Greasy membranes for clean separations

Significance 

Mimicking nature is one of science’s ways of providing ingenious solutions that in one way or the other improve the quality of human life. Case and point, the lotus-leaf whose study has led to the development of bio-inspired interfacial materials of non-wetting properties that vast technological implications in various fields of science. As anticipated, most auspicious developments possess one or more drawbacks: for this case, the super-hydrophobic surfaces developed are susceptible to failure due to elevated pressures and temperatures and by dissolution of the trapped air into the surrounding fluid. Recently, a novel class of functional surfaces known as slippery liquid-infused porous surfaces (SLIPS), inspired by the Nepenthes pitcher plants, has been introduced. SLIPS possess to capability of multiphase transport without clogging credit to an inherent gating mechanism they possess. The gating mechanism of these surfaces is expected to give rise to anti-fouling properties and multi-phase transport capabilities. Unfortunately, the long-term retention of the infusion liquid has not yet been explored.

To this note, a team of researchers led by Professor Rob Lammertink at University of Twente in The Netherlands investigated the retention of the infusion liquid in slippery liquid-infused membranes (SLIMs) via liquid–liquid displacement porometry (LLDP) experiments combined with microscopic observations of the displacement mechanism. Their work is currently published in the research journal, Soft Matter.

The research technique employed entailed liquid–liquid displacement porometry in a flux-controlled mode where pure water was pushed through polyvinylidene fluoride membranes infused with perfluoropolyether oil. The researchers further utilized a microfluidic chip consisting of square pillars resembling the porous medium to microscopically investigate the displacement mechanism under identical capillary number and viscosity ratio. Lastly, the permeation through SLIMs was related to two-phase flow in porous media so as to confirm the observed displacement mechanisms.

The authors observed that the pores opened in correspondence to the capillary pressure thereby leading to preferential pathways for water transport. Additionally, the LLDP results further suggested the presence of liquid-lined pores in SLIM. The remaining infusion liquid in the microfluidic chip was observed as pools, bridges and thin films around pillars which further confirmed liquid-lining. Moreover, the researchers also noted that the displacement patterns corresponded to capillary fingering in immiscible displacement in the porous media. Lastly, fractal dimension analysis, confirmed that the fractal patterns corresponded to capillary fingering, which was consistent with an invasion percolation with trapping model.

The study by Professor Lammertink and his research team successfully reported on the retention of the infusion liquid in slippery liquid infused membranes during water permeation and microscopic observation of the displacement mechanism. Generally, it was seen that the observed patterns along with the fractal analysis confirmed that the experiment was within the flow regime of capillary fingering which could be described by an invasion percolation with trapping model. Altogether, the presence of the liquid-lined pores after displacement with water has been seen to be crucial for anti-fouling characteristics of SLIM, which makes it a potential candidate for separation processes and future integration into related fields.

Reference

Hanieh Bazyar, Pengyu Lv, Jeffery A. Wood, Slawomir Porada, Detlef Lohse, Rob G. H. Lammertink. Liquid–liquid displacement in slippery liquid-infused membranes (SLIMs). Soft Matter, 2018, volume 14, page 1780

Go To Soft Matter

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