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Crystallization of Condensation Droplets on a Liquid Surface Olivier Pitois, Bernard François

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Crystallization of Condensation Droplets on a Liquid Surface Olivier Pitois, Bernard François Crystallization of condensation droplets on a liquid surface Olivier Pitois, Bernard François To cite this version: Olivier Pitois, Bernard François. Crystallization of condensation droplets on a liquid surface. Colloid and Polymer Science, Springer Verlag, 1999, 277, pp.574-578. hal-01990802 HAL Id: hal-01990802 https://hal.archives-ouvertes.fr/hal-01990802 Submitted on 31 Jan 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Colloid Polym Sci 277:574±578 (1999) Ó Springer-Verlag 1999 SHORT COMMUNICATION O. Pitois Crystallization of condensation droplets B. FrancËois on a liquid surface Abstract Highly ordered micropo- with time of the mean droplet radius Received: 4 January 1999 Accepted in revised form: 15 February 1999 rous two-dimensional membranes has been found to be described by a have been obtained from polymer power law with an exponent of 1/3, solutions (Widawski et al. (1994) proving that no coalescence pro- Nature 369: 397±399). Recently, a cesses occur. This particular behav- mechanism for the formation of iour is attributed to the precipitation such membranes was proposed, in- of the polymer at the water/solution volving water vapour condensation interface and to the formation of a O. Pitois (&) Laboratoire des Mate riaux et Structures (induced by the rapid evaporation of mechanically resistant polymer layer du Ge nie Civil the volatile solvent) onto the surface encapsulating each droplet. In this U.M.R. 113 (C.N.R.S./L.C.P.C.) of solutions and the formation of way, water droplets behave like solid 2, alle e Kepler ¯oating water droplets. Unfortu- particles, allowing compact sheets to F-77420 Champs-sur-Marne, France e-mail: [email protected] nately, the droplets growth process be formed. The presence of impor- Fax: +33-81-40 43 54 85 was not observed, and consequently tant surface currents is believed to only qualitative information was promote the formation of ``poly- B. FrancËois Laboratoire de Recherche sur reported. In the present paper, re- crystal'' and ``monocrystal'' les Mate riaux PolymeÁ res sults of light-scattering experiments patterns. C.N.R.S./U.P.P.A. with this system are reported. The Centre He lioparc Pau-Pyre ne es formation of water droplets growing Key words Condensation ± Mem- 2, avenue du Pre sident Angot F-64000 Pau at the surface of the solution has brane ± Scattering ± Droplet ± France been observed and the evolution Polymer Introduction on the surface of the solution [3]. The key element in this process was found to be related to the particular surface The formation of highly ordered microporous polymeric tension properties of this system. Firstly, it was shown membranes has previously been reported [1]. These that the shape of a water droplet ¯oating on the solution membranes consist of one (or possibly several) layer(s) surface is in accordance with the pore shape observed in of spherical micron-sized pores organized in a regular the membranes. In addition, PS-PPP was found to triangular lattice (an example of a membrane surface is precipitate at the solution/water interface, forming a shown in Fig. 1). They are obtained by rapidly evapo- solid polymer layer around each droplet. In this way, rating a thin layer of a solution of CS2 containing coalescence processes are prevented, allowing the drop- poly(p-phenylene)-block-polystyrene (PS-PPP). It was lets to behave like solid particles trapped at a liquid observed that the formation of the ordered structure interface. Then, capillary interactions and above all required the presence of moisture in the surrounding surface currents, contribute to organize the ¯oating atmosphere [1, 2]. Recently, a mechanism for the ``raft'' as packed sheets. In a previous paper [3], we formation of such structures was proposed, assuming reported optical microscopy observations of the solu- the formation of micron-sized water droplets condensed tion surface during the condensation process. Unfortu- 575 Fig. 1 Picture of a membrane surface formed from a solution of poly(p-phenylene)-block-polystyrene (Mw = 3000±30 000) in carbon disulphide (0.02 g/cm3) Fig. 2 Light-scattering experiment set-up. The patterns formed on the screen were ®lmed by means of a high-speed charge-coupled device camera 2 3 nately, the surface was made hardly visible with high magni®cation because of rapid surface currents (due to 6 XN XN Á7 ~à ~ 6 7 solvent evaporation