Cooperativity and segregation in confined flows of soft binary glasses
Laar, T. van de; Schroën, C.G.P.H. ; Sprakel, J.H.B. - \ 2015
Physical Review. E, Statistical nonlinear, and soft matter physics 92 (2015)2. - ISSN 2470-0045 - 8 p.
concentrated suspensions - particle migration - hard - microfiltration - fractionation - transition - diffusion
When a suspension containing particles of different sizes flows through a confined geometry a size gradient can be established, with large particles accumulating in the channel center. Such size separation driven by hydrodynamic interactions is expected to facilitate membrane filtration and may lead to the design of novel and innovative separation techniques. For this, a wide range of particle concentrations has to be investigated, in order to clarify whether shear-induced migration can be utilized at concentrations close to or above the colloidal glass transition, where particle motion is severely hindered and hydrodynamic interactions are screened. We explore this scenario by studying the flow of binary mixtures of soft colloidal microgels, well above their liquid-solid transition, through narrow microchannels. We find that, even though the flow becomes strongly heterogeneous, in both space and time, characterized by a large cooperativity length, size segregation still occurs. This suggests that even above the glass transition shear-induced diffusion could still be used as a fractionation mechanism, which is of great relevance for process intensification purposes.
Druppelen op het randje
Schroen, Karin - \ 2013
food technology - microanalysis - microtechniques - emulsions - microfiltration - droplets
Fouling mechanisms of dairy streams during membrane distillation
Hausmann, A. ; Sanciolo, P. ; Vasiljevic, T. ; Weeks, M. ; Schroën, C.G.P.H. ; Gray, S. ; Duke, M. - \ 2013
Journal of Membrane Science 441 (2013). - ISSN 0376-7388 - p. 102 - 111.
calcium-phosphate - skimmed-milk - osmotic distillation - whey proteins - ultrafiltration - microfiltration - adsorption - stability - spectroscopy - precursors
This study reports on fouling mechanisms of skim milk and whey during membrane distillation (MD) using polytetrafluoroethylene (PTFE) membranes. Structural and elemental changes along the fouling layer from the anchorpoint at the membrane to the topsurface of the fouling layer have been investigated using synchrotron IR micro-spectroscopy and electron microscopy with associated energy dispersive X-ray spectroscopy(EDS)Initial adhesion of single components on a membrane representing a PTFEsurface was observed in-situ utilizing reflectometry.Whey components were found to penetrate into the membrane matrix while skim milk fouling remained on top of the membrane. Whey proteins had weaker attractive interaction with the membrane and adhesion depended more on the presence of phosphorus near the membrane surface and throughout to establish the fouling layer. This work has given detailed insight into the fouling mechanisms of MD membranes in major dairy streams, essential for maintaining membrane distillation as operational for acceptable times, therewith allowing further development of this emerging technology.
Covalent functionalization of silicon nitride surfaces for anti-biofouling and bioselective capture
Nguyen, A.T. - \ 2011
Wageningen University. Promotor(en): Cees van Rijn; Han Zuilhof, co-promotor(en): Jos Paulusse. - [s.l.] : S.n. - ISBN 9789461730084 - 141
microfiltratie - ongewenste aangroei van levende (micro)organismen - aangroeiwerende middelen - oppervlaktechemie - microfiltration - biofouling - antifouling agents - surface chemistry
Microsieves – microengineered membranes – have been introduced in microfiltration technology as a new generation of inorganic membranes. The thin membranes are made of silicon nitride (SixN4), which gives the membranes outstanding features, such as chemical inertness and high mechanical strength. Microsieves have very well-defined pore size and pore shape, with an extremely homogeneous size distribution and high porosity. As a result, high-flux performance and excellent selectivity may be achieved. However, biofouling issues exert limitations on the application of microsieves in filtration and diagnostics. Surface functionalization was found to be a feasible way to minimize biofouling, but also to achieve biorecognition in microbiological applications. The aim of this thesis is to improve microsieve performance in biological applications by means of surface functionalization with organic coatings for protein repellence and selective capture of microorganisms.
