Advances in Colloid and Interface Science 107(2004) 1–7

doi:10.1016/S0001-8686(03)00101-5

Honorary note

Jacob Lucassen

Jacob or, as he is better known, Jaap Lucassen wasborn in the village of Zuilen, now part of the city ofUtrecht, on 19th October 1931. He was the first-born ofHerman Lucassen and his wife Antje Pranger, who werelater also to have two daughters. His time at school wasin part overshadowed by the European war, for most ofwhich The Netherlands was occupied territory.

In 1949 Jaap started to read chemistry at the Rijksu-niversiteit Utrecht where he graduated in 1957, majoringin Physical Chemistry and Selected Topics of Physics.His Ph.D. research, conducted at the Van’t Hoff Labor-atory under the supervision of the eminent colloid sci-entist J.Th.G. Overbeek, dealt with the kinetics of areaction used in the diagnosis of various types of jaun-dice, and was concluded with a thesis entitled ‘TheDiazo-reaction of Bilirubin and Bilirubin-diglucuronide’in 1961. During this research he briefly held a job inbiomedical chemistry at the then Utrecht City and Aca-demic Hospital.

However, Jaap soon developed second thoughts abouta career in clinical chemistry and he returned to funda-

mental research by joining the Unilever Research Lab-oratory at Vlaardingen in 1961. Here, the PhysicalChemistry Group led by Max van den Tempel wasengaged in studies of the properties of detergent foamsand food emulsions, the emphasis being on the colloidchemical principles underlying their formation and sta-bility, and their general properties. The year 1961 saw athird event of importance when Jaap married EmmieReynders, who had earlier joined the same Group andwas to co-author many papers with him(see later). Inhis first few Unilever years, he studied the properties ofcalcium soaps and acid soapsw1x. The Group’s interestin the dynamic parameters important for foam and emul-sion stability was reflected in Jaap’s first publication( jointly with van den Tempel), which dealt with theGibbs elasticity of thin liquid filmsw2x.

During the 1960s there was increased interest in non-equilibrium surface properties displayed by surfactantsolutions and in the use of surface waves as a tool tomeasure such dynamic properties. In those days, the only

2 Honorary note

Fig. 1. Example of wave numberk as a function of surface elasticityfor transverse(k ) and longitudinal(k ) surface waves. Parameter values:T L

vy2ps200ys; hs0.01 mPa s;rs1 gycm ; gs70 mNym. Redrawn from Lucassenw8x.3

waves considered by surface chemists were capillarywaves or ripples, i.e. the small-wavelength transversewaves governed mainly by surface tension(g) accordingto Kelvin’s equation:

3 2gk frv (1)T

(wave numberk s2pyl; l is the wavelength andvT

the angular frequency of the wave;r is the liquid den-sity). It was known that surface elasticity, defined as´sdgydln A (whereA is the surface area), had a significanteffect on the damping of ripples, but a full analysis ofthe dispersion equation for these systems only becamepossible with the advent of computer techniques of somesophistication. This led on to a number of studiesdescribing the effect of surface elasticity and viscosityon the wave propertiesw3–5x. A fruitful collaborationdeveloped when Jaap was given the opportunity to spenda year(1964–1965) in R.S. Hansen’s well-equipped sur-face chemical laboratory at Iowa State University in theUSA w6,7x. During this period, he established theoreti-cally that the Gibbs elasticity of a thin liquid film wasequivalent to the surface dilational modulus(´) meas-ured by small-amplitude oscillations in compression andexpansion of the surface area. However, evaluation ofthe modulus from measured ripple properties was shownto be possible only over a very limited range of surfac-tant concentration.

