Articles

Effects of Congo Red on the Drag Reduction Properties ofPoly(ethylene oxide) in Aqueous Solution Based on Drop

Impact Images

Melissa I. Alkschbirs, Vanessa C. Bizotto, Marcelo G. de Oliveira, andEdvaldo Sabadini*

Instituto de Quımica, Universidade Estadual de Campinas, Caixa Postal 6154, CEP13084-862, Campinas, SP, Brazil

Received May 4, 2004. In Final Form: August 9, 2004

The presence of very small amounts (ppm) of high-MW polymers in solution produces high levels of dragreduction in a turbulent flow. This phenomenon, often termed as the Toms effect, is highly dependent notonly on MW, but also on the flexibility of the macromolecular chain. The Toms effect can be studied throughthe images of the structures produced after the drop impact against shallow solution surfaces. The splashstructures composed of crown, cavity, and Rayleigh jet are highly dependent on the elongational propertiesof the solution. This work presents the effects of Congo red on the drag reduction properties of poly(ethyleneoxide) in aqueous solutions through the analysis of splash structures. Results obtained in this analysisindicate that Congo red molecules act as physical cross-linking agents, decreasing the polymer elasticityand its drag reduction capacity. It was observed that the maximum height of the Rayleigh jet can be usedas a sensitive parameter to the complexation between the dye and the polymer molecules.

Introduction

Frictional drag results in dissipation of energy, and, formany years, scientists and technologists have attemptedto devise methods to minimize this effect. In 1946, B. A.Toms found that a very dilute high-MW polymeric solutionin a turbulent flow required a lower pipe flow-pressuregradient than the pure solvent to produce the same flowrate.1 The phenomenon, often termed “Toms effect”, hasbecome of considerable engineering interest, mainly inpumping processes.2-5 Substances such as syntheticpolymers, biopolymers, or aggregates of surfactants canproduce the phenomenon described above.2,3,6,7 However,poly(ethylene oxide) (PEO), a flexible polymer, is the mosteffective drag reducing agent in aqueous systems.3

The drag reduction (DR) phenomenon is necessarilycomplex, as both turbulence effects and the extremelydilute nature of the solutions involved have to be takeninto account,2,8 and an understanding of the role ofpolymers in DR processes at the molecular level is still

primitive.9 One theory assumes that the added macro-molecules, under high shear, undergo a dynamic chainelongation, absorbing the energy of the eddies in the flow.This energy is then dissipated as elastic shear waves.10-12

Therefore, the DR phenomenon depends strongly on theintrinsic polymer flexibility.

Chain elongation occurs when the shear rate in aturbulent flow is greater than the reciprocal of themolecular relaxation time, 1/τ.12,13 The relaxation timefor a polymer in solution can be estimated from the Rouse14

and Zimm15 theories, in which both the MW and theconcentration of a polymer (C) are contributing factors,as shown in eq 1:

where Mv, ηsp, η0, R, T, and λi are the viscosity-averagedMW, the specific viscosity, the solvent viscosity, the gasconstant, the absolute temperature, and the eigenvaluesof Zimm theory, respectively. Each λi is associated withone specific linear normal mode due to the cooperativemotion of the polymer segments. A range of relaxationtime is possible, but the longest τ, where λ ) 1, is the mostimportant for DR.10-12

Experimentally, the studies involving this phenomenonaregenerallyperformedduringpipe flow.Recently, studies

* Corresponding author. E-mail: sabadini@iqm.unicamp.br.(1) Virk, P. S.; Merrill, E. W.; Mickley, H. S.; Smith, K. A.; Mollo-

Christensen, E. L. J. Fluid Mech. 1967, 30, 305.(2) McCormick, C. L.; Hester, R. D.; Morgan, S. E.; Safieddine, A. M.

Macromolecules 1990, 23, 2132.(3) Bailey, F. E., Koleske, V. J., Eds. Poly(ethylene oxide); Academic

Press: New York, 1976.(4) Figueredo, R. C. R; Sabadini, E. Collids Surf., A: Physicochem.

