Chapter 6 Stategies for Fouling and Wetting Mitigation in Membrane Distillation Opertation
6.3. Results and Discussion
6.3.6. Force Spectroscopy to Elucidate Oil-Membrane Interaction
To acquire fundamental understandings of the interaction between oil and membranes with different special wetting properties, underwater force spectroscopy was conducted for each membrane using an oil droplet as the force probe. The force curves from the force spectroscopy are shown in Fig. 6.6, showing a unique behavior for the interaction between each membrane with the oil droplet probe. In each measurement, the oil droplet probe first approached the membrane and retracted immediately after contacting the membrane, both at the same speed. Due to oil- membrane adhesion, the retraction stretched the oil droplet and eventually split the droplet into two portions with one remaining on the probe and the other retained by the membrane surface.
Physically, there should be no baseline adhesion force before the oil droplet contacted the membrane and after the split of the oil droplet, as there was no oil-membrane interaction in both cases. However, the measured “pseudo-adhesion-forces”, which were essentially the force exerted
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on the force sensor of the tensiometer, could be different for the two baselines in the receding stage due to the reduction of the oil droplet volume as a result of the split. The decrease of the oil droplet volume caused the simultaneous decrease of the flotation force, gravitational force, and viscous force (for constant-speed movement), which collectively led to a net positive pseudo-adhesion- force compared to the baseline before splitting. Such a baseline difference represents the amount of oil droplet retained by the membrane and thus complements the actual adhesion force in assessing the oil-membrane interaction.
Figure 0.6 Force curves for the interactions between the PVDF membrane (black curve), the composite membrane (red curve), and the omniphobic membrane (blue and green curves) and a mineral oil droplet. The force curves display the forces measured by the tensiometer sensor as a function of probe displacement. All force curves were collected for the oil-membrane interaction under DI water, with the exception for the one that was collected in a Triton-X solution (the green curve). The initial position of the oil droplet and the initial force applied on the oil droplet were both set as zero.
For the PVDF membrane, upon the physical contact between the oil droplet and the membrane surface, there was a drastic increase in the adhesion force. This strong attraction between the PVDF membrane surface and the approaching oil droplet is attributable to the strong and attractive hydrophobic-hydrophobic interaction[241]. The underwater adhesion force between the oil droplet and the PVDF membranes (~200 μN) was significantly stronger than that of the composite membrane and the omniphobic membrane. In addition, a large fraction of oil was retained by the PVDF membrane, which reduced the volume of the oil droplet probe that translated to a net force of ~130 μN for the new baseline in the receding stage.
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The oil droplet interacted with the composite membrane in a drastically different manner compared to that with the PVDF membrane. First of all, no strong adhesion force was detected upon the contact between the oil droplet and the superhydrophilic surface of the composite membrane. On the contrary, a small repulsion (reflected as negative adhesion force) was observed, most likely due to the repulsive hydration force. Stronger repulsion was likely to be observed if we further pressed the oil droplet probe against the membrane surface. However, we intentionally chose to retract immediately right after the oil-membrane contact because the ring-shaped tensiometer probe (to which the oil droplet was hung) could be forced into oil droplet and generate information irrelevant to the actual oil-membrane interaction. Furthermore, the pseudo-adhesion force for the baseline in the receding stage was very low compared to that with the PVDF membrane, suggesting little retention of oil on the membrane surface due to the very weak adhesion between the oil and the superhydrophilic surface of membrane.
The interactions between the oil droplet and the omniphobic membrane were the most interesting. For the interaction in DI water, similar to the interaction with PVDF membrane, a sudden increase in adhesion force was observed at the end of the advancing stage due to the hydrophobic-hydrophobic interaction. This is consistent with the results of our fouling experiments which showed similar fouling behaviors for both the hydrophobic PVDF membrane and the omniphobic membrane. However, the fraction of oil retained by the omniphobic membrane was significantly lower than that by the PVDF membrane, as indicated by the very low pseudo- adhesion force measured in the baseline of the receding stage. In fact, the fraction of oil droplet retained by omniphobic membrane was even similar to that retained by the composite membrane with a superhydrophilic surface, even though the two surfaces have drastically different upon- contact interactions with the oil droplet. In this regard, the omniphobic membrane can be considered “attractive but not adhesive” to oil, which is consistent with the observation in the CA measurements that the oil droplet spread reasonably well on the omniphobic membrane surface but did not penetrate into the membrane pores because of the reentrant structure.
The presence of the excess synthetic surfactants, Triton X-100, in the solution where the force curve was measured, drastically changed the oil-membrane interaction. The force curve for the interaction of the oil droplet with the omniphobic membrane in the presence of the surfactants was similar to that with the composite membrane in DI water. The observation of the oil-membrane interaction in the presence of surfactants was very interesting but not particularly surprising. As
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discussed in section 6.3.5, the surfactants ‘wrapped’ the oil droplets adsorbing onto the oil-water interface and rendered the oil droplets hydrophilic. Consequently, the attractive oil-membrane interaction was significantly reduced due to hydration force resulting from strong hydration of the hydrophilic heads of the adsorbed surfactants. Similar behavior of fouling mitigation has also been observed in a previous study in which Pickering emulsion stabilized by nanoparticles showed appreciably lower membrane fouling propensity in ultrafiltration compared to an emulsion stabilized simply by naturally occurring surfactants[148].