Scheid_The Role of Surface Rheology in Liquid Film Formation

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The role of surface rheology in liquid film formation

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2010 EPL 90 24002

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April 2010

EPL, 90 (2010) 24002 www.epljournal.orgdoi: 10.1209/0295-5075/90/24002


B. Scheid1,2(a), J. Delacotte3, B. Dollet4, E. Rio3, F. Restagno3, E. A. van Nierop1, I. Cantat4,

D. Langevin3 and H. A. Stone1,5

1 School of Engineering and Applied Sciences, Harvard University - Cambridge, MA 02138, USA2 TIPs - Fluid Physics unit, Universite Libre de Bruxelles - C.P. 165/67, 1050 Brussels, Belgium, EU3 Laboratoire de Physique des Solides UMR8502, Universite Paris-Sud - 91405 Orsay, France, EU4 Institut de Physique de Rennes UMR6251, Universite de Rennes - 35042 Rennes, France, EU5 Department of Mechanical and Aerospace Engineering, Princeton University - Princeton, NJ 08544, USA

received 26 March 2010; accepted 19 April 2010published online 6 May 2010

PACS 47.15.gm Thin film flowsPACS 47.85.mb Coating flowsPACS 68.03.-g Gas-liquid and vacuum-liquid interfaces

Abstract The role of surface rheology in fundamental fluid dynamical systems, such as liquidcoating flows and soap film formation, is poorly understood. We investigate the role of surfaceviscosity in the classical film-coating problem. We propose a theoretical model that predicts filmthickening based on a purely surface-viscous theory. The theory is supported by a set of newexperimental data that demonstrates slight thickening even at very high surfactant concentrationsfor which Marangoni effects are irrelevant. The model and experiments represent a new regimethat has not been identified before.

Copyright c EPLA, 2010

Introduction. Although the concepts of surfaceviscosity and elasticity date back to Plateau in the19th century, they have only recently been formalizedmathematically and subsequently been used in quanti-tative descriptions of surface flows (see [1] for a historicreview). The concepts of surface viscosity and elastic-ity (Marangoni) effects are intimately related, and arenot always distinguishable in an experimental setting.Even in some of the simplest situations, there is debateover the role of surface rheology. One such configurationis the Landau-Levich-Derjaguin (LLD) dip-coating flow,

wherein a film of wetting liquid is deposited onto a solidsubstrate as it is withdrawn from a bath. This process isso fundamental to coating flows that it is ubiquitous intodays coating technologies. The aim is always to deposita film with a desired thickness, which is achieved bycontrolling the substrate withdrawal speed and the (bulkand surface) rheology of the liquid. The film thickness h0in a dip-coating process at speed u0 usually follows theLLD-like law,

h0c

= 0.9458 Ca2/3, (1)

where c =/(g) is the capillary length and Ca =u0/ the capillary number, with the dynamic viscosity,(a)E-mail: [email protected]

the density, the surface tension, and g the gravitationalacceleration. Here is a thickening factor that is tied tosurface rheology and is a maximum of 42/3 for immobileinterfaces (in the frame of the film) [2].

There are open questions for flows where Marangonieffects are weak or are not expected to play a role. Inparticular, it is commonly assumed that, as for pureliquids, there is no thickening (i.e., = 1) for liquidscontaining large amounts of soluble surfactants, suchthat the substrate withdrawal does not induce Marangonistresses. However, in some cases thickening does occur

(e.g., [3]), again raising the prospect of a role for surfacerheology. In this letter we show experimentally and theo-retically that thickening in the absence of Marangonieffects can be explained by surface viscosity. Our work hasrelevance for dip-coating flows, as well as soap films forma-tion and other flows involving fluid-fluid interfaces withsurface-active materials. We first discuss the role of surfac-tant concentration on surface rheology, followed by thedevelopment of our model and a discussion of its resultsas compared to new experimental data.

Role of surfactant concentration. At high surfac-

tant concentration, exchanges between surface and volumeare fast and can suppress the Marangoni effect that iscaused by surface elasticity, thus rendering the interface

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B. Scheid et al.

more mobile. This phenomenon, referred to as surfaceremobilization [4], requires two conditions to occur thatare both fostered at high bulk surfactant concentration.First, the surfactant exchange rate between the interfaceand the sublayer has to be fast in comparison to the dilata-

tion/compression rate of the interface, e.g., [5] gives thedependence of surface elasticity with the rate of compres-sion. Second, the film should be thick enough such thatit contains enough surfactants to replenish the interface.Sonin et al. [6] showed that the drainage of very thin filmsis controlled by Marangoni effects and proceeds as if thesurfactants were insoluble, even at high bulk concentra-tion. This phenomenon was predicted earlier by Lucassen-Reynders and Lucassen [7], who proposed expressions forthe decrease of the effective film elastic modulus withincreasing film thickness.

