Effect of starch particles on foam stability and dilational viscoelasticity of aqueous-foam☆

Yongqiang Zhang ,Zhidong Chang ,*,Wenli Luo ,Shaonan Gu ,Wenjun Li,Jianbo An

1 School of Chemistry and Biological Engineering,University of Science and Technology Beijing,Beijing 100083,China

2 Research Institute of Petroleum Exploration and Development,PetroChina,Beijing 100083,China

Keywords:Foam stability Dilational viscoelasticity Starch particle Film

ABSTRACT Surface dilational rheological behavior and foam stability of starch/surfactant mixed solutions were studied at different starch concentrations and constant surfactant concentration.The results show that dilational viscoelasticity modulus,dilational elasticity modulus and dilational viscosity modulus increase with the concentration of starch particles.Foamstability increases with dilational viscoelasticity.Foamstrength also increases with starch concentration.Starch particles play a positive effect on foam stability and dilational viscoelasticity and the effect becomes more significant as drainage proceeds.Film pictures indicate that the film with 20%(by mass)starch particles is thicker than that with out starch.Starch particles gather in Plateau border and resist drainage,making the foam more stable.

1.Introduction

Foam is a dispersed system of gas and liquid,which plays an important role in a wide range of industrial applications[1-4].Thermodynamically,foam is unstable[5],so it tends to reduce its energy through rupture even without external disturbances.In many applications,such as fire control and foam flooding,persistent stability of foam is crucial and unstable foam limits its applications.Therefore,foam stability has been a research highlight in recent years[6-9].

In a foam system,dispersed gas phase is isolated by liquid film.Foam stability is greatly affected by the properties of foam film.Due to the relation between film stability and foam stability,many investigations have focused on enhancing the film stability.Avariety of factors influencing the film stability are proposed[10].Film dilational viscoelasticity is one of the most important factors with which the stability of foam the relaxation of surfactant and stabilizer molecules between surface and bulk solution.The analysis on film dilational viscoelasticity can provide information about configuration,interaction,aggregation of surfactant molecules and stabilizer in the film[13].Thus film dilational viscoelasticity is an important characteristic among film properties.

Film is mainly stabilized by surfactant and stabilizer[14].The addition of water-soluble polymer,a favorable stabilizer,has a prominent effect on film dilational viscoelasticity.Bhattacharyya et al.[15]reported that film dilational viscoelasticity largely depends on the formation of polyelectrolyte/surfactant complexes.The film surface structure is very complex.At low concentration of surfactant,the film dilational viscoelasticity of poly(styrenesulfonate)/dodecyltrimethylammonium bromide solutions is close to that of relatively concentrated pure poly(styrenesulfonate) solution.However,at intermediate surfactant concentrations,the film dilational elasticity drops abruptly[16],which appears in many systems containing surfactant and polyelectrolyte[17].This phenomenon is interpreted by different surface structures at higher and lower surfactant concentrations.

The addition of polymer increases the viscosity of foam solution and polymer has a significant influence on foam stability,but most of polymer solutions are pseudo-plastic fluid and their viscosity decreases as the solution is sheared[18].The viscosity of bulk solution containing polymer decreases when film drainages or is stretched,leading to unstable foam.We can deduce that the addition of dilatant fluid to foaming solution as a stabilizer can enhance the viscosity of bulk solution in stretching or draining film.However,the work about dilatant fluid as a foam stabilizer is rarely reported.

Addition of nano-particles or micron-particles can also improve foam stability and increase film dilational viscoelasticity,the effects of which have drawn more and more attention[19].It is found that nano-particles have a strong effect on film dilational viscoelasticity[20,21].

The viscosity of dilatant fluid increases with the shear rate.Due to its particular property,extensive researches have been carried out in the last years[22-25].Aqueous system with dispersed starch particles is a typical shear thickening fluid.When starch particles in water reach certain concentration,the viscosity of dispersed system increases with the shear rate.The rheology property of starch dispersion has been studied extensively,some of which focus on the shear thickening property[26-28].

Aqueous system with dispersed starch particles is easy to prepare and its shear thickening property is typical.Starch may be an excellent dilatant particle.In this study,starch particles are added into foaming solution as stabilizer.The effect of starch particles on film dilational viscoelasticity is studied and the mechanism of foam and film stability is investigated.

