of phospholipids on droplet formation and surface property evolution in microfluidic devices for droplet interface bilayer (DIB) formation

Despite growing interest in droplet microfluidic methods for droplet interface bilayer (DIB) formation, there is a dearth of information regarding how phospholipids impact device function. Limited characterization has been carried out for phospholipids, either computationally (in silico) or experimentally (in situ) in polydimethylsiloxane (PDMS) microfluidic devices, despite recent work providing a better understanding of how other surfactants behave in microfluidic systems. Hence, microfluidic device design for DIB applications relies heavily on trial and error, with many assumptions made about the impact of phospholipids on droplet formation and surface properties. Here, we examine the effects of phospholipids on interfacial tension, droplet formation, wetting, and hence device longevity, using DPhPC as the most widely used lipid for DIB formation. We use a customized COMSOL in silico model in comparison with in situ experimental data to establish that the stabilization of droplet formation seen when the lipid is dosed in the aqueous phase (lipid-in) or in the oil phase (lipid-out) is directly dependent on the effects of lipids on the device surface properties, rather than on how fast they coat the droplet. Furthermore, we establish a means to visually characterize surface property evolution in the presence of lipids and explore rates of device failure in the absence of lipid, lipid-out, and lipid-in. This first exploration of the effects of lipids on device function may serve to inform the design of microfluidic devices for DIB formation as well as to troubleshoot causes of device failure during microfluidic DIB experiments.