In their study of fiber coating, Quere and de Ryck [3]

defined a parameter = /(c h0), which measures thecapacity of the bulk reservoir to replenish the inter-face with surfactant, where is the surface concentrationand c the bulk concentration. Using the soluble surfac-tant DTAB (dodecyl trimethyl ammonium bromide) athigh concentration c = 5 cmc (cmc= 15mM), they iden-tified a dynamical transition of thickening by increas-ing the substrate speed (i.e. Ca), hence the film thickness,separating a region of large thickening ( 1.9) before thetransition, corresponding to > 0.1, from a region of smallthickening ( 1.15) beyond the transition, correspondingto < 0.01. This transition was shifted toward smaller Cawhen increasing the concentration to c = 25 cmc (see fig. 61in [3]). For both concentrations used by these authors, thethickening factor beyond the transition remained unex-pectedly larger than unity, and the thickening mechanismfor such thicker films has not been explained as of yet.As mentioned earlier, surface elasticity, responsible forthickening at lower Ca, should not play a significant roleafter the transition because of surface remobilization. Wedemonstrate here that thickening beyond the transitioncan instead be rationalized by surface-viscous effects.

Model. Consider a flat plate that is withdrawn froma bath of liquid and entrains a film of thickness h(x) that

eventually approaches a constant value h0 at distances farabove the static meniscus, as sketched in fig. 1. For a pureNewtonian fluid, this thickness is given by (1) with = 1and the region that connects the static meniscus withthe flat film, called the dynamic meniscus, has a typicallength of Ca = h0 Ca

1/3. Here we investigate how thethickening factor is affected by surface viscosity, denoted. As in the usual LLD problem the film is thin enoughin the dynamic meniscus region such that lubricationtheory applies, i.e. h0/ 1. We further neglect gravityin the dynamic meniscus as compared to viscous stress,i.e. g u0/h20. Under these assumptions, the velocity

profile tends toward a uniform flow as x and thepressure relative to the ambient pressure remains constantacross the film, i.e. p =xxh. The axial-force balance

h0

u0

g

us(x)h(x)

x

y

air

liquid

Fig. 1: Sketch of the dip-coating problem with surfactants athigh concentration; h(x) is the film thickness and us(x) thesurface velocity in the dynamic meniscus of length .

has the formyyu + xxxh = 0. (2)

The axial velocity u(x, y) is therefore parabolic in y and

using u|y=0 = u0, u|y=h = us(x) andh0

u(x, y) dy = h0u0,the velocity in the film can be expressed as

u

u0

= 1

1

usu0

y

h

3h 2h0

h+

usu0

y

h 1y

h,(3)

where us(x) is the unknown surface velocity. The systemis closed by the tangential stress balance at the interface(see, e.g., [8]):

yuy=h

= xxus. (4)

Substituting (3) into both (2) (evaluated at y = h(x)) and(4), and introducing the dimensionless variables H= h/h0,U= us/u0, and X= x/, leads to

H =12

H3 6 + 6U

H2, (5a)

U = 6H2

+2 + 4U

H, (5b)

where a prime denotes the X-derivative and hasbeen set to Ca such that = Bq Ca2/3 c/h0 is the soleindependent parameter, and Bq = /(c) is the Boussi-nesq number. The system (5) is solved with the followingboundary conditions: H, U 1 and H, H, U 0 asX. Furthermore, the curvature of the film profilemust match with the curvature of the static meniscusnear the bath, which yields the requirement xxh =

2/c

as x, orh0c

=H()

2Ca2/3. (6)

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1

1.5

2

2.5

10-2

10-1

100

101

102

103

104

105

106

107

108

-0.5

0

0.5

1

a42/3

b

Bq = c

U()

Fig. 2: Effect of surface viscosity on the thickening factor

and on the matching velocity U(), as calculated from themodel (5).

Using (6), we find Bq = H()/2, which is thusdetermined a posteriori from the solution ( being aninput of the calculation and H() being an output).

In the limit of 0, (5b) gives U|0 = (3H)/2H,which substituted into (5a) yields the familiar LLD equa-tion: H3H = 3 ( 1H). The solution to this equationgives H() = 1.3376 such that (6) leads to (1) with = 1. Moreover, as X approaches the static meniscus,H

1 such that U(

) =

1/2. Notice that the thicken-

ing factor for finite has the form = H

()/1.3376,and does not depend on Ca at constant Bq.

