Liquid-liquid displacement in slippery liquid-infused membranes (SLIMs)

Liquid-infused membranes inspired by slippery liquid-infused porous surfaces (SLIPS) have been recently introduced to membrane technology. The gating mechanism of these membranes is expected to give rise to anti-fouling properties and multi-phase transport capabilities. However, the long-term retention of the infusion liquid has not yet been explored. To address this issue, we investigate 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. Our results reveal that pores will be opened corresponding to the capillary pressure, leading to preferential flow pathways for water transport. The LLDP results further suggest the presence of liquid-lined pores in SLIM. This hypothesis is analyzed theoretically using an interfacial pore flow model. We find that the displacement patterns correspond to capillary fingering in immiscible displacement in porous media. The related physics regarding two-phase flow in porous media is used to confirm the permeation mechanism appearing in SLIMs. In order to experimentally observe liquid-liquid displacement, a microfluidic chip mimicking a porous medium is designed and a highly ramified structure with trapped infusion liquid is observed. The remaining infusion liquid is retained as pools, bridges and thin films around pillar structures in the chip, which further confirms liquid-lining. Fractal dimension analysis, along with evaluation of the fluid (non-wetting phase) saturation, further confirms that the fractal patterns correspond to capillary fingering, which is consistent with an invasion percolation with trapping (IPT) model

Universality in microdroplet nucleation during solvent exchange in Hele-Shaw like channels

Micro and nanodroplets have many important applications such as in drug delivery, liquid-liquid extraction, nanomaterial synthesis and cosmetics. A commonly used method to generate a large number of micro or nanodroplets in one simple step is solvent exchange (also called nanoprecipitation), in which a good solvent of the droplet phase is displaced by a poor one, generating an oversaturation pulse that leads to droplet nucleation. Despite its crucial importance, the droplet growth resulting from the oversaturation pulse in this ternary system is still poorly understood. We experimentally and theoretically study this growth in Hele-Shaw like channels by measuring the total volume of the oil droplets that nucleates out of it. In order to prevent the oversaturated oil from exiting the channel, we decorated some of the channels with a porous region in the middle. Solvent exchange is performed with various solution compositions, flow rates and channel geometries, and the measured droplets volume is found to increase with the P\'eclet number $Pe$ with an approximate effective power law $V\propto Pe^{0.50}$. A theoretical model is developed to account for this finding. With this model we can indeed explain the $V\propto Pe^{1/2}$ scaling, including the prefactor, which can collapse all data of the "porous" channels onto one universal curve, irrespective of channel geometry and composition of the mixtures. Our work provides a macroscopic approach to this bottom-up method of droplet generation and may guide further studies on oversaturation and nucleation in ternary systems.Comment: Published in Journal of Fluid Mechanics. 16 pages, 6 figure

Adaptive pores in next generation membranes

In this thesis a novel class of polymeric membranes known as slippery liquid-infused membranes are studied. In these membranes pores are filled with an infusion liquid which is typically an oil. The chemical compatibility between the oil and the membrane material is a requisite to ensure high affinity between these two. Once this criteria is satisfied, the infusion liquid spontaneously infiltrates the surface structure/pores and forms a smooth lubrication layer on the surface. This lubrication layer gives important features to these materials such as, low water contact angle hysteresis, self-healing and repelling a variety of liquids. Liquid-infused pores act like a gate that can open and close in response to the system pressure, thus called “Adaptive pores”. Once appropriate pressure is applied, certain species can be transferred through the pores while others are being retained. This is called gating capability which can be used for efficient sorting of immiscible fluids from a mixture in separation applications. After a general introduction in chapter 1, characterization, fabrication and gating mechanism of these membrane are studied in chapter 2. The oil retention inside the pores at high flux of permeating fluid (water) is investigated in chapter 3. Chapter 4 covers the application of these membrane for oil permeation from oil-in-water emulsions which requires movement of oil droplets towards the membrane. This is achieved here by gravity-driven creaming in dead-end filtration mode. The anti-biofouling performance is further studied in chapter 5 via long-term cross flow filtration. As an alternative technique for movement of oil droplets towards the membrane surface, an external field such as acoustic can be applied. The behavior of oil droplets in such field is investigated through a microfluidic study in chapter 6. Permeability is an important transport property of any porous media, including membranes. Numerical prediction of permeability suggest a universal scaling power-law for normalized permeability. This is experimentally validated via microfluidic experiments in chapter 7. The overall overview to the work together with reflections and perspectives are presented in chapter 8. The idea of gravity-driven oil permeation through the membranes is discussed in crossflow filtration mode at low oil concentrations. Original promotion date was May 20, 2020 (COVID-19

On the Gating Mechanism of Slippery Liquid Infused Porous Membranes

Slippery liquid infused porous surfaces owe some of their remarkable properties such as low fouling, multiphase transport without clogging, and self-healing to the presence of liquid lined pores. In this paper, the gating mechanism of liquid infiltrated porous membranes is investigated. The liquid film thickness lining the pores is experimentally obtained and correlated to annular two phase flow model

