Tackling slip effects in the nonlinear flow properties of gellan fluid gels

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is the peer reviewed version of the following article: García, Mª C., Sánchez, S., Trujillo-Cayado, L. A., Muñoz, J. and Alfaro, Mª C. (2018), Tackling slip effects in the nonlinear flow properties of gellan fluid gels. J. Appl. Polym. Sci., 136, 46900, which has been published in final form at https://doi.org/10.1002/app.46900. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self- Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited." Tackling slip effects in the non-linear flow properties of gellan fluid gels 1 Mª Carmen Garcíaa, Sergio Sáncheza, Luis A. Trujillo-Cayadoa, José Muñoza and Mª 2 Carmen Alfaroa* 3 aUniversidad de Sevilla, Departamento de Ingeniería Química, 41012, Seville, Spain 4 Corresponding author: e-mail: [email protected]s 5 6 Abstract 7 Gellan gum is a biopolymer widely used in fields such as food, pharmaceutics, chemical 8 or agrochemical. Its property to form strong gel makes possible to produce fluid gels. 9 These materials present an apparent yield stress but its value could be influenced by the 10 wall slip effect on performing the rheological measurements by which it is determined. 11 In this work, the influence of the measuring surface and gap on flow behaviour was first 12 determined. The tests revealed the need to use geometries with rough surfaces although 13 the sample thickness using a parallel plate no influence. Subsequently, the value of yield 14 stress was obtained by means of creep tests (was found to be 4.3 Pa) and, finally, the 15 effect of wall slip on the dynamic viscoelastic behavior was assessed. There was an 16 influence on the extension of the linear viscoelastic region, but not on the viscoelastic 17 functions of the mechanical spectra. 18 Keywords: Slip effects, Fluids gels, Yield stress, Creep test, Gellan gum 19 20 1. Introduction 21 Several polymers which produce strong gels subject to quiescent cooling such gellan 22 gum1-5, agar, k-carrageenan and agarose6-11 can form fluid gels. Such gels are normally 23 obtained on interrupting the formation of strong gels by means of applying shear when 1 the gelation is occurring in polysaccharide solution12. These samples exhibit intermediate 2 behaviour between weak and strong gels but are closer to the weak gels. 3 The essential applications of fluid gels are their capacity as a potential satiety agent13, as 4 a suspension agent3 and as an emulsion stabilizer14. 5 In this work, fluid gels obtained with low-acyl gellan gum were studied. The native gellan 6 gum is a biopolymer which is commercially produced from Sphingomonas elodea (ATCC 7 31461) by means of microbial fermentation3. Its molecular structure is constituted of a 8 tetrasaccharide repeat unit formed by D-glucose, D-glucuronic and L-rhamnose15. This 9 polymer is able to form strong gels under cooling. However, low-acyl gellan gum 10 solutions require cations to produce strong gels. Low-acyl gellan gum fluid gels are 11 aqueous dispersions based on the subsistence of a network consisting of gelled particles12. 12 As a consequence of its microstructure, it is necessary to subject the gel to a finite shear 13 stress so that the fluid gels can flow at a significant shear rate. This value of shear stress 14 is the so-called yield stress. Below this stress level, the fluid gel behaves like a solid while 15 above it, the fluid gel behaves like a viscous liquid. The determination of the apparent 16 yield stress is of great importance because its value directly influences the gel flow from 17 a container or the pumping of the fluid gel into a transportation pipeline. Moreover, the 18 application of fluid gels as a suspension agent for particulate matter depends on this yield 19 stress value since the suspension will remain stable if the stress exerted by the action of 20 gravity on the particles is less than the yield stress. For this reason, the main goal of this 21 study has been the determination of apparent yield stress of gellan gum fluid gels. 22 It is well documented that the yield stress can be determined using different methods. 