Eng. Asp. 2003, 215, 77.(5) Kulicke, W. M.; Gragem, H.; Kotter, M. Drag reduction phenom-

enon with special emphasis on homogeneous polymer solutions - PolymerCharacterization/Polymer Solutions; Springer-Verlag: Berlin, 1989.

(6) Sellin, R. H. J.; Hoyt, J. W.; Scrivener, O. J. Hydraul. Res. 1982,20, 29.

(7) Lin Z. Q.; Lu, B.; Zakin, J. L.; Talmon, Y.; Zheng, Y.; Davis, H.T.; Scriven, L. E. J. Colloid Interface Sci. 2001, 239, 543.

(8) Bonn, D.; Couder, Y.; van Dam, P. H.; Douady, S. Phys. Rev. E1993, 47, 28.

(9) Kim, O.-K.; Choi, L.-S.; Long, T.; Yoon, T. H. Polym. Commun.1988, 29, 168.

(10) Lumley, J. L. Appl. Mech. 1967, 20, 1139.(11) Peterlin, A. Nature 1970, 227, 598.(12) Kim, O.-K.; Choi, L. S.; Long, T.; McGrath, K.; Armistead, J. P.;

Yoon, T. H. Macromolecules 1993, 26, 379.(13) Shenoy, A. V. Colloids Polym. Sci. 1984, 262, 319.(14) Rouse, P. E. J. Chem. Phys. 1953, 21, 1272.(15) Zimm, B. H. J. Chem. Phys. 1956, 24, 269.

τ )Mv(ηsp/C)η0

0.586RTλi(1)

11315Langmuir 2004, 20, 11315-11320

10.1021/la0489007 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/25/2004

of DR have been focused on the visualization of splashstructures produced by the impact of a droplet against ashallow aqueous PEO solution.16,17 Splash is a term usedfor the formation of a crownlike structure, produced bythe impact of a droplet on a liquid surface (the targetliquid).18,19 In the milliseconds after droplet impact, thegravitational potential energy of the droplet provokes theformation of a “crown” on the liquid-air surface, and acavity in the target liquid. During the splash evolution,both the collapse of the crown and the closing of the cavityforce the liquid upward, forming a column called theRayleigh jet.20-22 The amplitude of the Rayleigh jetdepends on the elongational viscosity of the solution andcan be used to estimate the stored impact energy.17

We have already shown that the structures of the splashare sensitive to PEO MW, concentration, and chainflexibility.23 Splash investigations on the restrictions inPEO chain elongation induced by intramolecular chaincomplexation may add new information on this phenom-enon. The addition of very small quantities of Congo Red,CR (Figure 1), to a PEO aqueous solution greatly affectsthe polymer chain elongation, and therefore DR. A dipole-type cross-coupling formed between the two -NH2-groups of CR and the oxygen atoms of the PEO unitsproduces a restriction in the free stretching of the polymerchain.24

This work presents the effects of CR on the DR propertiesof PEO in aqueous solutions, using “frozen images” of thesplash captured with a high-speed CCD camera. Thecomplex formed between CR and PEO was also investi-gated using electronic fluorescence spectroscopy.

Experimental SectionMaterials and Methods. Aqueous PEO solutions were

prepared by an adaptation of the procedure suggested by Littleand Wiegard.25 Briefly, samples of PEO 1, 6, 9, 20, and 40 × 105

gmol-1 (Aldrich), weighted within (1 mg, were sprinkled overa large area of water to avoid clumping of the particles. At 3 hintervals, the solutions were gently stirred using a glass rod, toavoid polymer degradation, and this procedure was also repeatedon the following day. The final solutions were then prepared involumetric flasks. For experiments involving polymeric solutions,the same concentration of PEO was used in both the droplet andthe target solutions. Congo red (Carlo Erba) was used as received.All of the experiments were carried out using analytical gradewater from a Millipore Milli-Q Gradient filtration system.