High-flux membrane separation using fluid skimming dominated convective fluid flow
Dinther, A.M.C. van; Schroën, C.G.P.H. ; Boom, R.M. - \ 2011
Journal of Membrane Science 371 (2011)1-2. - ISSN 0376-7388 - p. 20 - 27.
particle trajectories - spherical-particle - laminar-flow - porous wall - microfiltration - suspensions - filtration - deposition - system - mechanics
We here report on the separation of yeast cells, with micro-engineered membranes having pores that are typically five times larger than the cells. The separation is due to neither shear-induced diffusion, nor initial lift, but to an effect similar to fluid skimming. The separation performance is linked to the ratio between cross-flow and transmembrane flux, and could be captured with a dimensionless number relating those. On the basis of this dimensionless number, flux and transmission of the cells could be predicted. The mechanism rests on having a sufficiently high cross-flow velocity, such that particles are not dragged too deep in the pore, but are dragged with the cross-flow back into the feed stream. The separation factor can simply be changed by changing the ratio between crossflow velocity and transmembrane flux. Since the membranes have very large pores, fouling does not play a role. Constant high transmembrane flux values of 200–2200 L/m2 h were reached for transmembrane pressures ranging from 0.02 to 0.4 bar (typical industrial fluxes are 150 L/m2 h bar with a maximum of 2000 L/m2 h bar for short periods of time, comparable to 50–400 L/m2 h  and ). Although the effect is strongest with monodispersed pores, it will be possible to exploit the mechanism with conventional membranes. As such, it may open up a new route towards non-fouling crossflow microfiltration
Effective use of enzyme microreactors : thermal, kinetic and ethical guidelines
Swarts, J.W. - \ 2009
Wageningen University. Promotor(en): Remko Boom; Michiel Korthals, co-promotor(en): Anja Janssen. - [S.l. : S.n. - ISBN 9789085853732 - 146
bioreactoren - enzymen - enzymactiviteit - microfiltratie - temperatuur - vloeistofmechanica - industriële enzymen - microtechnieken - microarrays - bioreactors - enzymes - enzyme activity - microfiltration - temperature - fluid mechanics - industrial enzymes - microtechniques - microarrays
Microreactor technology is reported to have many benefits over regular chemical methods. Due to the small dimensions over which temperature and concentration gradients can exist, mass and heat transfer can be very quick. This could minimize the time needed for heating and mixing, due to a reduction in diffusion limitation. Furthermore, a very low fluid to chip volume ratio could facilitate a very stable fluid temperature.
The goal of this thesis research was to investigate the effect of the use of microreactors on enzyme kinetics and the thermal behaviour of fluids inside the chip. First, the effect of the design and use of a microsystem on the fluid temperature inside the microfluidic chip was investigated experimentally and with computer models. A stable and predictable temperature is of great importance for running (enzymatic) processes in a microchip. Next, we used model enzyme reactions to investigate whether the enzyme kinetics were different on micro and bench scale, and when diffusion would play a role. Furthermore, some social and ethical aspects of microreactor technology applications were studied.
To ensure a stable and predictable temperature of the fluids inside the microreactor, the microsystem should be properly designed and used. To test these two aspects, we investigated the effect of practical use (chapter 2) and design parameters (chapter 3) on this fluid temperature. The micro system used in this research consisted of a PEEK chipholder, a relatively small heater, a glass microchip, and surrounding air. We conducted experiments and used computational fluid dynamics models to understand the effect of all varied parameters. In the design of the system, the chipholder shape and material (with its density, specific heat, and thermal conductivity) dominated the temperature of the fluid inside the chip. A temperature gradient as large as 40°C was observed over the length of the chip. This temperature profile at fluid level can be changed by adapting the geometry and material of the chipholder. The results show that a uniform temperature is highly dependent on the correct design of the integrated system of chip, chipholder, and heater. The practical use of the chip with moderate air flow over the chip and moderate fluid flow rates through the channel had no effect on the fluid temperature. A well designed micro system can therefore be considered thermally robust under moderate processing conditions.
The microsystem from chapters 2 and 3 was used for enzyme reactions on micro scale. The kinetic parameters of a lipase catalyzed esterification reaction (chapter 4) and a β-galactosidase catalyzed hydrolysis reaction (chapter 5) on this micro scale were the same as those found on bench scale. Kinetic and thermal (in-)activation results obtained on micro scale can be used for large scale processing. This can bring down optimization costs by reducing the required amount of enzyme and chemicals.
Next, we found that at residence times below a few seconds, diffusion effects limited the reaction rate and therefore reduced the conversion per volume of enzyme microreactor. This effect of diffusion on the conversion increased quadratically with channel width, increased with enzyme concentration, and decreased with substrate concentration. When an enzyme microreactor system should be run efficiently, these factors should be explored to avoid diffusion limitation and subsequent reduced volumetric productivity.
