Wetting of the membrane to displace air or conditioning liquids is important to exploit the complex porosity of a ltration membrane. This study reveals the details of wetting in multibore membrane based ltration modules. Using magnetic resonance imaging (MRI), we quantify the uid distribution patterns during initial membrane wetting in dead-end permeation mode. The spatio-temporal evolution of aqueous copper sulfate solution wetting the membrane bers was investigated as a function of the applied ux, packing density, and position along the membrane module length. Three initial wetting conditions were examined: delivery-state membranes, ethanol-washed and dried (air- lled) membranes, and ethanol- lled membranes. Signi cant changes in wetting patterns were observed due to interfacial and polymer swelling e ects. This in-situ investigation reveals a slow wetting progression over six hours and more to obtain complete wetting, even at high uxes of 200 LMH. However, an increased ux leads to faster wetting kinetics as the evolving wetting patterns are ux dependent. Packing density of the multibore bers additionally impacts the wetting kinetics by shifting the prevalent pressure conditions. Although in dead-end mode, the wetting progression is non-uniform along the membrane module length. In addition to this parameter study, di erent pre-wetting agents' e ect on the displacement behavior was inves- tigated in depth. This study helps to understand (a) complex wetting phenomena inside multibore membranes in dead-end ltration, (b) the membranes' interaction with their surroundings due to neighboring membranes, and (c) the e ect of the used uid system for displacement on the resulting wetting patterns. Keywords: Wetting, Multibore (Multichannel) Membrane, Magnetic Resonance Imaging (MRI), Fluid-Fluid Displacement, Filtration Processes 1. Introduction Micro ltration and ultra ltration membranes possess a porous nature that allows them to separate particular matter from a liquid system. One type of micro ltration membranes are These authors contributed equally to this work. Corresponding author: manuscripts.cvt@avt.rwth-aachen.de 1 D. Wypysek / Preprint (2021) 1{33 2 multibore membranes. Multibore membranes are monolithic and house several bore channels within one polymer matrix. The monolithic geometry results in higher mechanical robustness compared to single bore bers [ 1{3]. Those improved tensile properties allow multibore membranes for industrial-scale applications in drinking water production and seawater desalination [ 1,4,5], as well as wastewater and surface water ltration [ 2,6{8]. Multibore membranes have been subject to many studies regarding optimized geometry [ 2,4,7,9,10], membrane material [ 11{13], and experimental ow distribution [ 14{18], as well as ow distribution via computational uid dynamics (CFD) [ 16,19,20]. Especially the ow distribution across a multibore membrane has been proven to be very sensitive to non- idealities such as bending, skin damages, or excentric positioning. In turn, this altered ow distribution causes distinct changes in ow patterns across the whole module [16, 17]. A versatile method for the analysis of uid ow distribution and the online and in-situ characterization of membrane ltration is magnetic resonance imaging (MRI) and ow-MRI. MRI is a non-invasive tool to monitor uid velocities and particle depositions. Studies using MRI and ow-MRI for characterization of membrane ltration with liquid-liquid [ 21,22] as well as liquid-solid systems [23{26] have been presented in literature. As the MRI technology also allows the measurement of liquid content present in the membrane, wetted and non-wetted areas inside the porous structure can be distinguished. Incomplete wetting of membranes can in uence the permeation and performance of ltration process signi cantly [ 27{29]. Hence, quantifying the degree of wetting and wetting times are of major importance to judge the extent of permeable porosity. The majority of studies must assume that permeation experiments access a completely solvent-wetted membrane porosity. Comprehensive studies perform washing and wetting experiments prior to the nal transport studies. Here, we report MRI inspection studies on the degree of wetting and correlate these with permeation experiments. We investigate the wetting behavior of commercial polyethersulfone (PES) multibore membranes (SevenBore ™) inside custom made membrane modules. The e ect of packing density inside the module on the membrane wetting is analyzed in detail. Direct wetting of membranes in delivery conditions is compared to pre-wetted membranes. 