After his return to Unilever Research, Vlaardingen,Jaap opened up a much wider vista by his discovery ofa novel type of surface wave. This was the longitudinal

wave, the existence of which he predicted theoreticallyw8x and also demonstrated experimentallyw9x. In contrastwith the case of capillary ripples, propagation of the newwave depended mainly on gradients in tension,expressed in the viscoelastic modulus,´:

2 1y23) )) )´ k f rhv (2)Ž .L

(k is the complex wave number andh the liquid vis-L

cosity). The characteristic behaviour of the two types ofsurface wave is illustrated in Figs. 1 and 2. Clearly theinformation on surface(visco)elasticity to be gainedfrom the longitudinal wave is far greater than that con-tained in the transverse wave. Eq.(2) produced earlyvalues of the modulus,, through measurements of thelongitudinal wave propertiesk and b (the dampingL L

coefficient, the imaginary part of the complex wave-number) w9,10x.

One reason why this new wave eluded detection forso long a time lay in the fact, demonstrated theoretically,that it is heavily damped with increasing distance fromits source, much more so than is the case with transverseripples; another reason is that the wave cannot exist ona clean surface, because its wavelength is reduced to 0when´s0, as is the case for any pure liquid. In general,measurement of is most conveniently made under con-ditions where the wave motion results in uniform defor-mation of the area, i.e. when the length of the measuringarea is much smaller than the wavelength, as indicatedby a quantitative analysis of this conditionw11x. In thislimiting case, surface elasticity and viscosity follow

3Honorary note

Fig. 2. Example of damping coefficientb (i.e. imaginary part of complex wave numberk) as a function of surface elasticityfor transverse andlongitudinal surface waves.Thin line: approximation of Eq.(2). Bold line: from full dispersion equation. Parameter values as in Fig. 1. Redrawnfrom Lucassenw8x.

directly from amplitude and phase of the tension oscil-lations resulting from the imposed area changesw11x.This simplified technique was utilised with both low-molecular and macromolecular surfactantsw12,13x.

Longitudinal waves and the techniques based uponthem have been taken up by many other groupsw14–20x. In a separate development, the theory also provedvery fruitful in the study of surface shear waves, forwhich the dispersion equation was found by De Feijterto be equivalent to Eq.(2) w21x. Thus, a coherent frame-work had been developed for the evaluation of surfacerheological parameters from different types of surfacewaves. The theory was also extended to embrace thecase of thin filmsw22,23x.

In 1969 he transferred to the Port Sunlight laboratory,then headed by B.A. Pethica, joining the Basic DivisionSurface and Colloid Group, where his new colleaguesincluded Denver Hall, Jim Mingins and, later, BrendanCarroll and Peter Garrett. Here, he constructed apparatusfor measuring the dynamic surface tension of surfacesundergoing periodic area oscillation and, with variouscollaborators, embarked on a series of studies—underboth dynamic and static conditions—on nonionic surfac-tants w24x, mixtures of anionic and cationic surfactantsw25x, polymer–surfactant systemsw26x, proteins andmicellar solutionsw27x. In doing this, he demonstratedthat his proven theoretical skills were augmented by areal flair for practical work. The apparatus, of fairmechanical complexity, he constructed in the main part

using a model engineering fabrication kit(Fac) was rem-iniscent of, but more sophisticated than, Meccano. Anearly version of the apparatus also featured the use of adomestic Pyrex glass casserole dish with inverted lid,and modified in a simple way to act as a thermostattedtrough. In this way it proved possible to create well-functioning apparatus, the cost of which, on the scale oftoday’s commercially available equipment, was verysmall.

The fundamental process in emulsion formationinvolves the repeated formation and subsequent elonga-tion and breakdown of drops of the emulsificate(typi-cally oil), a process expected to be influenced by therheological properties of the interface, as was latershown to be the case in model experimentsw28x. Sim-ulation of this process can be achieved by use of theclassical four roller apparatus due to Taylorw29x but theexperimental procedure lacks flexibility and may becomplicated by the need for high speed photographictechniques, a serious drawback at that time. Jaap arguedthat liquid coatings on solid cylindrical supports oughtto exhibit similar break-up characteristics to the elon-gated, purely liquid drops but, owing to the presence ofthe solid support, should occur over a longer time scaleand thus ought to be amenable to observation with arelatively unsophisticated protocol. This insight provedto be fruitful and led to the publication of several paperson the kinetics of formation and break-up of thin cylin-drical films in the presence of surfactantw30–33x. An

