Janus Membranes via Diffusion‐Controlled Atomic Layer Deposition

Abstract

The first use of atomic layer deposition (ALD) to produce Janus membranes is reported, with an example system consisting of a compositional gradient ranging from hydrophilic Al2O3 on one face to hydrophobic poly(propylene) on the opposite face. Alternating pulses of trimethyl aluminum and water vapor lead to the growth of covalently bonded Al2O3 conforming to the membrane pore surfaces. Precise control of ALD parameters significantly affects the surface wetting of the modified membrane face and the depth of Al2O3 infiltration into the porosity. This depth control derives from slow precursor diffusion through the 200 nm membrane pores compared to much faster ALD surface reactions. For a given precursor exposure and purge time, increasing the number of ALD cycles decreases the water contact angle at the modified surface from hydrophobic to hydrophilic, until the water droplet is completely imbibed by the membrane. To demonstrate the utility of these Janus membranes, a hydrophilic/superaerophobic Janus treatment is shown to greatly reduce the size of air bubbles generated through the membrane, enabling faster mixing. This technique represents the first application of vapor‐deposited covalently bonded metal oxides to form Janus membranes. Further opportunities are afforded by the ability to laterally pattern Al2O3 across the membrane surface via physical masking.

Reducing Adhesion for Dispensing Tiny Water/Oil Droplets and Gas Bubbles by Femtosecond Laser‐Treated Needle Nozzles: Superhydrophobicity, Superoleophobicity, and Superaerophobicity

“…The generation of fine bubbles has a broad range of applications in aquaculture, environmental remediation, chemical engineering, and energy production. [5,6] The size of the bubbles has a huge impact on the process efficiency in those applications. Fine bubbles can greatly increase the gas/liquid contact area, thereby improving the gas/liquid mass transfer.…”

“…Generation and manipulation of tiny liquid droplets (down to micro-or nanoliter scale) and fine bubbles have attracted an ever increasing amount of interest because of the broad applications, such as liquid transportation, inkjet printing, highresolution three-dimensional (3D) printing, cell engineering, micro-reactor, bio-analysis, bio-sensing, energy production, chemical engineering, and environmental remediation. [1][2][3][4][5][6] Signicant efforts have been devoted from scientific and industrial communities to produce smaller droplets by reducing the nozzle size or through assistance of special driving mechanisms (e. g., mechanical-, electrical-, and thermal-driven equipments) in the past. [7][8][9][10] However, the traditional nozzles and dispensing methods still face many limitations.…”

“…[1][2][3][4][5][6] Signicant efforts have been devoted from scientific and industrial communities to produce smaller droplets by reducing the nozzle size or through assistance of special driving mechanisms (e. g., mechanical-, electrical-, and thermal-driven equipments) in the past. [7][8][9][10] However, the traditional nozzles and dispensing methods still face many limitations. For instance, the driving equipment usually makes the dispensing system become bulky and also increase the complexity of operation.…”

Asymmetric Surface Engineering for Janus Membranes

“…The diffusion confinement can also be achieved by single‐side vapor deposition [ 79–81 ] in which the diffusion area of the vapor containing reactive modifiers is imprisoned within those partially preactivated pores in the transmembrane direction. The construction of the opposite wettable layers relies on the reactions between the modifiers and active sites of membrane.…”

“…Another interesting application of Janus membranes with asymmetric wettability is for bubble aeration, showing superiority over the homogeneously hydrophobic or hydrophilic membranes with similar pore size (Figure 5b). [ 50,79 ] The hydrophilic layer is commonly aerophobic underwater. It is able to accelerate the gas escape from the aerophobic layer into water to form small‐sized bubbles which can facilitate the gas−liquid mass transfer in applications due to their large contact area and long retention time.…”