A review on nature-inspired gating membranes: From concept to design and applications

Nature has been a constant source of inspiration for technological developments. Recently, the study of nature-inspired materials has expanded to the micro- and nanoscale, facilitating new breakthroughs in the design of materials with unique properties. Various types of superhydrophobic surfaces inspired by the lotus/rice leaf are examples of nature-inspired surfaces with special wettability properties. A new class of functional surfaces whose design is inspired by the pitcher plant are the slippery liquid-infused porous surfaces (SLIPS). This Review summarizes the properties, design criteria, fabrication strategies, and working mechanisms of both surfaces with specific focus on SLIPS. The applications of SLIPS in the field of membrane technology [slippery liquid-infused membranes (SLIMs)] are also reviewed. These membranes are also known as liquid gating membranes due to the gating functionality of the capillary-stabilized liquid in the membrane pores leading to a smart gating mechanism. Similar to the gating ion channels in biological systems, the pores open and close in response to the ambient stimuli, e.g., pressure, temperature, and ions. Different types of stimuli-responsive smart gating membranes are introduced here, and their properties and applications are reviewed in detail. Finally, challenges and perspectives on both SLIPS and smart gating membranes are discussed. This Review provides a thorough discussion and practical applications of nature-inspired functional surfaces and membranes to pave the way for future research and further developments in this emerging field

Application of liquid-infused membranes to mitigate biofouling

Among different types of membrane fouling, biofouling is a critical issue which can significantly reduce the process productivity. If the initial phase of the microorganism attachment to the pore wall is prevented, a remarkable reduction in biofilm formation can be obtained. A novel approach to achieve this goal is the infusion of the porous membrane with an infusion liquid (oil) forming liquid-infused membranes (LIMs). It has been shown that the pore wall during permeation is still covered with the infusion liquid forming so-called liquid-lined pores. The liquid-lining can enhance anti-biofouling performance by preventing direct contact between the microorganisms and pore wall. Here, we investigate the capability of LIMs in mitigation of biofouling by conducting long-term cross-flow filtration experiments at constant flow rate for approximately 10-20 days. The results show significantly lower increase in transmembrane pressure (TMP) values for LIMs compared to non-infused counterparts (dry membranes). The bacterial growth curves are further investigated by fitting a sigmoidal function (logistic model). Approximately 4 times increase in the lag period λ and 7 times decrease in the bacterial growth rate μm are observed for LIMs compared to dry membranes revealing improved anti-biofouling performance of LIMs

Liquid-liquid displacement in slippery liquid-infused membranes (SLIMs)

Liquid-infused membranes inspired by slippery liquid-infused porous surfaces (SLIPS) have been recently introduced to membrane technology. The gating mechanism of these membranes is expected to give rise to anti-fouling properties and multi-phase transport capabilities. However, the long-term retention of the infusion liquid has not yet been explored. To address this issue, we investigate 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. Our results reveal that pores will be opened corresponding to the capillary pressure, leading to preferential flow pathways for water transport. The LLDP results further suggest the presence of liquid-lined pores in SLIM. This hypothesis is analyzed theoretically using an interfacial pore flow model. We find that the displacement patterns correspond to capillary fingering in immiscible displacement in porous media. The related physics regarding two-phase flow in porous media is used to confirm the permeation mechanism appearing in SLIMs. In order to experimentally observe liquid-liquid displacement, a microfluidic chip mimicking a porous medium is designed and a highly ramified structure with trapped infusion liquid is observed. The remaining infusion liquid is retained as pools, bridges and thin films around pillar structures in the chip, which further confirms liquid-lining. Fractal dimension analysis, along with evaluation of the fluid (non-wetting phase) saturation, further confirms that the fractal patterns correspond to capillary fingering, which is consistent with an invasion percolation with trapping (IPT) model

The permeability of pillar arrays in microfluidic devices: an application of Brinkman's theory towards wall friction

Darcy's law describes the flow of Newtonian fluids through bulk porous media as the product of the applied pressure difference, the fluid's viscosity and the medium's permeability. Brinkman extended Darcy's law with a viscous stress term, thereby enabling boundary conditions to the flow field at the surface of the medium. The validity of Brinkman's term, and the value of its effective viscosity, have been heavily debated since their introduction nearly 75 years ago. We use experiments and Multibody Dissipative Particle Dynamics (MDPD) simulations to study flows through ordered and disordered pillar arrays in microfluidic channels of limited height. We find that the simulated velocity profiles are well described by an expedient interpretation of Brinkman's theory. Depending on the solid volume fraction and pillar arrangement, the effective viscosity varies between two and three times the bulk fluid viscosity. The calculated effective permeabilities of the flow devices, combining the flow resistances due to the pillars and the walls by Brinkman's theory, agree well with the experimental data. This approach enables fast and accurate estimates of the effective permeability of micropillared chips. The simulated force distributions over the walls and pillars require an effective viscosity equal to the bulk viscosity and an elevation-dependent permeability of the pillar array