Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
Colloidal particles often display a surface topography that is smooth down to the nanometer scale... more Colloidal particles often display a surface topography that is smooth down to the nanometer scale. Introducing roughness at this length scale can drastically change the colloidal interactions, adsorption at interfaces and bulk flow behavior. We report on a novel, simple method to induce and control nano-scale roughness on (water based) polymer latex colloids. Reducing the amount of dissolved gases in the aqueous phase from the electrolyte solution surrounding the particles, generates self-structured surface asperities with an amplitude that can be tuned via temperature and repetition of the treatment. Due to the viscoelastic nature of the polymeric asperities, a mild thermal treatment below the glass transition temperature can be used for nanostructure relaxation, so that the particles can recover their original topography, making this method fully reversible. Roughness can thus be controlled without affecting the chemical composition of the colloidal surface. Experiments for varying particle size, polymer type and surface chemistry suggest a broad applicability of our method.
In microfluidic studies of improved oil recovery, mostly pore networks with uniform depth and sur... more In microfluidic studies of improved oil recovery, mostly pore networks with uniform depth and surface chemistry are used. To better mimic the multiple porosity length scales and surface heterogeneity of carbonate reservoirs, we coated a 2.5D glass microchannel with calcite particles. After aging with formation water and crude oil (CRO), high-salinity Water (HSW) was flooded at varying temperatures and durations. Time-resolved microscopy revealed the CRO displacements. Precise quantification of residual oil presented some challenges due to calcite-induced optical heterogeneity and brine–oil coexistence at (sub)micron length scales. Both issues were addressed using pixel-wise intensity calibration. During waterflooding, most of the ultimately produced oil gets liberated within the first pore volume (similar to glass micromodels). Increasing temperature from 22 °C to 60 °C and 90 °C produced some more oil. Waterflooding initiated directly at 90 °C produced significantly more oil than a...
Soft microgel particles inherently possess qualities of both polymers as well as particles. We re... more Soft microgel particles inherently possess qualities of both polymers as well as particles. We review the similarities and differences between soft microgel particles and stiff colloids at fluid-fluid interfaces. We compare two fundamental aspects of particle-laden interfaces namely the adsorption kinetics and the interactions between adsorbed particles. Although it is well established that the transport of both hard particles and microgels to the interface is driven by diffusion, the analysis of the adsorption kinetics needs reconsideration and a proper equation of state relating the surface pressure to the adsorbed mass should be used. We review the theoretical and experimental investigations into the interactions of particles at the interface. The rheology of the interfacial layers is intimately related to the interactions, and the differences between hard particles and microgels become pronounced. The assembly of particles into the layer is another distinguishing factor that separates hard particles from soft microgel particles. Microgels deform substantially upon adsorption and the stability of a microgel-stabilized emulsion depends on the conformational changes triggered by external stimuli.
Within reservoirs, spatial variations related to mineralogy and fluid chemistry determine the suc... more Within reservoirs, spatial variations related to mineralogy and fluid chemistry determine the success of improved oil recovery (IOR) techniques. However, the composition and structure of mineral-adsorbent-fluid interfaces, which fundamentally determine the initial and IOR-altered wettability of reservoir rocks as well as the displacement of crude oil (CRO), are unclear. Replicating the diagenetic alterations of carbonates, this study addresses the temperature dependence of the inorganic and organic modifications of calcite by reservoir pertinent fluids as well as its consequences on mineral wettability and reactivity. We utilize a suite of characterization methods, such as confocal Raman, scanning electron and atomic force microscopy as well as Fourier-transform infrared spectroscopy, to investigate the modifications of carbonates on aging in formation water (FW), CRO-equilibrated FW and FW-equilibrated CRO. The microscopic modifications of carbonates present positive correlations w...
