Foam stability in filtered lubricants containing antifoams

Nonaqueous foam stabilization mechanisms in the presence of volatile solvents

Hypothesis Nonaqueous foams are found in a variety of applications, many of which contain volatile components that need to be removed during processing. Sparging air bubbles into the liquid can be used to aid in their removal, but the resulting foam can be stabilized or destabilized by several different mechanisms, the relative importance of which are not yet fully understood. Investigating the dynamics of thin film drainage, four competing mechanisms can be observed, such as solvent evaporation, film viscosification, and thermal and solutocapillary Marangoni flows. Experiments Experimental studies with isolated bubbles and/or bulk foams are needed to strengthen the fundamental knowledge of these systems. This paper presents interferometric measurements of the dynamic evolution of a film formed by a bubble rising to an air-liquid interface to shed light on this situation. Two different solvents with different degrees of volatility were investigated to reveal both qualitative and quantitative details on thin film drainage mechanisms in polymer-volatile mixtures. Findings Using interferometry, we found evidence that solvent evaporation and film viscosification both strongly influence the stability of interface. These findings were corroborated by comparison with bulk foam measurements, revealing a strong correlation between these two systems.

Assessment of Cavitation Intensity in Accelerating Syringes of Spring-Driven Autoinjectors

PURPOSE

Cavitation is an undesired phenomenon that may occur in certain types of autoinjectors (AIs). Cavitation happens because of rapid changes of pressure in a liquid, leading to the formation of small vapor-filled cavities, which upon collapsing, can generate an intense shock wave that may damage the device container and the protein drug molecules. Cavitation occurs in the AI because of the syringe-drug relative displacement as a result of the syringe's sudden acceleration during needle insertion and the ensuing pressure drop at the bottom of the container. Therefore, it's crucial to analyze the potential effect of cavitation on AI. The goal of the current study is to investigate the effects of syringe and AI design parameters such as air gap size, syringe filling volume, fluid viscosity, and drive spring force (syringe acceleration) on the risk and severity of cavitation.

METHODS

A model autoinjector platform is built to record the syringe and cavitation dynamics which we use to estimate the cavitation intensity in terms of extension rate and to study the effects of design parameters on the severity of cavitation.

RESULTS

Our results show the generation of an intense shock wave and a high extension rate upon cavitation collapse. The induced extension rate increases with syringe acceleration and filling volume and decreases with viscosity and air gap size.

CONCLUSION

The most severe cavitation occurred in an AI device with the larger drive spring force and the syringe of a smaller air gap size filled with a less viscous fluid and a larger filling volume.

Uptake kinetics of spontaneously emulsified microdroplets at an air-liquid interface

Understanding the transfers occurring at the interfaces between emulsions and air is required to predict the properties of foamed emulsions, used for example as antifoaming lubricants or for oil extraction. Whereas bubbling oil-in-water emulsions have been studied in details, oil-in-oil emulsions have received less attention. We consider a phase-separating mixture of three oils being Polydimethylsiloxane (PDMS), decane and cyclopentanol. PDMS is dispersed as submicrometer-sized droplets by spontaneous emulsification. In bulk, we show that the time evolution of the emulsion is driven by undelayed coalescence of the Brownian microdroplets. At the freshly created interface of an air bubble created in the emulsion, we use tensiometry measurements to investigate the uptake kinetics of PDMS-rich microdroplets at the air-liquid interface. Specifically, we evidence two mechanisms of uptake: the advection of droplets at the interface during bubble swelling, followed by their diffusion on a longer time scale. We model the growth of the PDMS-rich layer at the interface and, finally, we establish the surface energy of a thin film of PDMS-rich phase squeezed between air and liquid as a function of its thickness.

Challenges in Mitigating Lubricant Foaming

Lubricant foaming and its mitigation is an active area of research driven by demands from modern machinery that require foam-free lubricant operation over extended periods and under adverse conditions. Tackling lubricant foaming has proven to be challenging due to interdependent foam stabilization mechanisms and a multitude of antifoam inactivation routes. This perspective briefly outlines the key challenges faced by researchers in this field. Overcoming these challenges to create lubricants with superior foaming characteristics requires the development of new lubricant and antifoam chemistry as well as a shift from the existing trial-and-error methods to mechanistic-insight-driven lubricant formulation and antifoam design.

Particle-stabilized oil foams

The area of oil foams although important industrially has received little academic attention until the last decade. The early work using molecular surfactants for stabilisation was limited and as such it is difficult to obtain general rules of thumb. Recently however, interest has grown in the area partly fuelled by the understanding gained in the general area of colloidal particles at fluid interfaces. We review the use of solid particles as foaming agents for oil foams in cases where particles (inorganic or polymer) are prepared ex situ and in cases where crystals of surfactant or fat are prepared in situ. There is considerable activity in the latter area which is particularly relevant to the food industry. Discussion of crude oil/lubricating oil foams is excluded from this review.