In the limit of , solving (5b) with the boundaryconditions for U yields U| = 1, which substituted into(5a) gives: H3H =12(1H), the solution of which leadsto (1) with = 42/3. In this limit, the interface moves atthe same speed as the substrate. It is not surprising that is identical for the cases of an infinite surface viscosityand an infinite surface elasticity or Marangoni effect(see, e.g., [9,10]), which renders in both cases the surfaceimmobile (relative to the moving plate). Finally, since forthe above limits, the matching velocity is constant, we also

require U() to be constant for any finite value of ,i.e. 12 1, so as to ensure continuity of the branchof solutions.

Numerical solutions. We solve (5) for finite values of and scrutinize the transition between the two limitsmentioned above. Solutions to (5) were found using ashooting technique (see details in the Appendix A) thatensures both a constant curvature H(X) and a constantsurface velocity U(X) for X. Results are shownin fig. 2. Figure 2(a) shows that the thickening factorincreases smoothly from 1 to 42/3 as Bq increases, demon-strating the possibility of thickening due to pure surface

viscosity effects. The best fit we found in the transi-tion region is Bq1/7 though it is only empirical.We also give in the Appendix B a nonlinear fit of the

h0

u0

6Bq = 1

Bq = 10 Bq = 100

Fig. 3: Streamlines for various Boussinesq numbers Bq. In eachplot, the vertical length is 6 = 6h0/Ca

1/3 and the dotted lineshows the position ofh0. Dots indicate stagnation points.

curve vs. Bq. Figure 2(b) shows the surface velocityU(), which varies monotonically from 12 to 1 as Bqis increased. We then observe that for Bq > 25, U(

)

becomes positive, indicating the absence of a stagnationpoint at the interface, in contrast to the pure LLD limit.We find that U() = 0 corresponds to 1.3, whichdiffers significantly from the value = 22/3 1.59 foundin the case of pure surface elasticity [11,12].

To understand the role of surface viscosity in the flowbehavior, we plot in fig. 3 the streamlines as reconstructedfrom the stream function defined by (x, y) =

u(x, y) dy.

The plot for Bq = 1 is comparable to the LLD situation forpure liquids, which includes the presence of a stagnationpoint at the interface. As the Boussinesq number increases,the stagnation point is displaced downwards (outside the

plot area for Bq = 10) and eventually disappears fromthe interface for Bq > 25 as found in fig. 2(b). Instead,for Bq > 25, the stagnation point moves into the interiorof the fluid and is displaced upwards for increasing Bq,as illustrated for Bq = 100. This behavior is qualitativelysimilar to the flow in the case of a pure elastic surface [12],even though the origin of the interfacial stress is different.

Applicability conditions. As conjectured in the intro-duction, the interface should essentially be viscous beyondthe dynamical transition of thickening, i.e. no Marangonieffects are expected in the region of large capillary

numbers. However, the present lubrication model (5) relieson the assumption that gravity is negligible, which is satis-fied only for small capillary numbers, namely Ca1/3 1

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B. Scheid et al.

-20 -15 -10 -5 00

0.1

0.2

X

dU

ds

Bq = 1

Bq = 10

Bq = 100

Fig. 4: Rate of stretching of a surface material element alongthe X-coordinate for various Bq.

assuming h0

c Ca

2/3. The region of applicability of the

present model is thus restricted to a narrow range of Ca,which only exists at high surfactant concentrations so as toensure complete surface remobilization and allow neglectof surface elasticity. This regime requires two conditionsto be satisfied. First, surfactants should have time to beadsorbed at the interface, and second they should be avail-able in sufficient quantity adjacent to the interface.

The first condition implies that when new interfaceis generated by stretching on a time scale s = /u0,surfactants should populate the interface by adsorbing ina time scale a s. Applying a result obtained in [3] forfiber coating to the case of planar coating, the condition

on time scales is as

c 1, (7)

where = (0 )0/(kc) is a length scale with 0the surface tension of the pure liquid, 0 the saturatedsurface concentration at equilibrium, and k the intrinsicsurfactant adsorption speed. The ratio /c does notdepend on u0 and is therefore only a property of thesurfactant solution. However, since stagnation points alsooccur at the interface, we cannot dismiss a priori thepossibility of having different time scales for stretchingalong the length of the film. We report in fig. 4 therate of stretching of a surface material element definedas dU/ds = U/1 + H2. We notice that as the valueof Bq increases, the maximum of the velocity derivativediminishes. Therefore, the convective transport becomesless important and the adsorption mechanism works evenmore efficiently to reduce the concentration gradients sincedU/ds = 1 was assumed when setting s = /u0.