23 Traditionally it has been obtained by using theoretical models based on the data obtained 24 from flow curve measurements16. Recently, various studies have shown other protocols 25 to obtain the yield stress17,18 and the thixotropy of fluid gels12,19. Nevertheless, this 1 measurement of yield stress is not easy because of wall-slip artefacts. Wall slip depletion 2 takes place in the flow of materials which consist of two or more liquid phases in 3 rheometers as a consequence of the dispersed phase being displaced from solid 4 boundaries, resulting in a lower viscosity liquid film. This phenomenon is a consequence 5 of steric, viscoelastic, hydrodynamic, gravitational and chemical forces which act over 6 the dispersed phase directly adjoining the walls16. Slip can be shown to occur in 7 rheological measurements when the results using smooth and serrated wall sensors are 8 compared20. Therefore, in the current investigation, in order to determine the apparent 9 yield stress of low-acyl gellan gum fluid gels without these wall-slip errors, firstly the 10 influence of different geometries and degrees of roughness of the surface on the flow 11 curves were studied. Moreover, the effect of the gap between the measuring plates, this 12 is to say, the effect of sample thickness on the flow behaviour was also considered. 13 Secondly, once the geometry and appropriate gap for this fluid gel were determined, the 14 yield stress was obtained using two different methods. On the one hand, from creep tests 15 in which a shear stress was instantaneously applied for a predetermined time, and, on the 16 other hand, from flow curves fitted to the Herschel-Bulkley equation. 17 There are few studies investigating the effect of wall slip phenomena on dynamic 18 viscoelastic behaviour. For this reason, an additional objective of this study consisted of 19 assessing this effect for low acyl gellan gum fluid gels. In order to achieve this purpose, 20 the extension of the linear viscoelastic region by stress sweep measurements at constant 21 frequency was studied as a function of the measuring geometry. Later, frequency sweep 22 tests were carried out within this range so as to ensure that the sample structure would not 23 be damaged by the stress imposed during the measurements. These frequency sweeps 24 were performed with the intention of knowing the influence of the frequency on the 1 storage modulus (G’) and the loss modulus (G’’) as a function of the geometry used. 2 Based on the results obtained in this work, a sensor system with a rough surface and a 3 measuring gap of 1000 m was employed to determine the yield stress by creep tests. Its 4 value was found to be 4.3 Pa. There was an influence of wall slip on the extension of the 5 linear viscoelastic region, but not on the G´ and G´´ of the mechanical spectra. 6 2. Materials and methods 7 2.1. Materials 8 Low Acil gellan (LA gellan), kindly supplied by CP-Kelco ( San Diego, USA), was 9 employed. Fluids gels were obtained containing 0.2 wt % gellan gum. In order to obtain 10 Ca2+ as a gel-promoting ion, CaCl2 (purity > 98%), supplied by Panreac (Barcelona, 11 Spain), was used. The utilized amount of CaCl2 was 0.15 wt%. To preserve the samples, 12 0.1 wt % azide sodium (Panreac) was included in the fluid gel formulation. In addition, 13 deionised water was always utilised (electrolytic conductivity: 2.1 mS/cm; calcium 14 concentration: 69.1 ppb; sodium concentration: 16.6 ppb. Room temperature). 15 16 2.2. Fluid gel preparation 17 Gellan gum fluid gels were obtained by the method suggested by Sworn (2009)3. LA- 18 gellan gum was slowly added to a vessel placed in a water-bath at 80ºC. The sample was 19 submitted to 700 rpm for 25 min in order to achieve hydration. For this purpose, an Ika- 20 Visc MR-D1 homogeniser (Ika, Germany) and a sawtooth-type impeller were employed. 