Microfluidics investigation of foam hysteresis – A short review

Leslie Labarre

In an article published by Leslie Labarre and Daniele Vigolo in the journal Microfluidics and Nanofluidics, the authors show that microfluidics allows for the qualitative investigation of 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.

ABSTRACT

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 deformation and to recover its initial properties after deformation. The technique developed here allows for a qualitative investigation of the property of foam recovery after a deformation in a flow-focusing microfluidic device. Foam hysteresis was evaluated by introducing the analogue of a standard three-step test in which the recovery of viscosity is commonly studied over three deformation stages. Foam behaviour is analysed over an induced cycle of ascendant and descendant deformation at the wall, well controlled by varying gas pressure for 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. 

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INTRODUCTION TO MICROFLUIDICS INVESTIGATION OF FOAM HYSTERESIS

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 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 equipment, demonstrates the limitations of macroscale rheology to study fragile and complex products such as foams.

Fig 1. Microfluidics investigation of foam hysteresis: Microfluidics three-step test schematic

Fig 1. A schematic of the three-step test consisting in an ascending and descending pressure ramps obtained by varying the gas pressure for a constant liquid pressure. CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/)

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 smaller 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 surface drag is characterised by controlling the level of deformation at the wall via fine-tuning of gas and liquid inlet pressures.

AIM & OBJECTIVES

  • To control 2D foam generation accurately via microfluidics.
  • To develop a microfluidics approach to measure foam hysteresis.
  • To identify the parameters influencing foam hysteresis.

KEY FINDINGS

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:

  • “bamboo” pattern = single layer of bubbles containing only one bubble in the full width of the channel.
  • “two-row” pattern = single layer containing two rows of bubbles in the width of the channel.
Fig 2. Microfluidics investigation of foam hysteresis: Foam regime maps plotting the gas inlet pressure (Pgas) versus the liquid inlet pressure (Pliq) for the following solutions: a 5 cmc SDS (reference system), b 5 cmc SDS + 20% (wt.) glycerol, c 5 cmc SDS + 40% (wt.) glycerol, and d 5 cmc SDS + 0.15 g L−1 DOH. The red arrows from left to right represent the low (P1) and high (P2) pressure ranges investigated for the formulation

Fig 2. Foam regime maps plotting the gas inlet pressure (Pgas) versus the liquid inlet pressure (Pliq) for the following solutions: a 5 cmc SDS (reference system), b 5 cmc SDS + 20% (wt.) glycerol, c 5 cmc SDS + 40% (wt.) glycerol, and d 5 cmc SDS + 0.15 g L−1 DOH. The red arrows from left to right represent the low (P1) and high (P2) pressure ranges investigated for the formulation. CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/)

Step 1: Two rows of bubbles generation via microfluidics. Courtesy of Leslie Labarre

Step 2: Transition from two rows of bubbles to single row of bubbles triggered via microfluidics. Courtesy of Leslie Labarre

Step 3: Single row of bubbles also called “bamboo” pattern generated via microfluidics. Courtesy of Leslie Labarre

Fig 3. Microfluidics investigation of foam hysteresis: Hysteresis evolution for 5 cmc SDS at low- (P1, black) and high-pressure (P2, grey) sets for both pressure ramps (0.5 and 2 mbar s−1). In the inset, a typical curve for P1 at 0.5 mbar s−1

Fig 3. Hysteresis evolution for 5 cmc SDS at low- (P1, black) and high-pressure (P2, grey) sets for both pressure ramps (0.5 and 2 mbar s−1). In the inset, a typical curve for P1 at 0.5 mbar s−1. CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/)

In the bamboo pattern, 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:

  • by changing pressure set (gas and liquid inlet pressures).
  • by altering pressure ramp (i.e., variation of pressure over time).

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 same area as 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.

If you’re interested in reproducing what Leslie Labarre achieved in her work, feel free to contact our team of experts for additional information about the OB1 pressure-driven flow controller for fine tuning of your fluid flow!

Fig 4. Microfluidics investigation of foam hysteresis: Hysteresis evolution for 5 cmc SDS + 40% (wt.) glycerol at low-pressure set (P1, black) for both pressure ramps (0.5 and 2 mbar s−1). In the inset, a typical curve for P1 at 2 mbar s−1.

Fig 4. Hysteresis evolution for 5 cmc SDS + 40% (wt.) glycerol at low-pressure set (P1, black) for both pressure ramps (0.5 and 2 mbar s−1). In the inset, a typical curve for P1 at 2 mbar s−1. CC BY 4.0( http://creativecommons.org/licenses/by/4.0/)

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REFERENCES TO MICROFLUIDICS INVESTIGATION OF FOAM HYSTERESIS

[1] Yekeen N et al (2018) A comprehensive review of experimental studies of nanoparticles-stabilized foam for enhanced oil recovery. J Petrol Sci Eng 164:43–74.

[2] Laporte M et al (2016) Characteristics of foams produced with viscous shear thinning fluids using microchannels at high throughput. J Food Eng 173:25–33.

[3] Andrieux S, Drenckhan W, Stubenrauch C (2017) Highly ordered biobased scaffolds: from liquid to solid foams. Polymer.

[4] Bekkour K, Scrivener O (1998) Time-dependent and flow properties of foams. Mech Time-Depend Mater 2:171–193.

[5] Mewis J (1979) Thixotropy—a general review. J Nonnewton Fluid Mech 6:1–20.

[6] Miquelim JN, Da Silva Lannes SC (2009) Egg albumin and guar gum influence on foam thixotropy. J Texture Stud 40:623–636.

[7] Mezger TG (2014) The rheology handbook, 4th Edition for users of rotational and oscillatory rheometers. 4th Edition. Edited by European Coating Tech Files. Vincents Network, Hanover.

[8] Asnacios RH, Cohen-Addad S, Asnacios A (1999) Rheological memory effect in aqueous foam. EPL (Europhys Lett) 48(1):93.

[9] Marze S, Guillermic RM, Saint-Jalmes A (2009) Oscillatory rheology of aqueous foams: surfactant, liquid fraction, experimental protocol and aging effects. Soft Matter 5:1937–1946.

[10] Guillot P et al (2006) Viscosimeter on a microfluidic chip. Langmuir 22:6438–6445.

[11] Koser AE et al (2013) Measuring material relaxation and creep recovery in a microfluidic device. Lab Chip 13:1850–1853.