While it is generally known that aging protocols have an important impact on the interaction betw... more While it is generally known that aging protocols have an important impact on the interaction between crude oil (CRO), brines, and mineral surfaces, the microscopic consequences of the various steps of aging have hardly been described. In this study, we characterize the properties of fluids and carbonate mineral surfaces throughout a series of equilibration steps at 95 °C and correlate these microscopic observations with macroscopic contact angle measurements. Chemical equilibration of CRO (eqCRO) and FW (eqFW) leads to transfer of organic molecules from the former to the latter, causing also a pH change in the eqFW. Confocal Raman microscopy, atomic force microscopy, and infrared spectroscopy are used to reveal how consecutive aging of calcite in eqFW and eqCRO induces: first, in eqFW, considerable surface reconstruction and precipitation of mineral particles with colocalized organic species, and second, upon exposure to eqCRO, the formation of a second adlayer primarily composed of polyaromatic hydrocarbon-rich particles. Our results show how these interconnected microscopic chemical and topographical surface modifications give rise to more “oil wetting” contact angles after the two-step aging procedure.
Abstract To understand the success of low salinity water flooding in improving oil recovery, it i... more Abstract To understand the success of low salinity water flooding in improving oil recovery, it is important to identify the molecular scale mechanisms that control the wettability and thus the adhesion between oil and rock. Previous experiments have attributed the wettability alteration in core flood experiments either to the expansion of the electric double layer or to multicomponent ion exchange reactions or a combination of both. Here, we explore changes of the contact angle of brine droplets on mica in ambient oil (n-decane plus added fatty acid) at variable temperature as a function of the concentration of mono- and divalent cations. For 20 °C and 40 °C, we find that the contact angle decreases by up to 30° with decreasing divalent cation concentration but remains constant upon decreasing the total salinity by removing only monovalent cations, i.e. upon double layer expansion at constant divalent cation concentration. At 60 °C, we find a remarkable increase of the water contact angle of artificial sea water to values of approximately 120°. This value decreases upon dilution to values in the range of 10–40 °C, where the lowest values are again only obtained upon removing the divalent cations. These findings corroborate the conclusion of earlier measurements at room temperature that divalent cations play an essential role in controlling the wettability of carboxylic acid groups to mineral surface, presumably in an ion bridging type mechanism. We also demonstrate that the contact angle reduction occurs very quickly upon flushing a sessile droplet of artificial sea water with divalent cation-free or simply diluted brine, suggesting fast equilibration as required for a successful tertiary water flooding process. Our experiments also demonstrate that, despite the simplicity of the present system, the origins of wettability alteration are rather complex and that synergistic effects can lead to dramatic variations such as the unexpectedly high contact angle at 60 °C.
To study the mechanism of enhanced oil recovery, it is important to characterize the three-dimens... more To study the mechanism of enhanced oil recovery, it is important to characterize the three-dimensional spatial distribution of various chemical species, especially water and oil, and their evolution during the course of water flooding. For example, visualizing the (selective) removal of oil from clay or silica substrates by low salinity water can yield important insights. Here, we present a platform that uses a microfluidic device (to represent water flooding at the pore scale) in combination with confocal Raman microscopy. Distributions of oil, water, and minerals are resolved at submicrometer resolution upon flooding water with changing composition. Using glass and gibbsite to mimic sandstone and clay, and water containing divalent cations (Ca 2+), we find that oil containing a fatty acid preferentially adsorbs on the gibbsite. Removal of the divalent cations leads to release of the oil droplet. This finding is consistent with the multiple ion exchange mechanism and underlines that the presence of clay is important for low salinity enhanced oil recovery. We expect that our platform will pave the road towards systematic screening of water flood compositions in more complex systems.
Langmuir : the ACS journal of surfaces and colloids, Feb 21, 2017
We studied the effects of shear and its history on suspensions of carbon black (CB) in lithium io... more We studied the effects of shear and its history on suspensions of carbon black (CB) in lithium ion battery electrolyte via simultaneous rheometry and electrical impedance spectroscopy. Ketjen black (KB) suspensions showed shear thinning and rheopexy and exhibited a yield stress. Shear step experiments revealed a two time scale response. The immediate effect of decreasing the shear rate is an increase in both viscosity and electronic conductivity. In a much slower secondary response, both quantities change in the opposite direction, leading to a reversal of the initial change in the conductivity. Stepwise increases in the shear rate lead to similar responses in the opposite direction. This remarkable behavior is consistent with a picture in which agglomerating KB particles can stick directly on contact, forming open structures, and then slowly interpenetrate and densify. The fact that spherical CB particles show the opposite slow response suggests that the fractal structure of the KB...
