© Imperial College LondonPage 1 Surfactant-Driven Thin Film Flows: Spreading, Fingering and Autophobing O. K. Matar Department of Chemical Engineering Imperial College London PASI on Interfacial Fluid Dynamics: From Theory to Applications Wednesday, 8 th August 2007 Correspondence to:
© Imperial College LondonPage 2 Outline Overview Previous work Models Predictions Open questions
© Imperial College LondonPage 3 Overview: Applications & Settings Engineering –Coatings –Paints –Adhesives –Foams –Reactors (falling film and spinning disc) –Heat exchangers and distillation columns Biology –Membranes –Lining of lungs –Tear films
© Imperial College LondonPage 4 Overview: Complex dynamics – examples I h min No h min Two fronts Fingering Instability time Hamraoui et al., 2004 Darhuber & Troian, 2003 Luckham et al., 2005
© Imperial College LondonPage 5 Autophobing & dewetting Two fronts Spreading Retraction Spreading and retraction of a drop of 0.4 cmc DTAB solution on a 100 m water film (Afsar-Siddiqui et al. 2004) Overview: Complex dynamics – examples II
© Imperial College LondonPage 6 Previous work –Spreading Grotberg et al. (1992) Starov et al. (1997) Dussaud, Matar & Troian (2005) –Fingering Marmur & Lelah (1981) Troian, Wu & Safran (1989) Frank & Garoff (1995a,b) He & Ketterson (1995) Afsar-Siddiqui, Luckham & Matar (2003a,b,c) Cazabat et al. ( ) Jensen & Naire (2005) –‘Running droplets’ Thiele et al. ( )
© Imperial College LondonPage 7 Previous work … –Dewetting & Autophobing Woodward & Schwartz (1996) Qu, Suter & Garoff (2002) Afsar-Siddiqui, Luckham & Matar (2004) Craster & Matar (2007) –Super-spreading Zhu et al. (1994) Stoebe et al. (1997) Churaev et. al. (2001) Chengara, Nikolov & Wasan (2002) Nikolov et al. (2002) Rafai et al. (2002) Kumar, Couzis & Maldarelli (2003)
© Imperial College LondonPage 8 Modelling Methodology: Problem Formulation I - Lubrication approximation. - Deposition is relatively thick. - Uncontaminated precursor layer. - Soluble surfactant. - Rapid vertical diffusion. - No adsorption of micelles at interfaces. - Micelles have same size. - Negligible intermolecular forces. - Non-linear equation of state. Assumptions: Schematic of surfactant-driven spreading problem Marangoni spreading Fingering
© Imperial College LondonPage 9 Modelling Methodology: Problem Formulation II Evolution equations for fingering problem Air-liquid interface (monomer) Bulk (monomer) Bulk (micelles) Air-liquid interface Fluxes + Sheludko E.O.S. Effects: Marangoni stresses Capillarity Diffusion (bulk and surface) Sorption kinetics Solubility Micellar formation and breakup Nonlinear E.O.S.
© Imperial College LondonPage 10 Modelling Methodology: Problem Formulation III Same as for fingering problem except: Introduce Disjoining pressure which should depend on surfactant adsorption substrate wettability can be altered during spreading. Autophobing Assumptions:
© Imperial College LondonPage 11 Air-liquid interface Bulk (monomer) Bulk (micelles) Liquid-solid interface (monomer) Air-liquid interface (monomer) Modelling Methodology: Problem Formulation IV Evolution equations for autophobing problem
© Imperial College LondonPage 12 L-W apolar component Short-range polar component Disjoining pressure depends on surfactant concentration At long times For stability Modelling Methodology: Problem Formulation V
© Imperial College LondonPage 13 b =0.05, C = 10 -3, Pe s =10 4, Pe b,m =10 2, =K s,b =1, =100, M=3, n=10, t= R=1R=100 For relatively small R, micelles present at late times, confined to the drop. Tendency for two-front formation increases with increasing R and M. Large concentration gradients at edges of drop and secondary fronts. Fingering Results I: Base state Edmonstone, Craster & Matar, JFM, 2006
© Imperial College LondonPage 14 Fingering Results II b =0.05, C = 10 -3, Pe s =10 4, Pe b,m =10 2, =K s,b =1, =100, R=100, M=3, n=10, t=10 4. Initial perturbations random Organisation into fingers Target: the secondary front Primary front Secondary front Edmonstone, Craster & Matar, JFM, 2006
© Imperial College LondonPage 15 Fingering Results III Fingering occurs in M=3 case despite apparent absence of ‘h min ’ Pronounced fingering for intermediate M. Edmonstone, Craster & Matar, JFM, 2006
© Imperial College LondonPage 16 Fingering Results IV Hamraoui et al. (2004) Experiment Initial condition Numerical simulations Branching & tip-splitting Craster & Matar, Phys. Fluids, 2006
© Imperial College LondonPage 17 Fingering: Open questions Is this the best way of modelling the presence of micelles? What is the mechanism that drives the instability? How good is the agreement between theory and experiment?
© Imperial College LondonPage 18 Autophobing Results I: LW = 4, P = 4 Rim formation Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 19 Autophobing Results II(a): LW = 16, P = Onset of retraction Early times Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 20 Autophobing Results II(b): LW = 16, P = Dewetting Retraction Late times Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 21 Autophobing Results III: LW = 16, P = Dewetting Retraction Above CMC Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 22 Autophobing Results IV: LW = -0.05, P = 4 Film rupture Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 23 Autophobing Results IV: LW = -1.1, P = Film rupture Craster & Matar, Langmuir, 2007
© Imperial College LondonPage 24 Autophobing: Open questions Is this the best of modelling the effect of surfactant on intermolecular interactions? Are we missing relevant physics? Should the structural component of be taken into account (esp. for C > CMC)? How does this change the predictions and the agreement with experiments?
© Imperial College LondonPage 25 Superspreading Surfactant-assisted spreading on hydrophobic substrates Only certain surfactants act as “superspreaders” Effect strongest for C > CAC R ~ tvs.R ~ t 1/10 or R ~ t 1/4 Mechanism: –Marangoni flow? –Unusual structure of trisiloxanes? –Direct adsorption of micelles? –Intermolecular effects near the contact line?
© Imperial College LondonPage 26 Superspreading: Open questions What is the mechanism? What happens at the advancing contact line? Are structural disjoining pressures important (esp. for C > CMC)? If so, how do we build them into our lubrication theory-based models? What molecular level information do we need?
© Imperial College LondonPage 27 General open problem Molecular scale information Micro-scale experiments Macro-scale experiments Theory (e.g. statistical mechanics, DFT…) Dependence of on surfactant Lubrication theories and simulations
© Imperial College LondonPage 28 Acknowledgements Collaborators –Richard Craster –Paul Luckham Students –Abia Afsar-Siddiqui –Mark Warner –Barry Edmonstone Funding agencies –EPSRC