4 Honorary note

Fig. 3. Model for motion of the TPL.u is the ‘equilibrium’ angle andq is the dynamic angle. Excess pressureDp due to the local surface0

curvature drives movement of the contact line at A.

initially unforeseen spin-off from this novel experimen-tal approach was that the end product of the break-up ofthe coating film, a series of drops adhering to a thin,solid cylinder, was, when suitably reduced in scale, itselfan excellent model for the study of the detergency ofoily soil in fabric systems. This line of thought was sub-sequently pursued in a series of papers by Carrollw33x.

Capillary phenomena formed another, not unrelated,interest for Jaap. This interest led to investigations ofconsiderable diversity that include the effect of surfacedynamic properties on foam coarseningw34x, capillaryforces between solid particles in Pickering-type emul-sions w35x, the effects of surface heterogeneity(e.g. ascaused by droplets floating on the surface) on thedynamic propertiesw36x and also to a theoretical descrip-tion of the kinetics of motion of a liquid meniscus on asolid substratew37x.

The last-mentioned work, developed at Unilever PortSunlight during the mid-1970s, anticipated much of thetheoretical work on the kinetics of wetting later pub-lished by de Gennesw38x, but for commercial reasons itspresentation to the scientific public was delayed until1988, when, after his retirement from Unilever, he wasinvited by de Gennes to spend 6 weeks at the College`de France as a guest lecturer on ‘Surfaces and Films inMotion’. The model selected for the movement of thethree phase contact line(TPL) region was both simpleand elegant. On a microscopic scale, the contact angleat the TPL retained its ‘equilibrium’ valueu (this being,0

depending on the direction of movement, either the(stat-ic) advancing or(static) receding value); in the dynamicsituation, the apparent(‘dynamic’) contact angleu dif-fers from this value and to explain this fact, it was pos-tulated that the meniscus close to the TPL was curved

into an arc, the length and radius of which determinedthe value of the apparent angle(Fig. 3). This meniscuscurvature gives rise to a Laplace pressure gradient thatdrives liquid from the TPL region, thus bringing aboutmovement of the TPL. The viscous resistance to thistransport process is ultimately to be linked to the rate ofmovement of the TPL. The overall process was likenedto the drainage of liquid from between parallel platessqueezed together by an external pressure, a classicalhydrodynamic problem analysed long ago by Lambw39x.The ensuing theoretical equation related the speed ofmovement,v, of the TPL with the dynamic and the equi-librium anglesu andu , with the liquid surface tension0

g and with bulk viscosityh and did not explicitlyinvolve the length and curvature of the postulated dis-torted arc of meniscus:

3{ }{ w x w x}vs gy2h sin (u yu)y2 sin (u yu)y4 y0 03{ w x w x}sin (u qu)y2 cos (u qu)y4) . (3)0 0

This equation was compared with experimental datapublished by other authorsw40,41x and was shown togive a good, universal fit(Fig. 4). There is no directcomparison of this equation with those later developed,independently, by de Gennes, but the underlying conceptof the role of viscous resistance to movement in the TPLregion is apparent.

Jaap transferred from Port Sunlight back to Vlaar-dingen in 1986. A further excursion into the field ofcapillarity theory appeared several years after the workon wetting just described and can be seen as a spin-offof Jaap’s final years of work for Unileverw42x. Thisconcerned the effect of a substrate’s geometry on itswettability. The work examined theoretically the stability

5Honorary note

Fig. 4. Comparison between experimental data and Eq.(3). The data are from independent investigatorsw40,41x.