Establishing and maintaining concentration gradients that are stable in space and time is critica... more Establishing and maintaining concentration gradients that are stable in space and time is critical for applications that require screening the adsorption behavior of organic or inorganic species onto solid surfaces for wide ranges of fluid compositions. In this work, we present a design of a simple and compact microfluidic device based on steady-state diffusion of the analyte, between two control channels where liquid is pumped through. The device generates a near-linear distribution of concentrations. We demonstrate this via experiments with dye solutions and comparison to finite-element numerical simulations. In a subsequent step, the device is combined with total internal reflection ellipsometry to study the adsorption of (cat)ions on silica surfaces from CsCl solutions at variable pH. Such a combined setup permits a fast determination of an adsorption isotherm. The measured optical thickness is compared to calculations from a triple layer model for the ion distribution, where su...
Surfactant adsorption in a three-phase system and its influence on wetting properties are relevan... more Surfactant adsorption in a three-phase system and its influence on wetting properties are relevant in various applications. Here, we report a hitherto not observed phenomenon, namely the retraction of an aqueous drop on hydrophilic solid substrates (which we refer to as 'autophobing') in ambient oil containing water-insoluble fatty acids, caused by the deposition of these fatty acids from the ambient oil onto the solid substrate. AFM measurements confirm that the surfactant is deposited on the solid by the moving contact line. This leads to a more hydrophobic substrate, the retraction of the contact line and a concomitant increase in the contact angle. The deposition process is enabled by the formation of a reaction product between deprotonated fatty acids and Ca(2+) ions at the oil/water interface. We investigate how the transition to a new equilibrium depends on the concentrations of the fatty acids, the aqueous solute, the chain lengths of the fatty acid, and the types of alkane solvent and silica or mica substrates. This phenomenon is observed on both substrates and for all explored combinations of fatty acids and solvents and thus appears to be generic. In order to capture the evolution of the contact angle, we develop a theoretical model in which the rate of adsorption at the oil-water interface governs the overall kinetics of autophobing, and transfer to the solid is determined by a mass flux balance (similar to a Langmuir Blodgett transfer).
We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow... more We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow. The experiments are carried out with a counterrotating Couette apparatus. The deformation depends on the mechanical properties of the lipid bilayer, the vesicle radius, and the viscosity of the surrounding Newtonian liquid. We show that the relevant mechanical parameter is the bending rigidity. A simple model has been developed that describes the deformation of a vesicle. This model takes thermal undulations of the bilayer into account. We have obtained a value for the bending rigidity of dimyristoyl-phosphatidylcholine bilayers and its value has been compared with literature data and with results from micropipette aspiration experiments. From the measurements we are able to discriminate between unilamellar and multilamellar vesicles. ͓S1063-651X͑97͒12212-7͔
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016
We chose water-soluble sodium hexanoate as a model organic molecule to study the role of salt ion... more We chose water-soluble sodium hexanoate as a model organic molecule to study the role of salt ions (Ca2+, Na+, Cl−) in the adsorption of carboxylates to mineral surfaces (silica, alumina, gibbsite) of variable surface charge and chemistry. Quartz crystal microbalance (QCM-D) measurements reveal a qualitatively different dependence of the adsorption behavior on the electrolyte composition for the different surfaces at near neutral pH. Overall, hexanoate adsorption is more pronounced on the positively charged alumina surfaces than on negatively charged silica surfaces. On silica, however, Ca2+ ions strongly enhance the adsorption of hexanoate, suggesting that the divalent cations act as bridges between carboxylate and deprotonated silanol surface groups. On alumina, hexanoate adsorption is found to depend only weakly on the salt composition, suggesting a direct interaction of the carboxylate group with the surface, consistent with a ligand-exchange mechanism. The adsorption behavior on partially gibbsite-covered silica surfaces is particularly rich and displays a strong non-monotonic dependence on the CaCl2 concentration. Comparison to earlier work and control experiments suggest an important role of Cl− anions, which compete with the carboxylate group for adsorption sites.
