Liquid uptake in porous cellulose sheets studied with UFI-NMR:Penetration, swelling and air displacement

Liquid penetration in porous cellulosic materials is crucial in many technological fields. The complex geometry, small pore size, and often fast timescale of liquid uptake makes the process hard to capture. Effects such as swelling, vapor transport, film flow and water transport within cellulosic material makes transport deviate from well-known relations such as Lucas-Washburn and Darcy's Law. In this work it is demonstrated how Ultra-Fast Imaging NMR can be used to simultaneously monitor the liquid distribution and swelling during capillary uptake of water with a temporal- and spatial resolution of 10 ms and 14.5–18 μm respectively. The measurements show that in a cellulose fiber sheet, within the first 65 ms, liquid first penetrates the whole sheet before swelling takes place for another 30 s. Furthermore, it was observed that the liquid front traps 15 v% of air which is slowly replaced by water during the final stage of liquid uptake. Our method makes it possible to simultaneously quantify the concentration of all three phases (solid, liquid and air) within porous materials during processes exceeding 50 ms (5 times the temporal resolution). We hence believe that the proposed method should also be useful to study liquid penetration, or water diffusion, into other porous cellulosic materials like foams, membranes, nonwovens, textiles and films.</p

Increasing particle concentration enhances particle penetration depth but slows down liquid imbibition in thin fibrous filters

The transport of particles within thin, porous media is a complex process which received growing attention due to its applications in filtration, printing and microfluidics devices. The effect of particles on liquid imbibition and particle clogging can reduce the performance and lifetime of these applications. However, these processes are still not clearly understood and are challenging to investigate. The goal of this study is to increase our understanding about the effect of particle concentration on the imbibition process in thin fibrous membrane filters. In this study, an Ultra-Fast Imaging NMR method is used to study the particle penetration inside nylon membrane filters for particle suspensions with varying particle concentrations (C0). The measurements revealed that increasing the particle concentration increases the particle penetration depth S(t) as governed by a Langmuir isotherm given by S(t)=l(t)(1+κC0)/1+κ(C0+Cb,m), with Cb,m the bound particles and κ the binding constant. Secondly, in droplet penetration, particles slow down liquid penetration in a Darcy like manner where effect on viscosity (η) and surface tension (σ) determine the penetration speed rather than changes within permeability (K0). The final liquid front (l), scaled according to l2∝σt/η. The particle penetration depths were verified using scanning electron microscopy images.</p

Magnetite-latex nanoparticle motion during capillary uptake in thin, porous layers studied with UFI‐NMR

The transport of nanoparticles in porous media has received growing attention in the last decades due to environmental concerns in, for example, the printing industry, filtration, and transport of pollutants. Experimental studies on the imbibition of particle dispersions in porous media with sufficiently high spatial and temporal resolution are still challenging. This study shows how Ultra-Fast Imaging (UFI) NMR is an ideal tool for studying Fe3O4-latex particles penetration with a temporal resolution of 15 ms and spatial resolution of 18 µm. In the first part, it is shown that a calibration curve between the UFI‐NMR signal intensity and the particle concentration exists. In the second part, UFI‐NMR is used to study the penetration of a particles inside a thin nylon membrane during capillary uptake, which revealed liquid-particle front splitting and an inhomogeneous buildup of the particle concentration. Both the liquid-particle front splitting and inhomogeneous build up could be verified by Scanning Electron Microscopy. Our method allows to determine particle concentration profiles during capillary uptake within thin, porous media. Therefore, the technique can be easily extended to study particle penetrations in a wide variety of systems such thin interfaces, biomaterials, films, and filter media.</p

Validation of FEM models describing moisture transport in heated concrete by Magnetic Resonance Imaging

Fire safety of buildings and structures is an important issue, and has a great impact on human life and economy. One of the processes negatively affecting the strength of a concrete building or structure during fire is spalling. Many examples exists in which spalling of concrete during fire has caused severe damage to structures, such as in the Mont Blanc and Channel Tunnel. Especially newly developed dense types of concrete such as HPC and SCC, have shown to be sensitive to spalling, hampering the application of these new concrete types. To reduce risks and building costs, the processes behind spalling need to be understood. Increasing our knowledge allows us to reliably predict the behaviour and take effective and cost friendly preventive measures. Moisture present in concrete is one of the reasons for spalling. When concrete is heated water will evaporate, which results in a high gas pressure inside the pores of concrete. This high gas pressure can induce spalling. To attain a better understanding of this process, a first step was taken to develop a finite element model (FEM) describing this transport of moisture in heated concrete. However, the validity of all current models (including our own) is unknown because of debatable input parameters and lack of experimental techniques to follow the transport process in situ. In cooperation with the Eindhoven University of Technology moisture transport in heated concrete can now be investigated with a home built dedicated 1D Magnetic Resonance Imaging (MRI) setup. Using the results of the MRI experiments the validity of our FEM models has been assessed for the first time. A general correspondence is observed. The FEM model described in this paper is a simplified FEM model compared to literature models. Already this simplified model shows a good correspondence with the MRI measurements

Modelling biocide release based on coating properties

Growth of micro-organisms on coated substrates is a common problem, since it reduces the performance of materials, in terms of durability as well as aesthetics. In order to prevent microbial growth biocides are frequently added to coatings. Unfortunately, early release of these biocides reduces the biocidal protection of these coatings. Furthermore, since biocides leach to the environment, their release rate is a key parameter determining the environmental impact of biocides. As a result of legislation, the use of biocides and the corresponding concentrations is restricted. Understanding how coating and biocide properties determine biocide release rate is crucial to understand durability, and enable new product development and evaluation of environmental impact. The present study aims to establish this connection via modelling of the release process. The proposed model is presented with an initial validation. The release is viewed as a combination of water diffusion, dissolution of the embedded biocide and release from the material. The key parameters necessary for the model are bulk parameters determined from experiments. The resulting model shows that release can be proportional to water exposure time or to its square root, depending on the dominating processes in a certain regime. In application conditions high variabilities exist in terms of materials and conditions, therefore the effect of environmental conditions is investigated by a statistical approach. The results show that the variability in the time of protection (the time during which biocides are still present in the coating) is high, due to the variabilities in the weather conditions. A variation of 1 year (95% interval) was seen on an average time of protection of 1.5 years. The model can be used to assess new approaches towards material development and helps underpinning product performance claims