of catenoidal foam films suspended between coaxialannular supports. For a stable foam film to exist, thegreatest possible separation of these supports for a par-ticular support radius was shown to be determined bythe so-called Lindelof envelope, a line which is tangent¨to all members of the particular family of catenoids. Thecase where one of the annular supports is replaced by asolid surface was then considered: the mathematicallyrequired shape of the substrate for which films of equalarea and which meet the substrate surface orthogonallywas derived, drawing upon advances in the mathematicaltheory of catenoids made some hundred years previously.A foam lamella necessarily contacts a solid at rightangles. It was shown how solids of a certain shape—termed Sinclair cuspoids—could thus establish a wettingequilibrium with a free foam lamella whose surface areadid not vary with position of the contact line on thesupporting surface. The predictions were convincinglyverified in an experiment in which a substrate of thetheoretical shape(Fig. 5a) was fabricated using a com-puter-driven lathe programmed with the correct coordi-nates. A foam lamella contacting this surface showed theexpected indifferent wetting characteristics as the sup-port separation was varied(Fig. 5b). This discussion wasthen extended to the case of liquidyair menisci under‘zero gravity’ conditions(i.e. for Bond Number Bos

wL y(grg)x, L being a characteristic dimension, much2

smaller than unity). The particular case where the liquidjust wets the solid(i.e. g qg sg ; us0) was ana-L SL SO

lysed, applying a theorem, due to Blissw43x, on the Lin-delof envelope properties in relation to changes in¨surface areas of revolution. The Bliss theorem, it wasdemonstrated, attributes to the cuspoids generated byrotation of the envelope, just the mathematical propertiesrequired to give indifferent wetting conditions, as wit-nessed by an invariant free energy change for differentposition of the meniscus on the cuspidal surface. Thus,surfaces of this nature ought to exhibit for wetting liq-uids the same kind of behaviour as was previously dem-onstrated for 908 angle foam films on Sinclair cuspoids.Thus, in principle, surfaces having geometries interme-diate between Sinclair and Lindelof cuspoids should¨exist to give analogous behaviour when the contact anglelies between the extrema of 08 and 908. Capillarity is apart of surface chemistry having a long and distinguishedpedigree of contributions, but this elegant paper mustindeed rank with the foremost. A recent publication co-authored by him revisited the earlier dynamic workw24x,confirmed the old viscoelastic moduli with a new tech-nique and proposed a quantitative theory for the staticand dynamic data measured over a large range of con-centrations and frequenciesw44x.

6 Honorary note

Fig. 5. (a) The experimental substrate having the form of a Sinclaircuspoid.(b) Indifferent wetting of a foam film on this substrate.(c)Lindelof cuspoid-covered surface expected to show critical wetting¨for liquids having zero contact angle.(Taken from Ref.w42x.)

Colleagues of Jaap were always impressed by his sci-entific curiosity and the care with which he studied anynovel phenomenon. He repeatedly demonstrated thesoundness of his belief that by taking time to understandobservations made in the lab and thus to describe themtheoretically, one could by-pass a lot of further experi-mental effort. Mark Twain once memorably jested ‘Get

your facts first and afterwards you can distort them asmuch as you please’. Jaap certainly established his factswith care; however, his subsequent theoretical delibera-tions frequently proved to be accurate descriptions of thephenomena observed.

As already noted, Jaap married Emmie Reynders, whowas already a member of van den Tempel’s team whenJaap joined this at Vlaardingen in 1961; from this pointonward, the two co-authored many papers and otherpublications. When Jaap transferred to Port Sunlight in1969, Emmie opted for the tasks of mother and house-wife whilst retaining her strong scientific interests. Thesuccess with which she managed this difficult dual rolecan be judged on the one hand by the distinguishedseries of publications of which she was editor or(co-)author and on the other by the subsequent academicdistinction later achieved by their two daughters, Annekeand Emy.

This uncommon and scientifically highly productivepartnership is amply worthy of the respect and thanks ofthe scientific community it belongs to and also of a sin-cere wish for happiness during the years of retirement.