Summary Most solid surfaces acquire a finite surface charge after exposure to aqueous environment... more Summary Most solid surfaces acquire a finite surface charge after exposure to aqueous environments caused by desorption and/or adsorption of ionic species. The resulting electrostatic forces play a crucial role in many fields of science and technology, including colloidal stability, self-assembly, wetting, and biophysics. Enhanced oil recovery (EOR) is an example of a large-scale industrial process that hinges in many respects on these phenomena. In this paper, we present a series of experiments illustrating fundamental aspects of low-salinity waterflooding in well-defined model systems. We show how pH and ion content of the water phase as well as the presence of model polar components (fatty acids) in the oil phase affect the wettability (i.e., contact-angle distribution) of oil/water/rock systems. Specifically, we discuss high-resolution atomic-force microscopy (AFM) experiments demonstrating the preferential adsorption of multivalent cations to mineral surfaces such as mica and g...
Analysis of the dynamics of colloids in shear flow can be challenging because of the superpositio... more Analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the timedependent particle coordinates. Restriction of the tracking to flow lanes and subtraction of estimated advective displacements are combined in an iterative scheme that eventually makes the spatial segmentation redundant. Tracking errors due to neglect of lateral diffusion are avoided, while drifts parallel and perpendicular to the flow are eliminated. After explaining the principles of our method, we validate it against both computer simulations and experiments. A critical overall test is provided by the Mean Square Displacement function at high Peclet numbers (up to 50). We demonstrate via simulations how the measurement accuracy depends on diffusion coefficients and flow rates, expressed in units of camera pixels and frames. Also sample-specific issues are addressed: inaccuracies in the velocity profile for dilute suspensions (Volume Fraction ≤ 0.03), and tracking errors for concentrated ones (VF ≥ 0.3). Analysis of experiments with colloidal spheres flowing through micro channels corroborates these findings, and indicates perspectives for studies on transport, mixing or rheology in microfluidic environments.
We studied both global and local effects of cylindrical confinement on the diffusive behavior of ... more We studied both global and local effects of cylindrical confinement on the diffusive behavior of hard sphere (HS) colloids.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2015
We studied the influence of salt on a commercially available carbon black (Ketjenblack 600, KB) i... more We studied the influence of salt on a commercially available carbon black (Ketjenblack 600, KB) in carbonate solvents commonly applied in rechargeable batteries. Adopting the typically used salts: lithium hexa-fluorophosphate (LiPF6), lithium bis(trifluoromethane sulfonyl) imide (LiTFSI), as well as sodium hexafluorophosphate (NaPF6) dissolved in mixtures of ethylene carbonate and propylene carbonate, we investigated both the zeta potential and the flocculation kinetics of the KB particles as a function of salt concentration between 0.01 mM and 1.0 M. Clear evidence was found for the preferential adsorption of cations. In the absence of salt, KB was found to carry a negative surface charge, but this gets neutralized by Li + at very low concentrations (~ 1 mM), and by Na + at intermediate concentrations (~ 30 mM). In the case of lithium ions, the increased adsorption at higher concentration led to a recovery of the colloidal stability around 3-30 mM, depending on the anion. At high concentrations exceeding 30-100 mM, all salts cause flocculation of the KB particles, due to a reduction of the electric double layer thickness. Since the charge neutralization of the KB by Na + takes place in the same concentration regime, no re-entrant stability is found for Na +. These findings could have implications in formulation protocols for semi-solid flow batteries, or other systems where an intermediate stable regime could assist mixing and/or structure formation at small length scales.
