Florent
Carn
*a,
Annie
Colin
b,
Olivier
Pitois
c and
Rénal
Backov
d
aLaboratoire Matière et Systèmes Complexes, Université Paris Diderot, UMR CNRS-Université Paris Diderot—7057, 10 rue A. Domon et L. Duquet, 75013, Paris, France. E-mail: florent.carn@univ-paris-diderot.fr
bLaboratoire du Futur, Université Bordeaux 1, UMR CNRS-Rhodia-Université Bordeaux 1 5258, 33608, Pessac, France
cLaboratoire Navier, IFSTTAR, Université Paris-Est, 2 allée Kepler, 77420, Champs-sur-Marne, France
dCentre de Recherche Paul Pascal, UPR CNRS 8641, 115 Avenue Albert Schweitzer, 33600, Pessac, France
First published on 16th November 2011
We investigate the drainage of a foaming solution during inorganic polycondensation by macroscopic measurements and local observations. We reveal an original mineralisation mechanism starting from Plateau border interfaces. This slow process is not able to counteract the destabilizing effects of foam drainage and we therefore propose a new strategy in which mineralisation is assisted by a biopolymer.
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Fig. 1 Evolution of the optical transmission (T) versus sample height (h) for a surfactant foam (TTAB at 10 CMC) at different times. |
This evolution has three different origins. First, the volume of water in the foam decreases due to drainage. Liquid flows in the downward direction due to gravity leading to liquid fraction gradient. Second, the mean size of the bubble grows due to Ostwald ripening. The increase of the mean droplet size accelerates the drainage process. Third, some coalescence events occur at the top of the foam column. We recall that coalescence in foams in the absence of Ostwald ripening occurs below a critical liquid fraction.7 This critical liquid fraction is equal to 2 × 10−4 for a solution of TTAB at 10 CMC.7 Such a low liquid fraction may be achieved at the top of the foam due to drainage and water evaporation.9 We then deduce the evolution of the relative volume of water released from the foam and the relative volume of foam as a function of time. Fig. 2 shows that all the foams release water with a rate scaling of 1/tα, with 0.3 ≤ α ≤ 0.5, until a final static equilibrium. This final value is increased by a factor of 3 when nanoparticles are present. In contrast, the kinetic of foam volume decrease seems to be faster for polymer foams than for surfactant and particulate foams. However, this effect is not very significant due to the experimental uncertainty arising from the determination of the foam/air interface by optical transmission measurements in the dry state (see Fig. 1c). In all cases the foam volume evolution is continuous as expected for such fine and wet foams. Fig. 3 shows bubble growth as a function of time for the three systems, indicating similar coarsening rates. The coarsening of such wet foams is dominated by Ostwald ripening rather than coalescence and theoretical evolutions are expected to be described by the following relation:
R(t)/R0 = (1 + K2t/2R20)1/2 | (1) |
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Fig. 2 Time evolution of the relative liquid volume contained in the foam (full symbols) and of the relative foam volume (open symbols) in free drainage configuration at 25 °C for the reference foaming solution (TTAB 10 wt%, black squares) and foaming solutions containing either TEOS (green circles) or silica nanoparticles (red triangles). |
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Fig. 3 Time evolution of the relative parietal bubble radius determined by image analysis for the reference foaming solution (TTAB at 10 CMC, black squares) and foaming solutions containing either TEOS (green circles) or silica nanoparticles (red triangles). The lines correspond to the best adjustment of eqn (1) for the different data. |
The above evolution law has been fitted for each set of data considering an average liquid fraction equal to ε ≈ 0.04, so that α(ε) ≈ 0.5,10 giving K2 ≈ 4.32 × 10−10, 3.72 × 10−10 and 3.42 × 10−10 m2 s−1 for TTAB, TEOS and Ludox© respectively. The corresponding values deduced for the thin film thickness are respectively 30, 33 and 36 nm. Note that the slight increase in film thickness between TTAB and Ludox© can be attributed to the slightly larger liquid fraction of foam made with Ludox©. This range of values is consistent with results already published for solutions of pure surfactant11 and suggests that inorganic additives are not present in foam films. It is in qualitative agreement with the systematic achievement of open porous network by the inorganic polycondensation approach3 and probably also for particulate approaches at low volume fraction.