The second condition implies that

=0ch0

0ccCa

2/3 0.01, (8)

where we assumed 0 for bulk concentrations much

above the cmc. A typical value is 0 = 1 molecule/50A

2

3 106 mol/m2 (see, e.g., [13]). Though (8) has beenobtained empirically by Quere and de Ryck [3] for DTAB,

0.0001 0.001

0.001

0.01

Ca = u0/

h0

c

= 0.99(sd= 0.02)

Fig. 5: Experimental results for silicon oil (Si47v20) with nosurfactant. The dashed line is a (2/3)-power law corresponding

to the mean value of the thickening factor , with sd thestandard deviation. Error on Ca values is smaller than the sizeof the symbols.

we assume this condition to be applicable for the surfac-tant used in the experiments we next describe, since themolecules are very similar.

Experiments. Experiments using water solutionsof the surfactant dTAB (decyl trimethyl ammoniumbromide) have been performed at high concentrations.This surfactant with cmc = 66mM was chosen forits greater solubility in water than DTAB. The surfacetension was measured by the Wilhelmy plate method. Theshear viscosity of the liquid was measured by a rheometer(Anton Paar Physica MCR 300) with an embedded doubleCouette cylindrical system (DG 26.7/TEZ 150 P-C) [14].The substrate used was a silicon wafer. The film thicknesswas measured using a multi-wavelength light reflected onthe film and analyzed with a spectrometer (Ocean Optics).The error on the thickness measurement was evaluated at5%, and is reported by the error bars in the subsequentplots. We validated the measurement technique with apure liquid by comparing the mean value of the thick-

ening factor obtained for each thickness measurement.The results are shown in fig. 5, where the dashed linecorresponds to the mean value for the thickening factor, = 0.99, which is 1% below the theoretical value, lying inturn within the standard deviation (sd) of 2%. The reasonwe performed the control experiment with silicon oil andnot with water is because water is easily contaminatedand it was impossible to keep pure water clean during acomplete set of measurements. Indeed, it is known thata very small amount of impurities at a water surface caninduce a strong Marangoni effect [15]. However, as long assurfactant (c > cmc) is added to water, these difficulties

disappear, as the impurities (mainly fatty substances) getdissolved in the micelles and the monolayer remains onlypopulated by added surfactants [16].

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0.0001 0.0010.001

0.01

Ca = u0/

h0

c

LLD

= 1.057(sd= 0.013)

= 42/3

Fig. 6: Experimental results for dTAB with c = 15 cmc. Solidlines are the bounds for the thickening factor. The dashed line

is a (2/3)-power law corresponding to the mean value of beyond the dynamical transition of thickening (dotted line).

Figure 6 shows the measured variation of the film thick-ness with the capillary number for c = 15 cmc. The dottedline separates the dynamical transition of thickening onthe left, to a zone of constant thickening on the right.Though large thickening is usually attributed to surfaceelasticity, the dynamical transition shows a decrease of thethickening with Ca. As mentioned in the introduction, thisphenomenon is due to the surface remobilization causedby the increase of the film thickness, which increases thenumber of surfactants available to replenish the interface(as quantified by the parameter ). Having also performedexperiments for smaller concentration (not shown), weobserved a shift of the dynamic transition of thickeningtoward smaller Ca values for increasing the concentra-tion. Consequently, the experiment for high concentra-tion (c = 15 cmc) allowed us to put in evidence a zone ofconstant thickening for almost a decade in Ca beyond thedynamical transition.

The mean value of the thickening factors for the exper-imental points lying on the right of the dotted line infig. 6 gives 1.06, with a standard deviation of about1%. Though small, this 6% thickening exceeds the errors

associated with our experimental method (1% for pureliquid). Such a result thus demonstrates the existence ofa thickening effect induced by the presence of surfactantin a regime where no concentration gradient exists, i.e.for which conditions (7) and (8) are satisfied. To eval-uate the intrinsic adsorption speed k, and consideringthe worst case, we added to the diffusion process (i.e.D/a 0.1 m/s, with D the diffusion coefficient and a themolecular size) a small electrostatic barrier ofV =100mV,common for large surfactant concentrations, which givesk = (D/a)eV/kbT 103 m/s, with kbT 25 mV. Taking0 = 0.072N/m, = 0.04 N/m, = 0.00335Pas and =

1000 kg/m

3

, leads, for c = 990 mM and Ca = 10

3

, toa/s 102 and 104, which indeed fully satisfiesconditions (7) and (8).