21 The impeller diameter to the vessel diameter ratio was 0.85. The required temperature 22 was exceeded by 10ºC21 to ensure perfect gum hydration. Finally, the required amount of 23 CaCl2 was added and the sample was maintained under mechanical treatment at 80ºC. 24 The quantity of water lost by evaporation was corrected by adding the necessary amounts 1 of water. Gelation was achieved by keeping sample vessel in a water-bath at 20ºC as a 2 coolant. The sample was subjected to mechanical treatment by the Ika-Visc equipment 3 for 1500s to obtain fluid gels. The samples were stored at 4.5ºC for 48 h before performing 4 the rheological study. 5 6 2.3. Methods 7 The rheological tests were performed with an AR-2000 controlled-stress rheometer (TA 8 Instruments, Crawley, United Kingdom). Temperature was set at 20 ºC ± 0.1 ºC with a 9 Peltier system assisted by a Thermo Scientific circulator. Every test was repeated at least 10 3 times with a fresh sample and the results are presented as an average. 11 2.3.1. Determination of the equilibration time 12 In order to avoid influences of the recent mechanical history after loading the sensor 13 system, the rest time necessary to achieve the equilibrium structure was determined. For 14 this purpose, time sweeps tests were carried out. These measurements were performed 15 with a parallel plate with a rough surface and 60 mm diameter (PP60R) with a measuring 16 gap of 1 mm. Since this test must be performed at shear stress within the linear 17 viscoelastic range (LVR), stress sweeps ranging from 8·10-3 to 10 Pa were carried out 18 previously. Once LVR was determined, time sweeps under oscillatory shear tests at a 19 stress of 0.04 Pa and a frequency of 1 Hz for 60 min were run to estimate the equilibration 20 time. The value obtained as equilibration time was 17 min (data not shown). Therefore, a 21 17-min rest phase was incorporated for all the rheological tests. 22 2.3.2. Steady shear flow tests 23 The measurements were carried out by the controlled-stress rheometer AR2000, in the 1 0.5-25 Pa shear stress range by a step-wise procedure, with 2 min at each shear stress to 2 reach the steady-state regime. Previously, fluid gels were maintained in the quiescent state 3 at the measuring position for 17 min to permit stress relaxation after the loading 4 procedure. In order to study the existence of wall slip during flow behaviour, the influence 5 of the geometry and measuring gaps on the flow curves were examined. For this, several 6 sensor systems with various geometries were used, namely two parallel plates systems 7 (60 mm diameter), one of them with serrated surface (PP60R) and the other with smooth 8 surface (PP60L), both with a measuring gap of 1000m, and a cone plate geometry, with 9 measuring gap 32m and angle 1.059º. In the same way, different measuring gaps 10 (500m, 750m and 1000m) were assessed with the serrated sensor system (PP60R). 11 2.3.3. Dynamic viscoelastic measurements 12 In order to study the viscoelastic behaviour, small amplitude oscillatory shear tests were 13 performed. Previously, stress sweeps in a range of 8·10-3 to 10 Pa at 1 Hz were carried 14 out to determine the linear viscoelastic range (LVR). Once the LVR was determined, 15 frequency sweeps from 3 Hz to 0.01 Hz at 0.02Pa were carried out. With the aim of 16 assessing wall-slip of the sample in both tests, several sensor systems with different 17 geometries (PP60R (measuring gap: 1000 µm), PP60L (measuring gap: 1000 µm) y CP60 18 (measuring gap: 32µm and angle: 1.0590) were used. 19 2.3.4. Creep tests 20 In these measurements, a constant stress from 1 to 8 Pa was applied for 120 seconds. The 21 sensor system PP60R with measuring gap of 1000 µm was selected to carry out this test. 22 For the reasons explained above, the sample was maintained at rest in the measuring gap 23 and for 17 minutes (the equilibration time) before beginning the test. 24 3. Results and discussion 1 3.1. Influence of the surface and geometry of the sensor system on the flow curves 2 Figure 1 shows the influence of the sensor geometry (PP60R, PP60L and CP60) on the 3 flow curves. As can be observed in Figure 1a, the samples exhibited a shear thinning 4 behaviour. In addition, the