These macroscopic measurements show that mineralisation occurs without significant modification of the macroscopic drainage kinetic and of the overall ageing dynamic whereas particulate foams containing the same silica content depict an improved stability compared to surfactant foams as already shown.
To get further insight into this puzzling interplay between drainage and mineralisation we performed direct visualisations at the Plateau border (Pb) scale, with the Plateau border apparatus12 (PbA), since macroscopic drainage is mainly determined by liquid flow in Pb (Scheme 1).
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Scheme 1 Schematic view of the experimental set-up used for the local flow observations. Adapted from ref. 12. |
In brief, we recall that in a PbA (Scheme S1 of ESI†), a Pb and the three adjoining films are formed upon withdrawing a tripod from a reservoir containing the foaming solution and that liquid can be injected through the channel at a controlled flow rate, Q, corresponding to typical liquid velocities encountered during foam drainage. Pictures of the Pb cross-sections could be obtained during the experiment using a camera. We present in Fig. 4 a typical snapshot taken during Pb mineralisation at low flow rate Q = 20 μL min−1.
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Fig. 4 Photographs of a Plateau border during TEOS polycondensation with a constant liquid flow (Q = 20 μL min−1) at different times: (a) 1 min, (b) 5 min and (c) 40 min. The scale bar represents 0.2 mm in (a) and (b). |
The as-observed mechanism of Pb mineralisation can be described as a four step process. For clarity, the different steps discussed in the following are illustrated in Scheme 2.
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Scheme 2 Tentative sketch of the 4 steps for Pb clogging during TEOS polycondensation: (a) formation of a polymer layer at Pb interfaces; (b) full coverage of Pb interfaces and thickening of the layer; (c) formation of SP by pull-out of the layer from the interface; (d) cork formation. |
Step 1. At first, one observes preformed “primary” particles (PPs) flowing in the Pb bulk. The majority of these particles crossover the whole Pb along the length without noticeable displacement perpendicular to the flow direction. When PP gets to the bottom node, a small fraction of PP is brought against the Pb interface where they come “to be stored” in the course of time (Fig. 4a–c). According to this scenario, an interfacial polymeric layer, with an open structure, results from individual PP aggregation. The adhesion seems to be ‘irreversible’ as long as the adsorbed PPs are weakly sheared by the bulk flow. In this respect, PP can be described as amphiphilic species.
Step 2. Then, the interfacial layer formed at the bottom of Pb is slowly carried along the Pb interface toward the top node by surface counter-currents. This ascension process is much slower than bulk velocity and is probably driven by the Gibbs–Marangoni effect.13 When the layer reaches the top of Pb interface, the counter-current vanishes thus promoting layer thickening and densification. At the end of this second step the Pb interface is covered by a polymeric layer composed of PP.
Step 3. When the interfacial layer reaches a critical thickness, the shearing action exerted by the bulk flow pulls out, fully or partially, the layer from the interface. Thus, large polymeric objects come into the Pb bulk through this erosion process. Their typical size is ∼100 μm and they will be called secondary particles (SPs). At the end of step 3, PP and SP are present together in the Pb bulk.
Step 4. after about 40 min of forced drainage at low flow rate, corks with diameter equal to Pb section are progressively formed by successive adhesive collisions between SP themselves in the bottom node area. Once a cork appears the continuous aggregation of PP contributes to expend the structure of the cork to the whole Pb (Fig. 4d). It is interesting to mention that we never observe the above described stratification process and consequently the formation of cork at high flow rate.