With microreactor technology reaching maturity, a wider application of the technology could be imagined. With increasing application the impact it will have on society will also increase. In chapter 6, three examples of microreactor technology applications in nutrition, in medicine, and in energy carrier supply were investigated. The benefits and costs, and their distribution were discussed for these examples. Furthermore, possible strategies of communication surrounding a public introduction of such a novel technology were considered. The applications proposed in this chapter were only three out of an infinite number of possibilities. However, the discussion of these examples can be used as a framework for discussing future applications as they might be developed in the future. A societal backlash as with the GMO-scare in 1990s, can be avoided when the relevant issues are communicated appropriately and timely. This could improve the chances of success of this technology in the market.
In this thesis we have shown that microreactors can be a useful tool for reaction engineering. Their use could reduce the required amount of enzyme and chemicals for optimization. Furthermore, they can be used to study processes with a very short residence time. To use microreactor technology effectively, one does have to consider whether the scale is appropriate, and whether that the system, including chipholder, interfaces to the outer world and thermal actuators, is properly designed and used.
Modification of silicon nitride and silicon carbide surfaces for food and biosensor applications
Rosso, M. - \ 2009
Wageningen University. Promotor(en): Han Zuilhof; Remko Boom, co-promotor(en): Karin Schroen. - [S.l. : S.n. - ISBN 9789085853794 - 221
organische verbindingen - unimoleculaire films - microfiltratie - nanotechnologie - oppervlaktechemie - oppervlakteverschijnselen - organic compounds - unimolecular films - microfiltration - nanotechnology - surface chemistry - surface phenomena
Silicon-rich silicon nitride (SixN4, x > 3) is a robust insulating material widely used for the coating of microdevices: its high chemical and mechanical inertness make it a material of choice for the reinforcement of fragile microstructures (e.g. suspended microcantilevers, micro-fabricated membranes-“microsieves”) or for the coating of the exposed surfaces of sensors (field-effect transistors, waveguide optical detectors). To a more limited extent, silicon carbide (SiC) can find similar applications, and this material also starts to be more and more applied in coating and sensor technologies.
In all these applications, control over the surface properties of inorganic materials is crucial, for example to avoid blockage of membranes during filtration, or to provide sensor surfaces with specific (bio-)recognition properties. In this thesis, a variety of methods is developed to obtain and study robust functional coatings on SixN4 and SiC. These enable a whole new range of applications involving biocompatible and bio-specific surfaces, while retaining the bulk mechanical, structural, electrical or optical properties of the inorganic substrates.
Chapter 2 and 3 of the thesis give an overview of the great potential of covalent organic monolayers: Chapter 2 presents the formation of alkylthiol, alkylsilane and alkene monolayers, as well as a number of applications in biocompatible surfaces, micro- and nanopatterning of surfaces and sensing. The emphasis of this review chapter is put on the possible combinations of the bulk properties of inorganic materials (electrical, optical, structural) and the surface properties of organic monolayers (wettability, biospecificity, biorepellence). Chapter 3 is focused on biorepellent surfaces in the field of filtration with microfabricated membranes. Indeed, silicon nitride microsieves, despite their high permeability and structural homogeneity, are prone to pore blocking, when submitted to biological solutions. The chapter gives a review of the available surface modification techniques involving organic coatings that can minimize or even prevent this surface contamination. These coatings involve highly hydrophilic oligomers and polymers, which have been widely explored for organic surfaces. Covalent organic monolayers formed onto inorganic surfaces can extend the applications of these biorepellent coatings to microdevices like SixN4 microsieves (as also discussed in Chapters 7 and 8)
Chapter 4 and 5 present the thermal functionalization with highly stable alkene-based organic monolayers of the surfaces of silicon-rich silicon nitride (Chapter 4) and silicon carbide (Chapter 5). This work was motivated by the substantial knowledge of similar monolayer formation on silicon surfaces1,2 and the initial success of simple functionalizations on silicon nitride.3 The strong covalent attachment of the coating molecules with the substrates makes the obtained hybrid structures much more resistant to chemical degradation than other types of monolayers on these substrates. The reaction proceeds via attachment of the terminal double bond of alkenes with the surface groups (Si-H in the case of silicon nitride surfaces or –OH for silicon carbide surfaces). Besides methyl-terminated surfaces, functional coatings can be obtained by the use of bi-functionalized alkenes (Figure 1), also allowing further surface reactions and the attachment of bio-recognition elements, through covalent attachment of diverse chemical (carboxylic acid, amine) or biological (oligo-peptides, protein) moieties.