results obtained by the parallel plate with serrated surface 5 (PP60R) showed a very shear thinning behaviour, that is to say, at lower shear stress, the 6 apparent viscosity has a tendency to achieve a Newtonian plateau where there is no 7 dependence of shear stress on the viscosity. Additionally, it is possible to observe a sharp 8 decrease in the viscosity by over five orders of magnitude over a small range of shear 9 stress. The values of shear stress at which this change of behaviour takes place is called 10 the yield stress. Not only does Figure 1a show that the flow curve performed with PP60R 11 sensor has a yield stress but the results obtained with this sensor also exhibited higher 12 values of viscosity than those obtained with the other geometries. This fact can be 13 explained by the existence of wall slip phenomena. This also occurred at higher shear 14 stress values but to a lesser extent. Therefore, it can be stated that there was slip and that 15 this was most pronounced at low stress. Similar results were shown in other studies with 16 different systems22-24. 17 18 Figure 1 19 20 In Figure 1b, shear stress as a function of shear rate is plotted. Just as in Figure 1a, the 21 very shear thinning behaviour, the existence of a yield stress and wall slip effects can be 22 observed. Moreover, it can be appreciated that the yield stress reached a higher value 23 when the rough sensor was utilized. These results support the need to use a rough 24 geometry in order to avoid the effects produced as a consequence of wall slip. 25 The experimental data of the shear stress as a function of shear rate has been fitted to the 1 Herschel-Bulkley model: 2 3 . 0·n k  += Eq. 1 4 Where  is the stress (Pa) as function of the shear rate ( .  ), 0 is the yield stress (Pa), k 5 is the consistence index (Pa·sn) and n is the flow index. 6 The fitting parameters to the Herschel-Bulkley model obtained with the different 7 geometries are shown in the Table 1. As can be observed, the higher values of the yield 8 stress and consistence index were achieved when the rough surface was used. This result 9 was a consequence of the wall-slip effect. 10 11 Table 1 12 13 14 3.2. Influence of the measuring gap on the flow curves 15 For these tests, the PP60R sensor system was utilized with different measuring gaps: 500, 16 750 and 1000m. 17 Figure 2a shows that the viscosity was not affected by the measuring gap. Therefore, it 18 can be stated that between 500 and 1000 microns, the measuring gap did not produce a 19 wall-slip effect in the gellan gum fluid gels. The same information can be deduced if 20 Figure 2b is analyzed. This result was supported by the fitting parameter to the Herschel- 21 Bulkley model (Table 2). 22 23 Figure 2 24 15. Lapasin, S. Pricl, Rheology of Industrial Polysaccharides. Theory and Applications, 1 Blackie Academic and Professional, London, 1995. 2 16. Barnes H.A. J. Non-Newtonian Fluid Mech. 1995, 56, 221-251. 3 17. García M.C., Alfaro M.C., Muñoz J. Biochem. Eng. J. 2016, 114, 257-261. 4 18. García M.C., Alfaro M.C., Muñoz J. J. Food Eng. 2015,159, 42-47. 5 19. Barnes H.A. J. Non-Newtonian Fluid Mech. 1997, 70, 1-33. 6 20. Wesche K., Flaatten H., Berg, W. vom. In Fresh Concrete, Important Properties and 7 their Measurement 2, ed. by Neville AM, Slater D, RILEM, Leeds, England, 1973; pp: 1 8 -11. 9 21. Valli R.C, and Miskiel F.J. In Handbook of Dietary Fiber Handbook of Dietary Fiber. 10 The Food Science and Technology Series. Marcel Dekker Inc., San Diego, California, 11 New York; Vol 113., 2001; pp. 695-720 . 12 22. Caufin B., Papo A., Zem.-Kalk-Gips 1984, 656- 661. 13 23- Coussot P., Piau J.M. J. Rheol. 1995, 39, 105-124. 14 24. Buscall R., McGowan J.I., Morton-Jones A.J., J. Rheol. 1993, 37, 621-641. 15 25. García M.C., Alfaro M.C., Muñoz J. Food Science and Technology International 16 2015, 22, 325-332. 17 26. Alfaro M.C., Guerrero A.F., Muñoz J. Langmuir 2000, 16, 4711-7719. 18 27. Saak A.W., Jennings H.M., Shah S.O. Cement and Concrete Research 2001, 31, 205- 19 212. 