Adsorption and Aggregation of Monoclonal Antibodies at Silicone Oil-Water Interfaces

Monoclonal antibody (mAb) therapies are rapidly growing for the treatment of various diseases like cancer and autoimmune disorders. Many mAb drug products are sold as prefilled syringes and vials with liquid formulations. Typically, the walls of prefilled syringes are coated with silicone oil to lubricate the surfaces during use. MAbs are surface-active and adsorb to these silicone oil-solution interfaces, which is a potential source of aggregation. We studied formulations containing two different antibodies, mAb1 and mAb2, where mAb1 aggregated more when agitated in the presence of an oil-water interface. This directly correlated with differences in surface activity of the mAbs, studied with interfacial tension, surface mass adsorption, and interfacial rheology. The difference in interfacial properties between the mAbs was further reinforced in the coalescence behavior of oil droplets laden with mAbs. We also looked at the efficacy of surfactants, typically added to stabilize mAb formulations, in lowering adsorption and aggregation of mAbs at oil-water interfaces. We showed the differences between poloxamer-188 and polysorbate-20 in competing with mAbs for adsorption to interfaces and in lowering particulate and overall aggregation. Our results establish a direct correspondence between the adsorption of mAbs at oil-water interfaces and aggregation and the effect of surfactants in lowering aggregation by competitively adsorbing to these interfaces.

Single bubble and drop techniques for characterizing foams and emulsions

The physics of foams and emulsions has traditionally been studied using bulk foam/emulsion tests and single film platforms such as the Scheludko cell. Recently there has been a renewed interest in a third class of techniques that we term as single bubble/drop tests, which employ isolated whole bubbles and drops to probe the characteristics of foams and emulsions. Single bubble and drop techniques provide a convenient framework for investigating a number of important characteristics of foams and emulsions, including the rheology, stabilization mechanisms, and rupture dynamics. In this review we provide a comprehensive discussion of the various single bubble/drop platforms and the associated experimental measurement protocols including the construction of coalescence time distributions, visualization of the thin film profiles and characterization of the interfacial rheological properties. Subsequently, we summarize the recent developments in foam and emulsion science with a focus on the results obtained through single bubble/drop techniques. We conclude the review by presenting important venues for future research.

Bubble Coalescence at Wormlike Micellar Solution-Air Interfaces

Surfactants in aqueous solutions self-assemble in the presence of salt, to form long, flexible, wormlike micelles (WLM). WLM solutions exhibit viscoelastic properties and are used in many applications, such as for cosmetic products, drag reduction, and hydraulic fracturing. Understanding the coalescence stability of bubbles in WLM solutions is important for the development of WLM based products that require a stable dispersion of bubbles. In this paper, we investigate the thin film drainage dynamics leading up to the coalescence of bubbles at flat WLM solution-air interfaces. The salts and surfactant type and concentrations were chosen so as to have the viscoelastic properties of the tested WLM solutions span over 2 orders of magnitude in moduli and relaxation times. The various stages in drainage and coalescence, the formation of a thick region at the apex (a dimple), the thinning and washout of this dimple, and the final stages of drainage before rupture, are modified by the viscoelasticity of the wormlike micellar solutions. As a result of the unique viscoelastic properties of the WLM solutions, we also observe a number of interesting fluid dynamic phenomena during the drainage processes including elastic recoil, thin film ripping, and single-step terminal drainage.

Breakup of Thin Liquid Films: From Stochastic to Deterministic

The thinning and rupture of thin liquid films is a ubiquitous process, controlling the lifetime of bubbles, antibubbles, and droplets. A better understanding of rupture is important for controlling and modeling the stability of multiphase products. Yet literature reports that film breakup can be either stochastic or deterministic. Here, we employ a modified thin film balance to vary the ratio of hydrodynamic to capillary stresses and its role on the dynamics of thin liquid films of polymer solutions with adequate viscosities. Varying the pressure drop across planar films allows us to control the ratio of the two competing timescales, i.e., a controlled hydrodynamic drainage time and a timescale related to fluctuations. The thickness fluctuations are visualized and quantified, and their characteristics are for the first time directly measured experimentally for varying strengths of the flow inside the film. We show how the criteria for rupture depend on the hydrodynamic conditions, changing from stochastic to deterministic as the hydrodynamic forces inside the film damp the fluctuations.

Hyperspectral imaging for dynamic thin film interferometry

Dynamic thin film interferometry is a technique used to non-invasively characterize the thickness of thin liquid films that are evolving in both space and time. Recovering the underlying thickness from the captured interferograms, unconditionally and automatically is still an open problem. Here we report a compact setup employing a snapshot hyperspectral camera and the related algorithms for the automated determination of thickness profiles of dynamic thin liquid films. The proposed technique is shown to recover film thickness profiles to within 100 nm of accuracy as compared to those profiles reconstructed through the manual color matching process. Subsequently, we discuss the characteristics and advantages of hyperspectral interferometry including the increased robustness against imaging noise as well as the ability to perform thickness reconstruction without considering the absolute light intensity information.