2.Experimental

2.1.Materials

Sodium dodecyl sulfate(SDS)with analytical grade was purchased from Sino Pharm Chemical Reagent Co.Ltd.,China.Edible starch made from sweet potato was purchased from the local market.KYPAM-2,a kind of polyacrylamide,was purchased from Beijing Hengju Oil field Chemistry Reagent Limited Company,China.The molecular weight of KYPAM-2 is 2.419×107and its degree of hydrolysis is 21.53%.The water used in this experiment was deionized water with a resistance value of≥18 MΩ·cm?1.

2.2.Experimental methods

20 g SDS[4%(by mass)]solution and 2 g KYPAM-2[0.5%(by mass)]were mixed and different amounts of starch particles and deionized waterwere added into the mixture with thoroughly stirring.The concentrations of SDS and KYPAM-2 were constant and the concentration of starch particles was varied.

Fig.1 is a sketch of film dilational rheology instrument,developed by CNPC.The dilational viscoelasticity modulus,dilational elasticity modulus and dilationalviscosity modulus of foaming solution were measured at 32°C.

Fig.1.Sketch of dilational rheology instrument for free liquid film.1—electronic analytical balance;2—thermostat;3—motor;4—position sensing unit;5—recorder;6—conductivity meter;7—immobile loop;8—mobile loop;9—slope system.

The characteristics of foam were investigated with Waring blender method[29].100 mlfoaming solution was added into a Waring blender,stirring at 7000 r·min?1for 1 min.For determining the foamability and foam stability,the foam was poured into a 1000 ml graduated cylinder,with foam volume and time recorded for drainage of 30,40,50,and 60 ml.The viscosity of foaming solution was measured with a Brook field DV-III viscometer(USA)at 10 r·min?1and 21.9 °C.The strength of foam was measured with a Brook field R/S plus rheometer(USA)at shear rate of 7 s?1and 21.9 °C.The optic images of foam film were obtained with a microscope(ZN17-TL60000,Beijing Chino Far East Technology Co.,Ltd.,China).

3.Results and Discussion

3.1.Foam stability

Fig.2 illustrates the effect of starch particles on foam drainage.The drainage without starch particles needs the least time.Drainage time increases with the concentration of starch particles.Addition of starch particles can improve foam stability.The differences between drainage time for 30,40,and 50 ml liquid are small,while the time for drainage of 60 ml increases apparently.In the drainage process,less starch particles are carried away by bulk solution and the concentration of starch particles in the bulk increases.As starch concentration reaches certain value,the solution becomes dilatant fluid,so the viscosity of bulk solution increases,slowing the film drainage.Dilatant fluid also improves film strength,so that the film is more capable of resisting disturbances.The disjoin pressure of the film increases,resisting the decrease of the distance between film surfaces.Shear thickening property of the starch dispersion is improved as drainage proceeds,making the film drainage more difficult.All these make the film more stable.

The effect of starch particles on the foamability of foaming solution is presented in Fig.3.The foamability without starch particles is the best,with a volume of 610 ml.The foamability decreases with the addition of starch particles.

To explain the effect of starch particles on foam stability and foamability,the viscosity of foam solution is measured.The effect of starch particles on solution viscosityμ is presented in Fig.4.The viscosity without starch particles is the smallest,4.38 mPa·s?1,and the viscosity increases with the addition of starch particles.When the solution is sheared,starch particles in the solution resist the fluid flow and increase the viscosity of solution.This effect is more significant as the concentration of starch particles increases.

The effect of starch particles on the foamability can be explained from two aspects.On the one hand,the viscosity of foaming solution increases with the addition of starch particles,so it is more difficult to bring gas into the solution at the same power of Waring blender,reducing the foam volume.On the other hand,the special structure of starch particles makes them absorb some surfactant molecules,increasing the surface tension of foam solution and decreasing the foam volume.

The addition of starch particles increases the viscosity of foam solution obviously,slowing gravitational drainage,and improves film strength,being more capable of self-healing and decreasing gas diffusion between foam bubbles.Thus the foam is more stable.

Fig.2.Effect of starch particles on foam drainage.Drainage volume/ml:▲60;▼50;●40;■30.

Fig.3.Effect of starch particles on foamability.

3.2.Dilational rheology of foam film

In this work, film stress,strain and phase angle are measured at 0.8 Hz.These data is analyzed and film dilational viscoelasticity modulus(E),dilational elasticity modulus(E′)and dilational viscosity modulus(E″)are obtained.The effect of starch particles on these moduli are presented in Fig.5.The values without starch particles are minimum,E=5.439 mN·m?1,E′=3.508 mN·m?1,and E″=4.156 mN·m?1.These modulus values increase with the concentration of starch particles.