20 28. Muñoz J., Alfaro M.C., García M.C., Claro J.G., Pérez L.M., Calero N. In product 21 design and engineering, Guerrero A., Muñoz J., Franco J.M. Eds, Grupo Español de 22 Reología. Real Sociedad Española de Química. Sevilla, 2008, pp. 17- 20. 23 24 Figure captions 25 26 Figure 1. Influence of the sensor system geometry and surface on the flow curves of fluid 27 gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a)Viscosity vs. shear 28 stress, b) Shear stress vs. shear rate. Temperature: 20ºC. 29 Figure 2. Influence of the measuring gap on the flow curves of fluid gels containing 0.2 1 wt % low acyl gellan gum and 0.15 wt % CaCl2. a)Viscosity vs. shear stress, b) Shear 2 stress vs. shear rate. Sensor PP60R. Temperature: 20ºC. 3 Figure 3. Influence of the sensor system geometry and surface on the determination of 4 the linear viscoelastic region at 1 Hz of the fluid gels containing 0.2 wt % low acyl gellan 5 gum and 0.15 wt% CaCl2. Temperature: 20ºC. 6 Figure 4. Influence of the sensor system geometry and surface on the mechanical spectra 7 of the fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. 8 Temperature: 20ºC. 9 Figure 5. Capacitance (J) as a function of the shear time in creep tests of fluid gels 10 containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a) At 1 Pa. b) At 4 Pa 11 Temperature: 20ºC. 12 Figure 6. Capacitance (J) as a function of the shear time in creep tests of fluid gels 13 containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. Average of three replicates 14 and standard deviation. Temperature: 20ºC. 15 Figure 7. Capacitance (J) as a function of the shear time in creep tests at 8 Pa of fluid gels 16 containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. Temperature: 20ºC. 17 Figure 8. Flow curve obtained from creep measurement results of fluid gels containing 18 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a) Viscosity vs. shear rate, b) 19 Viscosity vs. shear stress. Temperature: 20ºC. 20 Tables. 21 Table 1. Influence of the sensor system geometry and surface on the fitting 22 parameters to the Herschel-Bulkley model. Temperature: 20ºC. 23 24 25 26 27 28 29 30 31 32 Sensor system τ0±SD (Pa) k±SD (Pa·sn) n±SD PP60R 3.10±0.27 0.71±0.21 0.43±0.05 CP60 0.47±0.12 0.66±0.07 0.41±0.02 PP60L 1.68±0,26 0.33±0.08 0.53±0.036 Table 2. Influence of the measuring gap on the fitting parameters to the Herschel- 1 Bulkley model. Temperature: 200C. 2 3 Measuring gap (m) τ±SD (Pa) k±SD (Pa·sn) n±SD 500 3.66±0.29 0.49±0.06 0.47±0.03 750 3.30±0.32 0.58±0.24 0.45±0.07 1000 3.11±0.27 0.70±0.21 0.43±0.05 4 5 Table 3. Influence of the sensor system geometry and surface on the determination 6 of the linear viscoelastic region at 1 Hz. Temperature: 200C. 7 8 Sensor system Replicados τc(Pa)  c PP60R Mean value 0.15 0.005 SD 0.02 4E-04 CP60 Media 0,03 8E-04 SD 0,01 1E-04 PP60L Media 0,08 2E-03 SD 0,02 4E-04 9 Figure 1a Figure 1b Figure 1. Influence of the sensor system geometry and surface on the flow curves of fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a)Viscosity vs. shear stress, b) Shear stress vs. shear rate. Temperature: 20ºC. Figure 2a Figure 2b Figure 2. Influence of the measuring gap on the flow curves of fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a)Viscosity vs. shear stress, b) Shear stress vs. shear rate. Sensor PP60R. Temperature: 20ºC. Figure 3. Influence of the sensor system geometry and surface on the determination of the linear viscoelastic region at 1 Hz of the fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt% CaCl2. Temperature: 20ºC. Figure 4. Influence of the sensor system geometry and surface on the mechanical spectra of the fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. Temperature: 20ºC. Figure 5a Figure 5b Figure 5. Capacitance (J) as a function of the shear time in creep tests of fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. a) At 1 Pa. b) At 4 Pa Temperature: 20ºC. Figure 6. Capacitance (J) as a function of the shear time in creep tests of fluid gels containing 0.2 wt % low acyl gellan gum and 0.15 wt % CaCl2. Average of three replicates and standard deviation. Temperature: 20ºC.