Apparatus Used To Produce the Splash. A completedescription of the experimental procedure used to produce andto capture controlled splash images was described elsewhere.23

Droplets (with mass ) 25 ( 2 mg) were released from 184 cmabove the surface of the target liquid. The liquid film thickness(0.31 cm) was adjusted in each experiment. The drop impact

images were captured using a Sony CCD DXC-9000 camera, inwhich a shutter speed of 1 × 10-4 s was used, set at 30 framess-1. The captured images were recorded on a tape recorder(Panasonic S-VHS Ag-1980), interfaced with a computer, con-taining a frame grabber board (Media Cybernetic). Morphologicalparameters of the impact structures were obtained using thePro-Plus 3.0 software for image treatment. Averages of themorphologic parameters were obtained from 30 “frozen images”of each splash, and the error bars in the experimental datacorrespond to one standard deviation.

Electronic Spectra. UV-vis spectra of CR aqueous solutionwith and without PEO were obtained in the range from 200 to600 nm using a UV/vis spectrometer (HP8453). Fluorescencespectra in the range from 500 to 700 nm were obtained using aluminescence spectrophotometer LS55, Perkin-Elmer, withexcitation wavelength ) 500 nm.

Viscosity, Surface Tension, and Density Measurements.The shear viscosity of the solutions was measured using anOstwald viscometer-50. Surface tension measurements weretaken using a tensiometer (Sigma 701 System Unit) and theWilhelmy plate method. The density of the solutions wasmeasured using a density meter (Anton Paar, DMA 58). All ofthe experiments were conducted at 25 °C.

Results and DiscussionSplash Morphology. A three-stage schematic repre-

sentation of the structures produced after a drop impactagainst a shallow surface is shown Figure 2. The dropimpact energy (Ed) can be determined from eq 2:

where m and Rd are the mass and the radius of the drop,respectively, σ is the surface tension, and h is the heightin which the drop is released. In the experimentalconditions used in this work, Ed ) 4.5 × 10-4 J. The dropimpact produces a cavity and a crownlike structure, andan estimate of the energies of these structures is describedin ref 23. Figure 3 shows a comparison among typicalcrowns (filmed from side and inclined views) obtained forpure water (a), aqueous PEO 4 × 106 g mol-1 solution (40ppm) (b), and aqueous PEO 4 × 106 g mol-1 (40 ppm) plus8 ppm of CR (c). It can be seen that secondary jets producedat the top of the crown are smoother and less fragmentedfor both PEO aqueous solutions in comparison with thoseformed in water. However, in the presence of CR, thesemorphologic characteristics are less pronounced. Second-ary droplets are produced when the jets at the top of thecrown reach the maximum amplitude. At this point, thecapillary centripetal force overcomes the inertial force,creating capillary waves which travel back to the centerof the jet, creating droplets along the jet (Figure 3a).26 In

(16) Sabadini, S.; Alkschbirs, M. I. Journal Visualization 2001, 4,217.

(17) Sabadini, E.; Alkschbirs, M. I. Exp. Fluids 2002, 33, 242.(18) Worthingon, A. M. Proc. R. Soc. 1882, 1188, 217.(19) Cossali, G. E.; Coghe, A.; Marengo, M. Exp. Fluids 1997, 22,

463.(20) Hobbs, P. V.; Kezweeny, A. J. Science 1967, 155, 1112.(21) Hobbs, P. V.; Osheroff, T. Science 1967, 158, 1184.(22) Macklin, W. C.; Hobbs, P. V. Science 1969, 166, 107.(23) Sabadini, E.; Alkschbirs, M. I. J. Phys. Chem. B 2004, 108, 1183.(24) Inge, C.; Johansson, A. V.; Lindgren, E. R. Phys. Fluids 1979,

22, 824.(25) Little, R. C.; Wiegard, M. J Appl. Polym. Sci. 1970, 14, 409.

(26) Mourougou-Candoni, N.; Prunet-Foch, B.; Legay, F.; Vignes-Aadler, M.; Wong, K. J. Colloid Interface Sci. 1997, 192, 129.

Figure 1. Molecular structure of Congo red.

Figure 2. Schematic representation of a drop impact againsta shallow liquid surface: (a) just before the impact, (b) thecrown and cavity formed after the drop impact, and (c) theRayleigh jet impelled by the collapse of the crown and closingof the cavity.