We can therefore state that the constant thickeningobserved in fig. 6 should essentially be induced by surfaceviscosity effects. We then use our model to assess thevalue of surface viscosity corresponding to = 1.06,and from fig. 2(a), we get the corresponding Boussinesq

number Bq 3. Using c 2 mm, we finally obtain 2 105 Pa s m, which is consistent with typicalvalues reported in the literature for ionic surfactants (see,e.g., [17,18]). One should mention that this small valuewas at the limit of resolution of the surface rheometer(Anton Paar Physica MCR 301 with an embeddedinterfacial rheology system) available to us at the time ofthis study, which did not permit a direct measurement ofthe surface viscosity.

It is known that surface viscosity has two contributions,namely intrinsic viscosity associated with the shearbetween surfactants at the interface, and exchange

viscosity arising from the energy dissipation associatedwith the exchange of surfactants between the surface andthe sublayer (see, e.g., [7]). Though exchange viscosity isusually much higher than intrinsic viscosity at low c [19],it inherently vanishes with surface elasticity at large c (see,e.g., [20]) such that intrinsic surface viscosity alone shouldbe responsible for thickening in the parameter range thatsatisfies the conditions (7) and (8) for complete surfaceremobilization.

Conclusions. We conclude that the constantthickening observed in the dip-coating process at largeCa values with surfactants of high solubility and at high

concentration can be rationalized entirely by the effectof intrinsic surface viscosity. Moreover, based on ourfindings, we anticipate that the large constant thickeningobserved at small Ca values (see fig. 6) should not only bedue to surface elasticity, as generally proposed in the liter-ature, but to cooperative effects of both surface elasticityand surface viscosity, with an even bigger influence ofthe surface viscosity than in the present study due to itsexchange component. For instance, this argument couldbe a candidate to resolve the apparent paradox reportedby Krechetnikov and Homsy [21] between the strongthickening observed in experiments in contrast with thethinning predicted by their simulations. In any case, itis clear that surface rheology deserves closer study for abetter understanding of flows dominated by the presenceof fluid-fluid interfaces containing surface-active materials.

We thank Saint-Gobain Recherche for support of this

investigation. BS thanks the Brussels Region for thefunding through the program Brains Back to Brussels.

Appendix A: shooting procedure. We detailin this appendix the shooting procedure used in thenumerical resolution of (5). We first look for approximate

solutions in the vicinity of the flat film region, i.e. H, U1 as X. Those local solutions are sought in theform H= 1 + aeX and U = 1 + beX . Substituting those

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3

2

1

LLD

0.2 0.4 0.6 0.8 1.0

8

6

4

2

0

2

4

6

LLD

1

2

3

Fig. 7: Real eigenvalues vs. obtained from (A.1). Note aunique real eigenvalue is obtained in the LLD limit for 0.

expressions into (5) and linearizing with respect to thesmall amplitudes a and b, we find the characteristic

equation for the eigenvalue ,

42 3612 + 3

= 0. (A.1)

Among the five roots of this equation, three are real(one positive and two negative) and plotted in fig. 7 asa function of . The local solution should therefore bea linear combination of the solutions corresponding tothe negative eigenvalues, noted 1 and 2. We can thuswrite H= 1 + a1e1X + a2e2X for the local solution nearthe flat film region. The amplitudes b1 and b2 for thesurface velocity U are determined from bi =

6ai/(4

2i ). Since solutions are invariant by translation along theX-coordinate, we can fix one of the two amplitudes and usethe other as the shooting parameter. We chose in fact tofix the curvature of the solution, taking X= 0 as reference,such that H(0)= a1

21 + a2

22. For all calculations, we

have taken H(0)=103. The rest of the procedure thusconsists in shooting from X= 0 toward negative valuesby progressively extending the domain and adjusting thesingle shooting parameter such that H and U eventuallytend to constants, namely H() and U(). It wasfound that a domain of 15 was large enough to satisfythese criteria for any value of Bq.

Appendix B: nonlinear fit of vs. Bq. Wepropose here a nonlinear fit of the curve plotted in fig. 2(a),

= 12

+ 21/3 +

21/3 1

2

tanh

6.40 8.27Bq0.049 ,

which can be inverted to get a rough estimate of the valueof the Boussinesq number Bq, hence the surface viscosity, for a given thickening factor .

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Scheid_The Role of Surface Rheology in Liquid Film Formation