Impact of surface patterning on membrane performance: experimental analysis of CaSO4 scaling and hydrodynamic insights from PIV

Surface patterning of thin-film composite (TFC) membranes has emerged as a promising approach to enhance membrane performance in water treatment and desalination applications [1]. Numerous studies have demonstrated that imprinted surface patterns can induce local mixing effects, mitigating biofouling and scaling [2, 3], while also increasing the membrane’s active surface area, thereby improving pure water permeability (PWP) [4]. However, none of the respective studies investigated the performance of surface-patterned TFC membranes in combination with feed spacers, which are typical components in spiral-wound modules. Moreover, the direct patterning of dense reverse osmosis (RO) membranes via thermal embossing presents challenges, as significant plastic deformation may damage the crosslinked polyamide layer, negatively impacting PWP and/or permselectivity [5]. So far, the preparation of surface-patterned TFC membranes via a two-step approach [3, 4] (i.e., surface patterning of the membrane support followed by interfacial polymerization) has been the most reliable option. In this presentation, the development of surface-patterned RO TFC membranes with pronounced regular microstructures via the direct patterning thermal embossing method will be discussed. A systematic study of the imprinting conditions and their impacts on both the topographical characteristics and permselectivity of TFC membranes was conducted. Additionally, the results of lab-scale scaling experiments using surface patterned TFC membranes, combined with typical diamond-shaped feed spacer, will be presented. Furthermore, simulations and lab-scale experiments, conducted in spacer-free channels, revealed that surface-patterned membranes can modify fluid characteristics in direct membrane vicinity, and accordingly, decrease both membrane fouling and concentration polarization[4, 6]. Several mechanistic explanations (e.g., vortex shielding, secondary flow generation, shear-induced back-diffusion) were proposed in literature [1]; however, they were either rather case-specific or sometimes even contradictory. To date, there is no solid experimental evidence concerning the improved fluid characteristics in the immediate vicinity of surface-structured membranes. The current study aims at sufficient understanding of the effect(s) of topographical membrane surface modification on fluid characteristics and examinesreliable mechanisms for particles (or foulants) deposition on surface-patterned membranes in spacer-filled channels by means of real-time hydrodynamic analysis using Particle Image Velocimetry (PIV).

Experimental: Surface-patterned TFC membranes were prepared via hot embossing micro-imprinting lithography using commercial FilmTec™ LC LE-4040 membranes and a brass mold (27 × 7 cm²) featuring a regular lines-and-grooves pattern (line width: 20 μm, line-to-line distance: 20 μm, groove depth: 10 μm). The patterning fidelity was analyzed using 3D optical profilometer, while topographical characteristics (e.g., roughness) were analyzed using scanning electron microscopy and atomic force microscopy. Membrane performance was assessed via lab-scale testing of PWP and NaCl retention. Anti-Scaling performance, with/-out feed spacer, was compared vs. two reference flat-sheet membranes (pristine membrane and compacted membrane at analogous conditions as surface-patterned membrane). Hydrodynamic analyses were carried out with constant crossflow velocity of 0.15 m/s. Tracer particles with different characteristics were employed. Shake-the-Box (STB, Lagrangian Particle tracking) PIV with instantaneous measurement of three velocity components in a complete 3D measurement volume was employed. Different operating conditions (with/-out permeation, with/-out feed spacer) were examined.

Results and discussion: Surface-patterned TFC membranes with pronounced microstructures (pattern height up to 5 μm) were successfully achieved while maintaining dense polyamide layer, see Fig.1.(a), a correlation between patterns dimensions and PWP could be established. Additionally, surface-patterned membrane exhibited generally better antiscaling performance than flat membranes, while performance results in cases of feed spacer-free and feed spacer-filled channels differ substantially. Hydrodynamic measurement using STB in spacer-free channel at non-permeation and permeation conditions revealed that surface-patterning can indeed modify fluid characteristics in the feed-retentate channel. This was consistent with our earlier measurements using planar 2D-PIV[7], where vector velocity profiles showed substantially improved fluid characteristics for surface-patterned membranes compared to flat-sheet membranes. Acknowledgments This work is funded by the German Research Foundation (DFG) – project number: 499318495.