Published on 07 January 2020
In an article published by Leslie Labarre and Daniele Vigolo in the journal Microfluidics and Nanofluidics, the authors show that microfluidics can be used to evaluate qualitatively foam hysteresis. The demonstrated system employs a fine and smooth control of the two-phase flows to accurately tune the bubble size and resulting 2D foam generation.
Foam stability often describes the foam left to evolve with time in static conditions. Nevertheless, in everyday life, foams are subjected to a number of deformations. A feature of foam stability is represented by the foam’s ability to resist to the deformation and to recover its initial properties after deformation. The technique developed here allows for a qualitative evaluation of the property of foam recovery after a deformation in a flow-focusing microfluidic device. The foam hysteretic behaviour was evaluated by introducing the analogous of a standard three-step test in which the recovery of viscosity is commonly studied over three deformation stages. The foam behaviour is analysed over an induced cycle of ascendant and descendant deformation at the wall, well controlled by varying the gas pressure for a constant liquid pressure. Thus, the recovery of the two-row foam pattern used as reference is studied after a high deformation phase corresponding to the bamboo pattern and the level of hysteresis is measured qualitatively. The present study improves our understanding of the mechanisms triggering or enhancing foam hysteresis in a microchannel. The findings are interesting for many industrial processes where foams are submitted to a series of deformation steps along the process line from food industrial applications to biological systems.
Aqueous foams are dispersions of gas bubbles in a liquid continuous phase. They are used in a wide range of applications from enhanced oil recovery (EOR) [1] to food industry [2] and biological applications such as biocompatible scaffolds [3].
Aqueous foams can behave like a shear-thinning fluid over time after being subjected to deformation [4]. This property, called “thixotropy”, plays an important role in foam stability [5]. Thixotropy implies the creation of a change in foam properties before and after the deformation, thus, hysteresis.
Foam rheology and more precisely foam thixotropy or hysteretic behaviour is mainly studied by rheometry [6]. The two most common tests employed to study how a fluid recovers after deformation are the three-step test and the creep test [7]. The three-step test consists of three consecutive stages of rest, deformation and rest. The initial rest viscosity is taken as reference for the study of the fluid recovery after a gradual or sharp deformation. This test has been adapted to fragile products such as foams by applying a non-destructive deformation to preserve the sample [8].
However, foams evolve over time due to the simultaneous destabilising events which are the key actors of its stability kinetic (e.g., drainage, coalescence and coarsening) [9]. This ageing process, in addition to the quantity of product required and the cost of the equipment, demonstrates the limitations of macroscale rheology to study fragile and complex products such as foams.
A decade ago, a novel and innovative use of microfluidics was developed at first to measure the viscosity of single phase fluids [10] and further to measure the properties of viscoelastic fluids such as the relaxation times [11].
Microfluidics rheometry matches the standards of its macroscopic counterpart with the advantages of requiring much less amount of sample and lower cost.
Here, this use of microfluidics is extended to two-phase fluids: a new way to study and to evaluate the parameters influencing the property of recovery of foam after a gradual deformation at the wall is implemented. These parameters comprise a range of properties affecting the foam microstructure from the continuous phase viscosity, the surface tension and the interfacial properties such as the surface elasticity.
In the present work, the goal is to identify the parameters influencing the induced microstructural hysteresis during the deformation cycle in a fixed geometry. The influence of the surface drag is characterised by controlling the level of deformation at the wall via the fine-tuning of the gas and liquid inlet pressures.
The foam was generated and studied within a PDMS flow-focusing microfluidic device by co-injection of air and foaming solution containing 5 cmc SDS and various additives to tune the properties.
Two reservoirs of air and liquid are connected via a pressure controller (OB1 MK3, Elveflow) to accurately control the gas and liquid inlet pressures.
A foam regime map (FRM), which gives a detailed view of all the different foam patterns available for a specific geometry, is first generated for each solution by changing the gas and liquid inlet pressures between 200 and 1400 mbar as shown in Fig 2.
Areas of steady patterns of bamboo and two-row foam can be identified and selected for the study:
00:00 – 00:06
Step 1: Two rows of bubbles generation via microfluidics. Courtesy of Leslie Labarre
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Step 2: Transition from two rows of bubbles to single row of bubbles triggered via microfluidics. Courtesy of Leslie Labarre
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Step 3: Single row of bubbles also called “bamboo” pattern generated via microfluidics. Courtesy of Leslie Labarre
In the bamboo pattern, the deformation at the wall during the continuous foam generation and flow in the channel is considered as higher than the two-row pattern due to the largest surface area in contact with the wall.
Two ways can be employed to vary the degree of deformation at the wall:
The evolution of the apparent mean bubble diameter during one deformation cycle is obtained via image analysis of snap shots collected by a high-speed camera at up to 20,000 frames per second (fps) via an inverted optical microscope for specific pressure ratio in the ascending and descending pressure ramps.
From each snap shot, the apparent mean bubble diameter is obtained by calculating the equivalent diameter of a circle having the same area of the one obtained for the bubble and normalised by the channel width.
By plotting the normalised apparent mean bubble diameter versus the pressure ratio, the hysteresis is then evaluated by measuring the area between the ascending and descending curves.
Taken together, these findings suggest that microfluidics is a powerful tool to evaluate qualitatively foam hysteresis in a microchannel.
The foam response to a gradual deformation at the wall was analysed based on the transition from the two-row to the bamboo pattern.
It was observed that the viscosity of the continuous phase had the strongest impact on foam hysteresis.
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