Analysis of the dynamics of colloids in shear flow can be challenging because of the superpositio... more Analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the time-dependent particle coordinates. Restriction of the tracking to flow lanes and subtraction of estimated advective displacements are combined in an iterative scheme that eventually makes the spatial segmentation redundant. Tracking errors due to neglect of lateral diffusion are avoided, while drifts parallel and perpendicular to the flow are eliminated. After explaining the principles of our method, we validate it against both computer simulations and experiments. A critical overall test is provided by the Mean Square Displacement function at high Peclet numbers (up to 50). We demonstrate via simulations how the measurement accuracy depends on diffusion coefficients and flow rates, expressed in units of camera pixels and frames. Also sample-specific issues are addressed: inaccuracies in...
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
Colloidal particles often display a surface topography that is smooth down to the nanometer scale... more Colloidal particles often display a surface topography that is smooth down to the nanometer scale. Introducing roughness at this length scale can drastically change the colloidal interactions, adsorption at interfaces and bulk flow behavior. We report on a novel, simple method to induce and control nano-scale roughness on (water based) polymer latex colloids. Reducing the amount of dissolved gases in the aqueous phase from the electrolyte solution surrounding the particles, generates self-structured surface asperities with an amplitude that can be tuned via temperature and repetition of the treatment. Due to the viscoelastic nature of the polymeric asperities, a mild thermal treatment below the glass transition temperature can be used for nanostructure relaxation, so that the particles can recover their original topography, making this method fully reversible. Roughness can thus be controlled without affecting the chemical composition of the colloidal surface. Experiments for varying particle size, polymer type and surface chemistry suggest a broad applicability of our method.
In microfluidic studies of improved oil recovery, mostly pore networks with uniform depth and sur... more In microfluidic studies of improved oil recovery, mostly pore networks with uniform depth and surface chemistry are used. To better mimic the multiple porosity length scales and surface heterogeneity of carbonate reservoirs, we coated a 2.5D glass microchannel with calcite particles. After aging with formation water and crude oil (CRO), high-salinity Water (HSW) was flooded at varying temperatures and durations. Time-resolved microscopy revealed the CRO displacements. Precise quantification of residual oil presented some challenges due to calcite-induced optical heterogeneity and brine–oil coexistence at (sub)micron length scales. Both issues were addressed using pixel-wise intensity calibration. During waterflooding, most of the ultimately produced oil gets liberated within the first pore volume (similar to glass micromodels). Increasing temperature from 22 °C to 60 °C and 90 °C produced some more oil. Waterflooding initiated directly at 90 °C produced significantly more oil than a...
Soft microgel particles inherently possess qualities of both polymers as well as particles. We re... more Soft microgel particles inherently possess qualities of both polymers as well as particles. We review the similarities and differences between soft microgel particles and stiff colloids at fluid-fluid interfaces. We compare two fundamental aspects of particle-laden interfaces namely the adsorption kinetics and the interactions between adsorbed particles. Although it is well established that the transport of both hard particles and microgels to the interface is driven by diffusion, the analysis of the adsorption kinetics needs reconsideration and a proper equation of state relating the surface pressure to the adsorbed mass should be used. We review the theoretical and experimental investigations into the interactions of particles at the interface. The rheology of the interfacial layers is intimately related to the interactions, and the differences between hard particles and microgels become pronounced. The assembly of particles into the layer is another distinguishing factor that separates hard particles from soft microgel particles. Microgels deform substantially upon adsorption and the stability of a microgel-stabilized emulsion depends on the conformational changes triggered by external stimuli.
Within reservoirs, spatial variations related to mineralogy and fluid chemistry determine the suc... more Within reservoirs, spatial variations related to mineralogy and fluid chemistry determine the success of improved oil recovery (IOR) techniques. However, the composition and structure of mineral-adsorbent-fluid interfaces, which fundamentally determine the initial and IOR-altered wettability of reservoir rocks as well as the displacement of crude oil (CRO), are unclear. Replicating the diagenetic alterations of carbonates, this study addresses the temperature dependence of the inorganic and organic modifications of calcite by reservoir pertinent fluids as well as its consequences on mineral wettability and reactivity. We utilize a suite of characterization methods, such as confocal Raman, scanning electron and atomic force microscopy as well as Fourier-transform infrared spectroscopy, to investigate the modifications of carbonates on aging in formation water (FW), CRO-equilibrated FW and FW-equilibrated CRO. The microscopic modifications of carbonates present positive correlations w...