Ed ) mgh + mgRd + 4πRd2σ (2)

11316 Langmuir, Vol. 20, No. 26, 2004 Alkschbirs et al.

the case of the PEO solutions, the stretching of thepolymeric chains produces secondary droplets linked bya thick liquid filament (Figure 3b). The interaction of CRwith PEO molecules reduces the chain elongation, in-creasing the fragmentation of the jets (Figure 3c).

The collapse of the crown and the cavity was observedto start approximately 20 ms after the drop impact23 andforce a liquid column, known as Rayleigh jet, against thegravitational force. The maximum height of the jet isstrongly dependent on the presence of PEO and is affectedby the presence of CR (Figure 4).

The splash, and mainly the Rayleigh jet, is dependenton the surface tension, shear viscosity, and the elonga-tional properties of the solution. As observed in Table 1,the values of surface tension and shear viscosity of theaqueous PEO solutions are just slightly affected by the

presence of Congo red. This means that the Rayleigh jetis dominated by the elongational viscosity of the solutions.

The present interpretation of Rayleigh jet elasticity isfundamentally based only on bulk solution effects. How-ever, the dilatational effects of the surface, due to thePEO molecules adsorbed at the solution-air surface,should also be considered. Typically, the presence of PEO(in the concentration range used) reduced the surfacetension by approximately 10 mN m-1 relative to purewater, and this value is practically independent of CRconcentration (Table 1). The splash structures are pro-duced so fast that it is possible to assume that there is nochange in PEO concentration at the solution-air surface.It is expected that there will be no changes in the PEOexcess surface concentration due to the increase in thesolution-air surface area during the splash. The short

Figure 3. Representative “frozen” pictures (side and inclined views) of crowns formed in the impact of a drop against a liquid targetsurface for: (a) pure water, (b) 40 ppm of aqueous PEO, and (c) 40 ppm of aqueous PEO solution with 8 ppm of CR. PEO MW )4 × 106 g mol-1.

Effects of Congo Red on the Drag Reduction of PEO Langmuir, Vol. 20, No. 26, 2004 11317

lifetime of the splash (some milliseconds) is not enoughfor the PEO molecules to diffuse from the bulk to thesolution-air surface. Therefore, we have assumed thatthe dynamic surface tension is not important.

Although the understanding of splash is complex, ithas been demonstrated that the height of the Rayleigh jetdepends, up to certain limits, on the drop impact energystored in the process. A linear relationship between theheight of the Rayleigh jet of the solvent (Hjs) and thepolymeric solution (Hjp) allows an estimation of thepercentage of DR, according to eq 3:17

Molecular Considerations. Studies of drop impactof diluted solution of flexible polymers against dry surfacesprovide significant insight into the fluid dynamics andthe stretching-contraction motion of the polymerchain.27-29 In the present case, we have estimated elonga-tion rates (obtained from the diameter of the cavity duringits formation) higher than 3500 s-1.23 This rate is higherthan the onset rate in which PEO chains start to stretch.27

During the crown and cavity collapses, similar rates aredeveloped.23 The PEO chains stretch again within anapproximately uniaxial flow regime produced during theRayleigh jet formation. The high local hydrodynamicviscosity experienced by water molecules around thestretched PEO chains reduces not only the probability ofeddies being created but also dissipates the energies ofeddies, resulting in high levels of DR.

The mixing degree of droplet and target liquids hasalready been determined from experiments using inkeddroplets. The average area was measured from images ofthe ink mark taken immediately after the movement ofthe liquid has stopped. The extent of mixing of the dropand target liquid during the turbulent drop impact forpure water was observed to be almost twice that for anaqueous PEO solution. The global deformation of theprimary drop and target liquids is visually higher in thecase of PEO solutions. This means that, for water, asignificant fraction of the impact energy is dissipatedduring the mixing of both liquids, resulting in a loweramplitude of the Rayleigh jet.16

The interaction between PEO and CR involves theformation of hydrogen bonds between EO units and NH2groups of the dye, and the position of the -SO3

- groups

(27) Bergeron, V.; Bonn, D.; Jean-Yves, M.; Vovelle, L. Nature 2000,405, 772.