2 D. Wypysek / Preprint (2021) 1{33 3 Wetting degrees and wetting times are measured. With this study, we aim to provide a more detailed understanding of initial permeation conditions and suggest process conditions allowing a full wetting of the membranes, which in turn leads to overall homogenized transport properties. 2. Background on wetting Membrane wetting is mostly addressed for membrane contactors [ 30], membrane dis- tillation [ 27,31,32], microsieves [ 33], and monoliths [ 34]. However, literature states that micro ltration and ultra ltration membrane performance is signi cantly in uenced by its wetting state [ 35{38]. Conventionally, commercially available ultra ltration membranes are pre- lled with a pore stabilizer such as glycerol [ 39]. This uid needs to be replaced by the feed solution before the lled pore can contribute to the ltration performance. The results presented in the literature indicate that even at advanced stages of ltration, the membranes remain partially non-wetted with water, especially for hydrophobic membranes in microporous parts of the structure [ 35,37]. This incomplete wetting is caused by the non-uniform pore size distribution in polymer networks produced via phase separation. Non- uniform pores that are connected and simultaneously wetted by a uid form communicating networks. This network communication leads to non-uniform wetting [ 40] and e ects like uid droplet trapping and bypassing ows [ 41]. In those cases, the necessary hydrodynamic pressure to wet the pores cannot be reached with the macroscopically applied transmembrane pressure (TMP). Hence, macroscopically non-uniformly wetted structures arise. On a pore-scale level, the necessary pressure for pore wetting  pBdepends on the surface tension , the macroscopic contact angle , and the local pore diameter dPoreof the speci c membrane and liquid system. It can be derived from the Young-Laplace equation (see Equation 1). However, Equation 1 can only describe large pore diameters of macropores and above. At the meso- and micropore level, additional e ects such as van der Waals and electrostatic double-layer forces can have a dominating e ect on the interface [ 42]. As the here investigated membranes are ultra ltration membranes with a nominal pore size of 40 nm, we consider Equation 1 to be valid for our experiments. 3 D. Wypysek / Preprint (2021) 1{33 4 pB=4 cos dPore(1) At the membrane surface - namely where the retention of particular matter takes place - liquid only penetrates the membrane trough wetted pores. Hence, the total liquid transport can be calculated by the sum of all ows through single pores. The volume ow _Vthrough a single pore can be calculated using the law of Hagen-Poisseuille as described in Equation 2. Here, the ow depends on the pore radius r, the local pressure gradient  p, the liquid viscosity, and the pore length L. _V=r4 8p L(2) A non-uniformly wetted membrane decreases the ux across the membrane in two ways. First, the number of pores contributing to the ux is reduced. Thus, the overall ux linearly decreases with an increasing number of non-water-wetted pores. Second, a non-uniformly wetted membrane only allows distinct ow paths for the liquid to pass through the membrane. Hence, ow patterns inside the membrane change, which causes unfavorable pathways limiting the liquid mass transport by increased pressure drop. [16] 3. Materials and methods 3.1. Experimental setup Membrane .The investigated inside-out ultra ltration membranes with a nominal pore size of 40 nm are commercially available PES SevenBore ™ bers supplied by SUEZ Water Technologies & Solutions. The membrane cross-section is provided with a central bore, surrounded by six symmetrically arranged bores close to the outer ber diameter, which is approximately 4 mm . Each bore has an diameter of approximately 0:9 mm . To reveal the membrane pore structure, we used a eld-emission scanning electron microscope (FeSEM, Hitachi High-Technologies Corporation Model S-4800) to image the membrane cross-section. Figure 1 a) depicts the unique pore structure of the investigated membrane bers. While blue framed areas mark the active membrane layer with smaller pore sizes, white framed 4 D. Wypysek / Preprint (2021) 1{33 5 areas depict the areas with bigger pore sizes representing the membrane's support structure. Domains with bigger pore sizes provide mechanical stability of the membrane ber and do not contribute to particular matter separation. Module .For the examination of wetting morphologies inside the pore structure, individual bers must be incorporated into modules. Thereby, three packing densities are investigated, which contain either one, seven, or 16 membrane bers accounting for packing densities of 5 %, 35 %, and 79 %, respectively. The corresponding membrane