Figure 1. Modification of SiC and Si¬xN4 surfaces with alkyl monolayers
Chapter 6 describes a modification of this method, where UV irradiation is used instead of heat to initiate the modification of both silicon nitride and silicon carbide. For both materials, this method allows the grafting of heat-sensitive compounds, needs less starting material (using only a liquid film) and provides monolayers with higher quality (as e.g. indicated by grafting density and stability) and higher reproducibility. Here again the attachment of diverse functionalities is possible, via formation of activated esters. After hydrolysis and activation of such grafted ester, amines can be attached in high yield (> 80 %), as demonstrated using X-ray photoelectron spectroscopy (XPS). Besides the homogeneous modification of plain surfaces, this method also opens the way to surface patterning of silicon nitride and silicon carbide and the modification of mechanically sensitive microfabricated devices.
In Chapters 4 to 6, the chemical functionalizations are studied using X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and static water contact angles. Si-C bonds are formed preferentially upon reaction of SixN4 surfaces with alkenes, similarly to what is reported for pure silicon surfaces, albeit that no measurement could totally exclude the presence of C-N bonds. The wet etching of SiC yields hydroxyl-terminated surfaces, and an IRRAS study reveals the attachment of alkenes via a Markovnikov-type addition (O-C bond formed on the second carbon of the double bond). The stability of these monolayers is reported in acidic and basic conditions, and it was shown that UV initiation yields even more stable monolayers, probably due to some cross-linking of the alkyl chains.
Chapter 7 explores the biorepellence of UV-initiated monolayers on silicon nitride surfaces Oligomers of ethylene glycols (3 or 6 units: methoxy-tri(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)3(CH2)9CH=CH2; EO3, and methoxy-hexa(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)6(CH2)9CH=CH2; EO6) are attached on the silicon nitride surfaces. The adsorption of two proteins, bovine serum albumin (BSA) and fibrinogen is used to test the biorepellence of the monolayers, in comparison with bare oxidized silicon nitride. Both proteins adsorb readily onto bare SixN4 surfaces, with adsorbed amounts of 1.25 and 2.7 mg.m-2 for BSA and fibrinogen, respectively, of which more than 80 % is irreversibly bound. In contrast to this, when oligomers are attached to the surface, this adsorption decreases to under the detection limit of the method used for this experiment (optical reflectometry). The ex situ study of surfaces with AFM and water contact angles also indicates that some of the monolayers completely prevent the adsorption of proteins.
Figure 2. Biorepellent behavior of oligoethylene oxide coated SixN4 surfaces
Chapter 8 describes the applications of the biorepellent coatings used in Chapter 7 (EO6) to silicon nitride microsieves, in order to improve the filtration of biological solutions and liquid food products. The EO6 coatings are successfully formed on microfabricated membranes with pore diameters of 0.45 micrometer, using the UV-initiated monolayer formation described in Chapter 6. This work shows that these coatings could be applied without loss of permeability due to wettability or pore blocking. Moreover, AFM showed that these coatings significantly decrease the adsorption of proteins on the surface between the pores.
Chapter 9 describes an alternative functionalization technique for inorganic surfaces, namely the use of plasma oxidation of alkyl monolayers to reproducibly form aldehydes (among other oxidized species) onto surfaces. The method described here for silicon and silicon nitride surfaces, is developed for the functionalization of sensitive devices and substrates. The formation of methyl-terminated alkyl monolayers from linear terminal alkenes is one of the easiest to perform, since linear monofunctional alkenes are readily available, their purification is easy (distillation) and their grafting conditions are very flexible (liquid state, heat-resistant, UV-resistant > 250 nm). Once these stable monolayers are formed, a short plasma treatment (0.5 to 2 s) is able to form oxidized functionalities within the top few angstroms of the surface, while the underlying alkyl chains retain their initial packing and insulation properties of the inorganic substrate. The grafting of gold nanoparticles shows that micron-sized patterns can be formed using a soft contact mask to protect a limited area of the monolayer. Alternatively, the aldehydes can be used to attach biotin and avidin onto SixN4 surfaces. The selective adsorption of biotinylated BSA onto the avidin-modified surfaces shows that the plasma treatment of methyl-terminated monolayers is a fast and efficient method to produce surfaces displaying high specific biochemical interactions.