References

w1x J. Lucassen, Hydrolysis and precipitates in carboxylate soapsolutions, J. Phys. Chem. 70(1966) 1824.

w2x M. van den Tempel, J. Lucassen, E.H. Lucassen-Reynders,Application of surface thermodynamics to Gibbs elasticity, J.Phys. Chem. 69(1965) 1798.

w3x R.S. Hansen, J.A. Mann, Propagation characteristics of capil-lary ripples I, J. Appl. Phys. 35(1964) 152.

w4x M. van den Tempel, R.P. van de Riet, Damping of waves bysurface-active materials, J. Chem. Phys. 42(1964) 2769.

w5x E.H. Lucassen-Reynders, J. Lucassen, Properties of capillarywaves, Adv. Colloid Interface Sci. 2(1969) 347.

w6x J. Lucassen, R.S. Hansen, Damping of waves on monolayercovered surfaces. I Systems with negligible surface dilationalviscosity, J. Colloid Interface Sci. 22(1966) 32.

w7x J. Lucassen, R.S. Hansen, Damping of waves on monolayercovered surfaces. II Influence of bulk to surface diffusionalinterchange on ripple characteristics, J. Colloid Interface Sci.23 (1967) 319.

w8x J. Lucassen, Longitudinal capillary waves. I Theory, Trans. Far-aday Soc. 64(1968) 2221.

w9x J. Lucassen, Longitudinal capillary waves. II Experiments,Trans. Faraday Soc. 64(1968) 2230.

w10x J. Lucassen, M. van den Tempel, Longitudinal waves on visco-elastic surfaces, J. Colloid Interface Sci. 41(1972) 491.

w11x J. Lucassen, G.T. Barnes, Propagation of surface tension chang-es over a surface with limited area, J. Chem. Soc., FaradayTrans. I 68(1972) 2129.

w12x J. Lucassen, M. van den Tempel, Dynamic measurements ofdilational properties of a liquid interface, Chem. Eng. Sci. 27(1972) 1283.

w13x M. Blank, J. Lucassen, M. van den Tempel, The elasticities ofspread monolayers of bovine serum albumin and of ovalbumin,J. Colloid Interface Sci. 33(1970) 94.

w14x P. Joos, The Elasticity of Insoluble Monolayers, Berichte vomVI. Internationalen Kongreß fur Grenzflachenaktive Stoffe,¨¨Zurich, 1972(Carl Hanser Verlag, Munchen, 1973) Band II, p.¨ ¨113.

7Honorary note

w15x G. Loglio, U. Tesei, R. Cini, Surface compressional modulusof surfactant solutions. A time domain method of measurement,Berichte Bunsen-Gesellschaft fur Physikalische Chemie 81¨(1977) 1154.

w16x H.C. Maru, D.T. Wasan, Dilational viscoelastic properties offluid interfaces—II Experimental study, Chem. Eng. Sci. 34(1979) 1295.

w17x C. Stenvot, D. Langevin, Study of viscoelasticity of solublemonolayers using analysis of propagation of excited capillarywaves, Langmuir 4(1988) 1179.

w18x Q. Yiang, Y.C. Chiew, J.E. Valentini, An apparatus for the studyof surface longitudinal waves at the airywater interface, J. Col-loid Interface Sci. 159(1993) 477.

w19x A. Dussaud, M. Vignes-Adler, Surface properties of proteinalcoholic solutions II. Surface dilational rheology, J. ColloidInterface Sci. 167(1994) 256.

w20x B.A. Noskov, D.A. Alexandrov, R. Miller, Dynamic surfaceelasticity of micellar and nonmicellar solutions of dodecyldi-methyl phosphine oxide. Longitudinal wave study, J. ColloidInterface Sci. 219(1999) 250.

w21x J.A. de Feijter, The propagation of surface shear waves I. The-ory, J. Colloid Interface Sci. 69(1979) 375.

w22x J. Lucassen, M. van den Tempel, A. Vrij, F.Th. Hesselink,Waves in thin liquid films. I. The different modes of vibration,Proc. Koninkl. Ned. Akad. Wetensch. B 73(1970) 110.