(28) Cooper-White, J. J.; Crooks, R. C.; Boger, D. V. Colloids Surf.,A 2002, 210, 105.

(29) Richard, D.; Clanet, C.; Quere, D. Nature 2002, 471, 811.

Figure 4. Comparison of some representative pictures of the Rayleigh jet for: (a) pure water, (b) 40 ppm of aqueous PEO solution,(c) 40 ppm of aqueous PEO solution containing 8 ppm of CR. PEO MW ) 4 × 106 g mol-1.

Table 1. Surface Tension and Shear Viscosity for 40 ppmof Aqueous Solutions of PEO of Different Molecular

Weights with and without CR

PEO MW/105 g mol-1

CR concentration/ppm

surface tension/mN m-1

shear viscosity/cP

1 0 62.70 0.9168 64.00 0.891

6 0 62.56 0.9088 66.33 0.908

9 0 62.79 0.9148 66.02 0.910

20 0 62.76 0.9368 66.12 0.921

40 0 62.58 0.9141 66.13 0.8954 66.15 0.9098 64.65 0.922

12 60.70 0.91416 64.15 0.91920 65.10 0.92240 65.32 0.91660 65.75 0.92580 66.71 0.955

100 67.03 0.947

%DR ) (1 -Hjs

Hjp)100 (3)

11318 Langmuir, Vol. 20, No. 26, 2004 Alkschbirs et al.

was already shown to have an important role in theintensity of this interaction.30

In very diluted solution of polymer and dye, it is moreprobable that the two NH2 groups of a CR molecule interactwith two EO units in different parts of the PEO molecule.Therefore, the CR molecules act as physical cross-linkingagents, reducing the elasticity of the PEO chain underflow. The rupture of such interactions has been demon-strated under energetic flow.24

Fluorescence measurements were carried out using theluminescent properties of CR to investigate the interactionbetween PEO and CR molecules. As shown in Figure 5,the intensity of the CR band at ca. 614 nm is clearly higherin PEO solution than in pure aqueous solution (in bothcases, the CR concentration was kept at 8 ppm). The insetin this figure shows the fluorescence intensity measuredat 614 nm for CR in 40 ppm of PEO solution as comparedto CR in water as a function of CR concentration.

Figure 6 shows the concentration effect of CR in themaximum amplitude of the Rayleigh jet, in which theconcentration of PEO (4 × 106 g mol-1) was kept constantat 40 ppm. In the range of CR concentration between 0and 12 ppm, there is a sharp decrease in the height of thejet, and consequently in the capacity of PEO to act as adrag reducing agent. Beyond this point, a plateau isobserved. For comparison, the results of CR fluorescencewere plotted in the same figure. The ratio between the

fluorescence intensities for CR with (FIPEO+CR) and withoutPEO (FICR) as a function of CR concentration is correlatedwith the results of the Rayleigh jet height. The minimumin the height of Rayleigh jets corresponds to a maximumin the intensity of CR fluorescence ratio. This means thatwhen CR molecules complexes with some EO units, notonly the elasticity of PEO but also the translational androtational freedom-degrees of CR molecules decrease.Therefore, the nonradiative decay is reduced, favoringthe luminescent process. An increase in the intensity ofCR fluorescence was also observed when CR is in thepresence of a peptide solution, and this is attributed tothe formation of a self-assembled complex between CRand the helical peptide.31

The capacity of PEO to act as a drag reducing agentdecreases from approximately 70% to 50% when ∼10 ppmof CR is present. In this concentration, the ratio of CRmolecule/PEO units is low; however, it is enough to changesignificantly the elasticity of PEO. A decrease in therelaxation time of PEO chains was observed by Bermanet al. when CR is present in aqueous PEO solution. Thismeans that the polymer chains become stiffer and morecompact. However, when the stress is high enough, a coil-stretching transition occurs.32

