Characterization of super-oil-wettability under water

Title: Synthesis of Chemically-Reactive Polymeric Multilayer Coating *

2.3. Results and Discussions

2.3.1. Characterization of super-oil-wettability under water

deposition of 20 bilayers of NC, an arbitrary hierarchical and porous topography was observed under FESEM (Fig. 2.5C,G). Whereas, the topography of the multilayer of BPEI was noticed to be featureless and smooth (Fig. 2.5D,H), even after repeating deposition cycles for 20 times. The gradual change in the morphology in the multilayer of NC was exploited later in revealing the role of surface topography behind the underwater oil-wettability property.

strategically selected glucamine molecule through 1,4-conjugate addition reaction provided underwater superoleophobicity with an advancing oil contact angle (OCA) of 170.8° (Fig. 2.7F). This post-chemical modification is referred as ‘Glu-treated’ in rest of the discussion. In comparison to that, after same post-

chemical modification (with glucamine) of multilayer (20 bilayers) of BPEI, the featureless (Fig. 2.5D,H) and inherently underwater oleophilic (OCA of 25.5°; Fig. 2.7G) coating became underwater oleophobic

with an OCA of 123.8° as shown in Fig. 2.7H. Thus, this study further revealed the need for a porous hierarchical topography to achieve underwater superoleophobicity. Moreover, it was noticed that the underwater oil-wettability of the ‘Glu-treated’ multilayer of NC was varied by increasing the number of bilayer depositions. The underwater static OCA gradually increased from 135.6° (2 bilayers; Fig. 2.8A) to 165.2° (20 bilayers’ Fig. 2.8E) for the ‘Glu-treated’ multilayer of NC on increasing the number of

Figure 2.8: CA images showing the change in static OCA of beaded oil droplets on post-modified (A-E: glucamine) multilayer of NC with increasing the LbL deposition cycles.

Figure 2.7: (A-F) The CA images of beaded water (A-B) and oil (C-F) droplets on multilayer of NC (A-F) in air (A-D) and under water (E-F), before (A,C,E) and after (B,D,F) post modification with glucamine. (G-H) CA images of beaded oil droplet on untreated (G) and glucamine treated multilayer (H) of BPEI under water.

bilayer depositions (Fig. 2.8) from 2 to 20. Thus, this current study confirmed the need for appropriate chemical (hydrophilic/hydrophobic) and physical (surface topography) optimizations to achieve the

desired underwater anti oil-wettability. Moreover, these ‘Glu-treated’ multilayer of NC were capable of displaying underwater super oil repellency with a wide range (in terms of surface tension) of water immiscible model oils (Fig. 2.9) starting from hexane (17.89 mN m^-1; Fig. 2.9A) to DCE (32.2 mN m^-1;

Fig. 2.9G). On dropping of a model oil (DCM) droplet (11 mL) on to the ‘Glu-treated’ multilayer (20

Figure 2.10: (A-E) CA images accounting bouncing of DCM (model oil) droplet (11 μl of DCM) on ‘Glu-treated’

multilayer (20 bilayers) of NC.

Figure 2.9: A-G) CA images of various water immiscible organic solvents that were beaded on the post-functionalized (glucamine) multilayer of NC. The surface tensions of the selected model oils, varied from 17.89 mN m^-1 (hexane) to 32.2 mN m^-1 (DCE).

bilayers) of NC from a height of 12.5 mm, it was noted that the oil droplet bounced (Fig. 2.10) multiple times before eventually settling on the multilayer with an OCA of 165° (Fig. 2.10E). On the other hand, beaded oil droplet readily rolled off (Fig. 2.11) when the surface was tilted at 3°. This extreme oil- repellency under water is often explained using the Cassie–Baxter model,⁴ where the heterogeneous

wetting of oil on a multilayer of NC is due to the impregnation and confinement of aqueous phase within the hierarchically featured surface. The confined water layer within the ‘Glu-treated’ porous NC multilayer reduced the contact area between the beaded oil droplet and ‘Glu-treated’ multilayer of NC.

Furthermore, the fractional area of contact between the liquid droplet (model oil DCM droplet) and the

‘Glu-treated’ multilayer of NC was estimated using eqn (2.1) and eqn (2.2),

cos θ

CB

= f

1

cos θ

1

+ f

2

cos θ

2………... (2.1)

f

1

+ f

2

= 1

………

(2.2)

where,

θ

1 and

θ

CB are the oil CAs on the multilayer (20 bilayers) of BPEI (smooth surface, glu- treated:123.8°) and NC (porous, glu-treated: 165.2°), respectively. The fraction of contact areas of the beaded oil droplet with both the ‘Glu-treated’ multilayer of NC and trapped aqueous phase were labelled as f1 and f2, respectively. The estimated fraction of the contact area of the beaded oil droplet on the ‘Glu- treated’ NC multilayer (20 bilayers) was 0.075, whereas the values were 0.621 and 0.86 for the 15- and 10-bilayers coatings (where each coating was ‘Glu-treated’), respectively. The increase in fraction of contact area between the beaded oil droplet and the solid NC multilayer induced more adhesive interaction between the liquid and solid phases. This was mostly because of the difference in the surface topography

Figure 2.11: CA images illustrating the rolling of 11 μl DCM droplet at tilting angle of 3, where DCM droplet was dispensed from a height of 0.7 cm.

of the multilayer as confirmed by the FESEM study (Fig. 2.5). The topography of multilayer coatings plays a critical role in controlling the adhesive interaction of the beaded liquid droplet on the solid interfaces,³¹ and the appropriate hierarchical topography is essential for minimizing (Cassie state) the contact area between the beaded oil droplet and the solid interface. After 20 bilayers of NC depositions (Fig. 2.5C,G), the multilayer of NC was inherently decorated with an essential hierarchical topography, and as a result, such interface displayed non-adhesive superoleophobicity under water. However, a less prominent microdomains in the multilayer of NC having 10 (Fig. 2.5A,E) and 15 (Fig. 2.5B,F) bilayers of depositions were attributed to the higher contact area between the beaded oil droplet and the NC multilayer, and eventually the multilayer adopted a Cassie–Wenzel transition state.³¹