w23x A. Vrij, F.Th. Hesselink, J. Lucassen, M. van den Tempel,Waves in thin liquid films. II. Symmetrical modes in very thinfilms and film rupture, Proc. Koninkl. Ned. Akad. Wetensch.B 73 (1970) 124.

w24x J. Lucassen, D. Giles, Dynamic surface properties of nonionicsurfactant solutions, J. Chem. Soc., Faraday Trans. I 71(1975)217.

w25x E.H. Lucassen-Reynders, J. Lucassen, D. Giles, Surface andbulk properties of mixed anionic–cationic surfactant systems,J. Colloid Interface Sci. 81(1981) 150.

w26x J. Lucassen, J.H. Buckingham, F. Hollway, Surface propertiesof mixed solutions of poly-L-lysine and sodium dodecyl sul-phate. II. Dynamic surface properties, J. Colloid Interface Sci.67 (1978) 432.

w27x J. Lucassen, Adsorption kinetics in micellar systems, FaradayDiscuss. 59(1975) 76.

w28x J.J.M. Janssen, A. Boon, W.G.M. Agterof, Influence of dynam-ic interfacial properties on droplet breakup in simple shearflow, AIChE J. 40(1994) 1929.

w29x G.I. Taylor, Formation of emulsions in definable fields of flow,Proc. Roy. Soc. A 146(1934) 501.

w30x B.J. Carroll, J. Lucassen, Capillarity-controlled entrainment ofliquid by a thin cylindrical filament moving through an inter-face, Chem. Eng. Sci. 28(1973) 23.

w31x J. Lucassen, B.J. Carroll, Effect of surface dynamics on theprocess of droplet formation from supported and free liquidcylinders, J. Chem. Soc., Faraday Trans. I 70(1974) 1228.

w32x B.J. Carroll, J. Lucassen, The effect of equilibrium and dynam-ic interfacial properties on the elementary emulsification pro-cess, in: A.L. Smith(Ed.), Theory and Practice of EmulsionTechnology, Academic Press, London, New York, 1977, p. 29.

w33x B.J. Carroll, Physical aspects of detergency, Colloids Surfaces74 (1993) 131.

w34x J. Lucassen, Dynamic properties of free liquid films and foams,in: E.H. Lucassen-Reynders(Ed.), Anionic Surfactants; Phys-ical Chemistry of Surfactant Action, Dekker, New York, 1981,p. 217.

w35x J. Lucassen, Capillary forces between solid particles in fluidinterfaces, Colloids Surfaces 65(1992) 131.

w36x J. Lucassen, Dynamic dilational properties of composite sur-faces, Colloids Surfaces 65(1992) 139.

w37x J. Lucassen, Internal Unilever reports, 1977; The Rate of Move-ment of a 3-phase Contact Line. Abstracts of Papers of theAmerican Chemical Society 1988 196, 165-Coll.

w38x P.G. de Gennes, Rev. Mod. Phys. 57(1985) 827.w39x H. Lamb, Hydrodynamics, Cambridge University Press, 1916.w40x W. Rose, H.W. Heins, J. Colloid Sci. 17(1962) 39.w41x R.J. Hansen, T.Y. Toong, J. Colloid Interface Sci. 36(1971)

410.w42x J. Lucassen, E.H. Lucassen-Reynders, A. Prins, P.J. Sams, Cap-

illary engineering for zero gravity. Critical wetting on axisym-metric solid surfaces, Langmuir 8(1992) 3093.

w43x G.A. Bliss, The Calculus of Variations, The Open Court, LaSalle, IL, 1925, p. 102.

w44x E.H. Lucassen-Reynders, A. Cagna, J. Lucassen, Gibbs elastic-ity, surface dilational modulus and diffusional relaxation innonionic surfactant monolayers, Colloids Surfaces, A 186(2001) 63.

Brendan Carroll*

Unilever Research Laboratory, Quarry Road East,Bebington, Merseyside L63 3JW, UK

*Tel.: q44-151-641-3000; fax:q44-515-641-1826.