The film dilational modulus is defined as the ratio of the change in surface tension and the corresponding change in surface area.It can be divided into dilational elasticity modulus(E′)and dilational viscosity modulus(E″).

The film dilational elasticity is related to the ability of self-healing.When a film is stretched,dilational elasticity modulus and surface tension gradient increase,so surfactant molecules transfer to the regions with lower surfactant concentration.Surfactant molecules carry surrounding water molecules with them,recovering the film thickness.

Increase of surfactant concentration has two effects on surface properties:higher surfactant concentration on the surface and larger concentration gradient between surface and bulk liquid,enhancing the transfer of surfactant molecules.When a film is stretched,higher surfactant concentration on the surface leads to a larger surfactant concentration gradient,which improves the surfactant transfer and increases film dilational elasticity.Larger concentration gradient between surface and the bulk makes surfactant transfer from the bulk to new surface,reducing the concentration gradient on the surface and weakening film dilational viscoelasticity.

Fig.4.Effect of starch concentration on foam solution viscosity.

Fig.5.Effect of starch particles on film dilational viscoelasticity modulus(a),dilational elasticity modulus(b)and dilational viscosity modulus(c).

For low concentration of surfactant,the addition of surfactant mainly affects the surfactant concentration on the surface.Its increase enlarges the film dilational viscoelasticity and strength.When the adsorption capacity is saturated,addition of surfactant can make surfactant molecules transfer to new surface more easily,reducing film dilational viscoelasticity and strength.

In this work,the concentration of surfactant is much higher than CMC.The adsorption capacity of surfactant is saturated and starch particles added in the solution adsorb some surfactant molecules in the bulk solution,reducing the surfactant transfer from bulk solution to new surface and increasing the film dilational viscoelasticity.This effect strengthens with the increase of starch particles.Particle cluster theory indicates that the shear thickening property is related to the formation of particle clusters in the solution.In this study,when film is stretched or drainages,starch particles will gather and form particle clusters[30],impeding surfactant transfer from bulk solution to new surface.This accounts for the increment of film dilational viscoelasticity modulus.The formation of particle clusters also explains the increase of film dilational viscosity.Particle clusters resist the transfer of surfactantfrom bulk solution to new surface and make this process consume more energy.As a result, film dilational viscosity increases.

Table 1 Effect of starch particles on foam strength

The foaming process in the blender can be regarded as a process of film stretch.With the addition of starch particles, film dilational viscoelasticity increases,so that the foaming process is more difficult at certain blender power.This explains the decrease of foam volume with the increase of starch concentration in Fig.3.The increase of film dilational viscoelasticity can improve the capacity of self-healing,which also slows the drainage and makes film more stable.

Fig.6.Foam images without starch(a)and with 20%starch(b).

3.3.Foam strength

Foam strength G represents the maximum stress before foamrupture and it also affects foam stability.The effect of starch particles on foam strength is presented in Table 1.Foam strength enhances as the amount of starch particles increases,which is similar with film dilational viscoelasticity modulus in Fig.5.Addition of starch particles increases the viscosities of bulk solution and surface,which is related to foam strength.Starch particles can form dilatant fluid.When a film is stretched or drainages,the viscosity increases,enhancing film strength.Higher film strength makes foam resist larger stress before rupture and reduces gas diffusion in the film,so foam becomes more stable.

3.4.Optic image of foam film

The effect of starch particles on foam feature and film thickness is illustrated in Fig.6.The film thickness without starch particles is only about 0.1 mm,while that with 20%(by mass)starch particles is about 0.6 mm.Thus the addition of starch particles could maintain film thickness,slow the liquid drainage and make foam more stable.The film in Fig.6(a)is transparent,while that in Fig.6(b)is different.This suggests that in the drainage process,starch particles aggregate in Plateau border and the fluid becomes dilatant,increasing the viscosity of film solution and making film and foam more stable.

4.Conclusions

With the addition of starch particles, film drainage reduces,foam strength and viscosity of foaming solution increase.These improve the foam stability.Addition of starch particles can increase dilational viscoelasticity,dilational elasticity and dilatational viscosity,which are appropriate parameters to characterize foam stability.The effect of starch particles on foam stability is more significant as drainage proceeds.