arrangements are presented in Figure 1 b). We used 3D-printed spacers to ensure constant positioning of the membrane bers inside the single and seven ber modules. The manufactured modules are 50 cm in length and have an internal diameter of 18 mm . Furthermore, the membrane modules contain one connection for the feed stream and one connection for the permeate and retentate, respectively. The wetting behavior is monitored at three positions along the module length (see Figure 1 c)). Distances from the module inlet are8:3 cm for position A-A, 16:6 cm for position B-B and 25 cm for position C-C. Magnetic resonance imaging setup .Figure 1 c) represents the developed experimental setup. All MRI measurements are carried out on a Magritek low- eld NMR tomography system combined with a KEA2low-frequency spectrometer. To control the hardware, Magritek's software Prospa is used. The tomograph operates with a eld strength of 0:56 T at a Larmor frequency of 23:8 MHz . Depending on the experimental procedure, MRI-parameters have to be adjusted. Within the 60 mm inner diameter imaging probe, the membrane module is placed horizontally. Measurements in the tomograph are carried out in the x-y-plane, being the cross-section of the membrane module. All images are obtained using a 2D spin-echo pulse sequence adapted from our previous studies [ 16,26,43]. In all experiments, a uid circuit is established with an ISMATEC®ISM405A gear pump. Thereby, uxes of 50 LMH, 100 LMH, and 200 LMH are applied to the membrane modules. For wetting experiments, a copper sulfate solution ( 3:12 g L1copper(II) sulfate pentahydrate ( CuSO 45 H 2O,99.5%, Carl Roth, Germany) in de-ionized water) was used as aqueous phase, hereafter termed aqueous solution. This concentration is equivalent to 0.05 M ionic strength. The use of CuSO 4-solution 5 D. Wypysek / Preprint (2021) 1{33 6 has a potential in uence on the wetting progress as it might shift the contact angle of the used material system. However, CuSO 4is indispensable for MRI, since the required time to capture a magnetic resonance image is signi cantly reduced. Corresponding mass ow rates across the membrane are monitored with a mini CORI-FLOW ™M14 (Bronkhorst High-Tech B.V., Netherlands). The resulting transmembrane pressure (TMP) is recorded with a Wika pressure transmitter S-11 (Wika, Germany). To ensure comparability between experiments on the impact of ux and packing density, we fabricated an individual module with new membranes for each experiment. Figure 1: Presentation of (a) a FESEM image of the cross-section of the investigated SevenBore ™™membrane bers, (b) three considered membrane packing densities and the correlating membrane arrangements in the membrane module, (c) the experimental setup for the investigation on the wetting behavior of SevenBore ™ membranes via MRI. 3.2. Wetting procedures Typically, ultra ltration membranes are factory- lled with a pore stabilizer, as the pore network in the selective layer tends to collapse when dried. We assume this pre-wetting agent to be glycerol as this is known to be a prominent choice for pore network stabilization. Glycerol as a pre-wetting agent can be removed by washing with ethanol. To investigate the amount of contained pre-wetting agent, as well as to evaluate the approach of ethanol-washing, we immersed single membrane ber samples in EtOH for two hours and dried these in an exsiccator for 30 min. The sample masses were compared to factory- lled membranes, also dried in an exsiccator for the same time. The weight loss of 5 cm membrane samples can be 6 D. Wypysek / Preprint (2021) 1{33 7 seen in Table 1 and con rms that the pre- lling agent is washed-out successfully. Table 1: Weight of 5 cm membrane samples for factory- lled membranes, dried membranes, and washed and dried membranes and the corresponding weight loss after treatment. Con guration Mass [mg/cm] Weight loss [mg/cm] Weight loss [%] Factory- lled 35.54 1.7 - - Dried 33.04 2.50 7.03 Washed and dried 16.94 18.60 52.34 To correlate the MRI data obtained from membrane investigation, we used MRI reference measurements of di erent uid systems to compare signal strengths of the di erent uids used in this study. Details of those reference experiments are shown in Supplement 1. In fact, this data implies that main parts of the membrane are factory- lled with a pore-stabilizing agent that can be removed with ethanol-washing. We, hence, conclude that at least the selective layer of the investigated SevenBore ™membranes is fully pre- lled with a pore-stabilizing agent. However, no de nite conclusion can be drawn about the wetting state of the more macroporous support structure. Based on the ndings of sample masses and MRI