In the chapter 10, some of the most striking effects that are described in the previous chapters are put into a wider perspective. Especially the formation and stability of monolayers is discussed, also in relation to biofunctionalization, biorepellence, and opportunities for surface engineering are proposed.
Classification and evaluation of microfluidic devices for continuous suspension fractionation
Kulrattanarak, T. ; Sman, R.G.M. van der; Schroën, C.G.P.H. ; Boom, R.M. - \ 2008
Advances in Colloid and Interface Science 142 (2008)1/2. - ISSN 0001-8686 - p. 53 - 66.
field-flow fractionation - deterministic lateral displacement - continuous particle separation - colloidal particles - dielectrophoretic separation - membrane - size - manipulation - microfiltration - filtration
Membrane processes are well-known for separating and fractionating suspensions in many industries, but suffer from particle accumulation on the membrane surface. Currently, there are new developments using microfluidic devices for cell/DNA sorting and fractionation. We anticipate these devices are also applicable to fractionation of polydisperse and concentrated suspensions (e.g. foods), and may potentially have fewer problems with particle accumulation compared to membranes. This review article presents an overview of relevant microfluidic devices. We focus on their performance with respect to concentrated suspensions, as one finds in food industry. We give quantitative estimates on their yield, selectivity, and the potential for large-scale application. From this evaluation follows that deterministic ratchets seem most promising.
Membrane emulsification with vibrating membranes: A numerical study
Kelder, J.D.H. ; Janssen, J.J.M. ; Boom, R.M. - \ 2007
Journal of Membrane Science 304 (2007)1-2. - ISSN 0376-7388 - p. 50 - 59.
cross-flow membrane - dynamic interfacial-tension - droplet size - microfiltration
Membrane emulsification of oil in water may be enhanced by mechanically exciting the membrane, thereby enabling the formation of smaller droplets of a narrower size distribution, combined with higher specific production rate. To evaluate this potential, a force balance model was developed that includes the additional forces induced by the transversal membrane movement. This model yielded the ranges of interest of the excitation amplitude A and frequency f. In these ranges of interest, 3D transient simulations were carried out to predict the 3D droplet formation and detachment on a single pore under constant cross-flow. For conditions without membrane excitation, the force balance model agreed satisfactorily with experimental results reported in the literature and with the current 3D computations. The model also predicts a non-linear dependence of the droplet size on the pore diameter. However, under membrane excitation the extended force balance model does not seem to give reliable results. This is probably due to the simplifications in this model, which does not include the effects of the dispersed phase flux and viscosity, and the strongly non-spherical droplet geometry upon detachment. Moreover, for large vibration-induced forces coalescence occurred in the 3D model, which leads to much larger droplets. Thus, fully transient 3D CFD simulations appear to be required for reliable predictions. Ideally these should account for surfactant dynamics and a variable surface tension coefficient. The simulations show that membrane excitation potentially has a strong effect on the average droplet size in membrane emulsification, but that successful exploitation will require careful design of membrane and process. First estimates seem to indicate that systems with lower excitation frequency and larger excitation amplitude may perform better, but this will require experimental verification.
On-line cake-layer management by trans-membrane pressure steady state assessment in Anaerobic Membrane Bioreactors for wastewater treatment
Jeison, D. ; Lier, J.B. van - \ 2006
Biochemical Engineering Journal 29 (2006)3. - ISSN 1369-703X - p. 204 - 209.
critical flux - submerged membrane - microfiltration - aeration
Membrane bioreactors have been increasingly applied for wastewater treatment during the last two decades. High energy requirements and membrane capital costs remains as their main drawback. A new strategy of operation is presented based on a continuous critical flux determination, preventing excessive cake-layer accumulation on the membrane surface. Reactor operation is divided in cycles of 500 s filtration followed by a short back-flush of 15 s. If cake-layer formation is detected during continuous operation, a decrease in flux or an increase in cross flow velocity is performed. The proposed approach keeps reactor operation oscillating around the critical flux, minimizing reactor maintenance and maximizing performance. An easy to operate statistical steady state determination tool for the trans-membrane pressure was used to detect cake-layer formation. The developed control approach was tested on two Anaerobic MBRs equipped with submerged membranes. Despite the existence of very different critical fluxes and cake-layer formation characteristics, proposed approach was able to keep pressure increase during filtration cycles below 20 mbar. The developed approach is an efficient tool for on-line control of cake-layer formation over the membranes, changing cross flow velocities by manipulating gas sparging in submerged MBRs
Evaluation of microsieve membrane design
Brans, G.B.P.W. ; Kromkamp, J. ; Pek, N. ; Gielen, J. ; Heck, J. ; Rijn, C.J.M. van; Sman, R.G.M. van der; Schroën, C.G.P.H. ; Boom, R.M. - \ 2006
Journal of Membrane Science 278 (2006)1-2. - ISSN 0376-7388 - p. 344 - 348.