and to the gas ¯ow carrying water I hE hÁE hIa hÁ4N exp i~q Á~rij 5 ; i1 j1 vapour). Nevertheless, ¯oating micron-sized droplets j6i were distinguished under particular experimental con- ditions, allowing some qualitative information to be 2 gathered about the mechanism of formation of the ~ where Ea h and Ia h are the electric ®eld and the structure. In the present paper, we report recent results intensity scattered by one droplet, respectively, and of light-scattering experiments performed with the ~q ~k ~k is the scattering momentum transfer (where system previously described. In contrast to previous 0 d ~ observations in direct space, we have been able to kd is the wave vector of the scattered light) with observe the condensation process under optimum j~qj2k sin h=2 4p=k sin h=2. The ®rst term in conditions for preparing well-ordered membranes. In Eq. (2) represents incoherent scattering and the second the next section, we brie¯y present some theoretical accounts for the interference of radiation scattered by aspects of correlated light scattering by a two-dimen- the droplets. Converting the sums to integrals over the sional assembly of droplets. The experiment performed surface coordinates and assuming a constant number and the corresponding results are presented in subse- density of droplets (r) over the surface, the following quent sections. expression is obtained for an isotropic system [5] Z 1 I hNIa hÁ 1 2pr r drgÁ r1 J0 2kr sin h Scattered light by a two-dimensional assembly 0 of droplets NIa hS h ; 3 Consider N droplets of radius a on a surface and where where J0 is the zero-order Bessel function and S(h) is the the centres of the droplets are located by the position scattering structure factor, which is, with the exception of the leading factor of unity and the number density r, vector ~ri with 1 £ i £ N. Light of wavelength k and ~ the Fourier transform of g(r) ) 1, where g(r) is the pair wave vector k0 is sent normal to the surface (see Fig. 2). The electric ®eld and the intensity scattered can correlation function. be written [4] XN Experimental ~ ~ ~ ~ E h Ea hÁexpi k0 kdÁ~ri 1 Experiments were performed under optimum conditions for i1 preparing the membranes. A solution (50 ll) was spread on a 576 glass plate at room temperature. A carrier gas (nitrogen) was for r >2a and I(h) » NIa(h). Then, the intensity of the bubbled through a ¯ask of distilled water at room temperature. central peak decreased strongly and spread on the disk The jet of saturated nitrogen was then sent from a nozzle onto the liquid surface (the average velocity of the gas ¯ow was approx- surface (Fig. 3A). The droplets were no longer arranged imately 20 cm/s). Moisture condensation on the bottom side of the randomly and so coherent scattering appeared. As r glass plate (which could induce parasite scattering during the increased, the droplet arrangement became semi-ordered cooling of the solution due to evaporation of the solvent) was locally. At this stage g(r » 2a) > 1 and so a peak prevented by sending dry gas onto this side. A laser beam (He-Ne, appeared in the structure function S(h), and since the k = 0.6328 lm, beam width » 1 mm) was directed towards the sample, normal to the glass plate (Fig. 2). Scattered light was scattering system constituted by the droplets remained projected on a screen and the patterns so formed were recorded by isotropic, a ring (®rst Debye±Scherrer ring) was ob- means of a charge-coupled device camera. The solution was served on the projection plane (Fig. 3B). The maximum 3 composed of CS2 and PS-PPP (Mw = 30 000±3000, 0.02 g/cm ). in the structure function rises up at Pure solvent was also used for comparison. 4p h 2p q sin max : 4 max k 2 2a Results The patterns revealed a drastic dierence between Four typical patterns, corresponding to four stages of polymer solutions and pure solvent. In the ®rst case, the condensation process, are presented in Fig. 3. About the sharpness of the ®rst ring and the presence of a well- 3 s after the gas ¯ow had been sent, the ®rst pattern to de®ned third ring (see Fig. 3C) prove that the assembly appear on the projection plane was a diuse central peak of droplets is a ``polycrystal'' [6], while only typical (not presented in Fig. 3). At this stage, water droplets liquid-like scattering patterns were produced with pure were randomly arranged on the surface and the surface solvent (the ring was generally badly de®ned, which fraction occupied by the droplets remained small. The accounts for the large polydispersity in the droplet correlation function was well approximated by g(r)=1 radius).
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