We have carried out an experiment with higher con-centrations of CR to investigate the intermolecularinteraction between complexed PEO molecules coupledby CR (Figure 7). A minimum in the height of the Rayleighjet is achieved when CR concentration is in the range of10-20 ppm. Beyond this range, the height of the jetincreases slightly. This result is in agreement with theDR results observed by Berman et al. for experimentsdeveloped in a pipe flow rheometer.30 The increase in thesize of the complex PEO-CR-PEO due to the intermo-lecular complexation overcomes the decrease in the PEOchain flexibility observed for CR concentrations below 12-20 ppm, resulting in a slight increase in the Rayleigh jet.The CR effect on the elasticity of PEO with different MWwas also investigated (Figure 8). For this study, theconcentrations of PEO and CR were fixed at 40 and 8ppm, respectively. For comparison, the height of the jetwith and without CR is presented. The height of the jetfor the PEO-CR system is almost the same for PEO withdifferent molecular weights. The height of the Rayleighjet cannot be sensitive enough to differentiate the com-plexation of PEO with different molecular weights. Theshear viscosity of the studied solutions (inset in Figure 8)

(30) Berman, N. S.; Berger, R. B.; Leis, J. R. J. Rheol. 1980, 24, 571.(31) Cooper, T. M.; Stone, M. O. Langmuir 1998, 14, 6662.(32) Lee, J. K.; Berman, N. S. J. Rheol. 1996, 40, 1025.

Figure 5. Comparison between the fluorescence spectra of CRin pure aqueous solution (bottom) and in aqueous solution inthe presence of 40 ppm of PEO (top). CR concentration ) 8 ppmin both cases. Inset: Dependence of the maximum intensity ofCR fluorescence in water and in 40 ppm of PEO on the CRconcentration. The curves are drawn as a guide to the eye.

Figure 6. Dependence of the maximum height of the Rayleighjet and of the ratio of CR fluorescence intensity in aqueousPEO solution (FIPEO+CR) to pure aqueous solution (FICR) on theCR concentration (FI measured at 614 nm). The dashed curvesare drawn as a guide to the eye.

Figure 7. Dependence of the maximum height of the Rayleighjet in aqueous solutions containing 40 ppm of PEO and CR onthe CR concentration. The dashed curve is drawn as a guideto the eye.

Effects of Congo Red on the Drag Reduction of PEO Langmuir, Vol. 20, No. 26, 2004 11319

is lower for the PEO-CR solution in comparison to thepure aqueous solution of PEO, and the effect is moreintense for PEO with high MW. Therefore, it is possiblethat the elasticity of the PEO-CR is almost the same, inthis PEO MW range.

Conclusions

The elasticity of PEO chains is reduced by the intramo-lecular complexation between the EO units and bothextremities of CR. In this case, CR acts as a cross-linkingagent. The effects of the complexation can be clearlyobserved by the changes in the morphology of thestructures developed during the short lifetime of thesplash. The typical smooth and long secondary jetsproduced at the top of the crown for PEO aqueous solutionbecome more fragmented when CR is added to the polymersolution. The Rayleigh jet is sensitive to the CR com-plexation, revealing the decrease of PEO capacity to actas a drag reducing agent, and the effect is more pronouncedwith high MW PEO. The complexation between PEO unitsand CR is also observed in CR fluorescence experiments,revealing a good agreement with the results of the dropimpact experiments.

Acknowledgment. We would like to thank CNPq andFAPESP for financial support. M.I.A. and V.C.B. heldgraduate fellowships from Fundacao de Amparo a Pes-quisa do Estado de Sao Paulo, FAPESP, and Coordenacaode Aperfeicoamento de Pessoal de Nıvel Superior, CAPES,respectively.

LA0489007

Figure 8. Height of the Rayleigh jet for 40 ppm of aqueousPEO solution (O) and 40 ppm of aqueous PEO solutioncontaining 8 ppm of CR (9) as a function of PEO MW. Inset:Dependence of the shear viscosity of the solutions as a functionof PEO MW. The dashed curves are drawn as a guide to the eye.

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