model - microfiltration
In principle, microsieve membranes have high fluxes, due to their extremely low flow resistance and their uniform pore size. However, it was found experimentally, that the design of the support structure, even though its flow resistance is negligible, had great effect on the flux and the evolution of pore blocking. This finding was quantified using computational fluid dynamics (CFD) simulations of the flow through the microsieve. From the CFD calculations, we could conclude that the design of a microsieve should necessarily encompass the design of the top layer and the support structure together. The first design of the microsieve only had 30% of the maximum possible flux. It was shown that the channel height between the pore field and the support structure should be at least 150 ¿m for optimal use of the microsieve
Particle separation and fractionation by microfiltration
Kromkamp, J. - \ 2005
Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen; Ruud van der Sman. - [S.l.] : s.n. - ISBN 9789085042396 - 184
microfiltratie - membranen - deeltjes - deeltjesgrootte - simulatiemodellen - computersimulatie - microfiltration - membranes - particles - particle size - simulation models - computer simulation - cum laude
cum laude graduation (with distinction) For the production of present-day dairy products, raw milk is often considered an entity. However, a large quality improvement could be reached if selected constituents were available. In order to achieve this, milk will have to be fractionated prior to use in dairy products. Microfiltration is an important technique for the fractionation of milk; the pore size typically being in the order of micrometers. However, due to insufficient separation caused by blockage of the filter, the potential of microfiltration is still hardly used. This instigated the Ph.D. research project of Janneke Kromkamp which aims at using microfiltration for fractionation to its fullest potential. The interaction between the different microparticles in milk and the surrounding liquid were studied at a fundamental level by means of computer simulation techniques. By coupling this information to observations on the microfiltration of milk, important new insights were obtained which can substantially improve the fractionation process. Paradoxically enough, the liquid flow was better able to fractionate particles than the membrane alone. Because the particles organise in the liquid flow, the large ones moving to the centre of the channel, the smaller ones can be separated easily. Herewith, membrane blockage, which was the biggest challenge in this thesis, is prevented. An additional advantage is that the fractionation process can be completely controlled by easily controllable process parameters. The high-quality dairy products mentioned earlier have now come within reach.
Why liquid displacement methods are sometimes wrong in estimating the pore-size distribution
Gijsbertsen-Abrahamse, A.J. ; Boom, R.M. ; Padt, A. van der - \ 2004
AIChE Journal 50 (2004)7. - ISSN 0001-1541 - p. 1364 - 1371.
ultrafiltration membranes - microfiltration - morphology - emulsification
The liquid displacement method is a commonly used method to determine the pore size distribution of micro- and ultrafiltration membranes. One of the assumptions for the calculation of the pore sizes is that the pores are parallel and thus are not interconnected. To show that the estimated pore size distribution is affected if this assumption is not satisfied, we developed two models: (1) a model describing the flow through an isotropic porous membrane with uniform pores, and (2) a two-layer model for uniform pores that approximates the first model if the membrane thickness is larger than 10 times the pore radius. In the two-layer model the membrane skin layer is divided into two parts: the unconnected pore layer and a sublayer. This model is extended to describe pore activation as a function of pressure with a pore size distribution in the unconnected pore layer (that is, membrane surface). It is shown that, depending on the membrane thickness or the sublayer resistance, the transmembrane pressure needs to be much larger than the critical pressure of the pores, to activate all the pores. If the sublayer resistance is over 10% of the resistance of the unconnected pore layer, the pore size is underestimated with the liquid displacement method; thus the number of pores is overestimated. Because the sublayer resistance is always larger than the unconnected pore layer resistance in an isotropic membrane with interconnected pores, we conclude that the estimated pore size distribution is always shifted toward smaller pore sizes than they really are. To use the liquid displacement method correctly, we suggest either counting the number of (active) pores or measuring the flux-pressure curve several times, while covering each time a different fraction of the membrane surface.