This local mechanism is in qualitative agreement with our macroscopic measurements concerning the dynamic of ageing. Indeed, the slow growth of the inorganic network localised at Pb interfaces does not influence the bulk flow and thus drainage for a long time. Our local observations also indicate that inorganic entities are almost absent in the foam films in agreement with the macroscopic measurements concerning the foam volume and the bubble size evolution. This process may also explain why freeze drying is needed to realize solid foams via dilute solutions of molecular precursors since we show here that the mineralisation of the Pb is probably not completed at the time of their freezing. The small size of the final solid foams and their open porosity should be also a consequence of this incomplete and probably heterogeneous polymerisation state across the foam structure. However, more experiments are needed to fully transpose our local observations at the macroscale and notably to understand why the macroscopic measurements do not detect any effect on 2 h measurements while the clogging of single Pb occurs within 40 min? An answer may be that the time necessary for a measurable clogging effect at the scale of the whole foam channel network is strongly delayed compared to one channel due to the different orientations of Pb in real foam volume that must modify the stratification kinetic and the size distribution of the PP as a function of the foam altitude due to progressive adsorption of the smaller PP. One has also to consider the possible artifact arising from the flowing condition in the upper part of the PbA tripod with respect to real Pb.
As the reported mineralisation process is not able to counteract the destabilizing effects of foam drainage, we introduce an alternative strategy in which the mineralisation is assisted by a biopolymer that increases foam stability. To achieve the role of surfactant, thermo-gelator and mesostructuring agent we propose to use gelatin in place of classical molecular surfactants like TTAB. The interest of this kind of biopolymer toward the first two roles are already known. We briefly recall that gelatin gelation is thermoreversible with a sol state above Tgel consisting in a solution of single chains of denatured collagen whereas below Tgel, renaturation of the native triple helix forms a percolating network of helical segments connected by single strand coils. Concerning the last role, it is expected from recent studies14 that such biopolymer could act as template for producing mesoporosity in the wall of the inorganic monolith. To the best of our knowledge gelatin or other biopolymers were never used to realize these three roles simultaneously in the field of HPS elaboration. To validate this alternative we studied the ageing dynamic of gelatin foams (Cgelatin = 10 wt%) and gelatin foams containing TEOS in the course of polycondensation. The gelatin considered here was derived from porcine skin (Tgel ≈ 29 °C, Mw = 1 × 105 g mol−1) nicely provided by Rousselot. We performed measurements on foams prepared at 30 °C and quenched at T = 8 °C < Tgel for the measurements. The time evolution of the relative liquid volume contained in the foam and of the relative foam volume in free drainage configuration at 8 °C (see Fig. 5) indicate that the ageing dynamic is fully arrested from at most t = 400 s, with and without TEOS until several days.
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Fig. 5 Time evolution of the relative liquid volume contained in the foam (full symbols) and of the relative foam volume (open symbols) in free drainage configuration at 8 °C for the reference foaming solution (TTAB, black squares) and foaming solutions containing either pure gelatin (green circles) or gelatin with TEOS (red triangles). |
The present study has unveiled an unexpected mechanism of Pb mineralisation during drainage starting from a dilute solution of inorganic molecular precursors. By observations at the Pb scale, we have shown that polymerisation starts at the Pb interface until the formation of a thick coating that is then pulled-out by the shearing action of the bulk flow. Thus large particles appear in the bulk of Pb and promote Pb clogging by successive adhesive collisions in the bottom node area. This local mechanism is in qualitative agreement with the macroscopic ageing dynamic of the corresponding foams that is almost not affected by the polycondensation process. It may also explain why lyophilisation is often necessary to obtain well defined porous solid by the sol–gel approach starting from a dilute solution of inorganic precursors. As a green alternative to freeze drying and to usual synthetic surfactants, we propose to use a natural water-gelator like gelatin that could achieve the role of surfactant, thermo-gelator and mesostructuring agent.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1sm06778h |
This journal is © The Royal Society of Chemistry 2012 |