While it is generally known that aging protocols have an important impact on the interaction betw... more While it is generally known that aging protocols have an important impact on the interaction between crude oil (CRO), brines, and mineral surfaces, the microscopic consequences of the various steps of aging have hardly been described. In this study, we characterize the properties of fluids and carbonate mineral surfaces throughout a series of equilibration steps at 95 °C and correlate these microscopic observations with macroscopic contact angle measurements. Chemical equilibration of CRO (eqCRO) and FW (eqFW) leads to transfer of organic molecules from the former to the latter, causing also a pH change in the eqFW. Confocal Raman microscopy, atomic force microscopy, and infrared spectroscopy are used to reveal how consecutive aging of calcite in eqFW and eqCRO induces: first, in eqFW, considerable surface reconstruction and precipitation of mineral particles with colocalized organic species, and second, upon exposure to eqCRO, the formation of a second adlayer primarily composed of polyaromatic hydrocarbon-rich particles. Our results show how these interconnected microscopic chemical and topographical surface modifications give rise to more “oil wetting” contact angles after the two-step aging procedure.
Abstract To understand the success of low salinity water flooding in improving oil recovery, it i... more Abstract To understand the success of low salinity water flooding in improving oil recovery, it is important to identify the molecular scale mechanisms that control the wettability and thus the adhesion between oil and rock. Previous experiments have attributed the wettability alteration in core flood experiments either to the expansion of the electric double layer or to multicomponent ion exchange reactions or a combination of both. Here, we explore changes of the contact angle of brine droplets on mica in ambient oil (n-decane plus added fatty acid) at variable temperature as a function of the concentration of mono- and divalent cations. For 20 °C and 40 °C, we find that the contact angle decreases by up to 30° with decreasing divalent cation concentration but remains constant upon decreasing the total salinity by removing only monovalent cations, i.e. upon double layer expansion at constant divalent cation concentration. At 60 °C, we find a remarkable increase of the water contact angle of artificial sea water to values of approximately 120°. This value decreases upon dilution to values in the range of 10–40 °C, where the lowest values are again only obtained upon removing the divalent cations. These findings corroborate the conclusion of earlier measurements at room temperature that divalent cations play an essential role in controlling the wettability of carboxylic acid groups to mineral surface, presumably in an ion bridging type mechanism. We also demonstrate that the contact angle reduction occurs very quickly upon flushing a sessile droplet of artificial sea water with divalent cation-free or simply diluted brine, suggesting fast equilibration as required for a successful tertiary water flooding process. Our experiments also demonstrate that, despite the simplicity of the present system, the origins of wettability alteration are rather complex and that synergistic effects can lead to dramatic variations such as the unexpectedly high contact angle at 60 °C.
To study the mechanism of enhanced oil recovery, it is important to characterize the three-dimens... more To study the mechanism of enhanced oil recovery, it is important to characterize the three-dimensional spatial distribution of various chemical species, especially water and oil, and their evolution during the course of water flooding. For example, visualizing the (selective) removal of oil from clay or silica substrates by low salinity water can yield important insights. Here, we present a platform that uses a microfluidic device (to represent water flooding at the pore scale) in combination with confocal Raman microscopy. Distributions of oil, water, and minerals are resolved at submicrometer resolution upon flooding water with changing composition. Using glass and gibbsite to mimic sandstone and clay, and water containing divalent cations (Ca 2+), we find that oil containing a fatty acid preferentially adsorbs on the gibbsite. Removal of the divalent cations leads to release of the oil droplet. This finding is consistent with the multiple ion exchange mechanism and underlines that the presence of clay is important for low salinity enhanced oil recovery. We expect that our platform will pave the road towards systematic screening of water flood compositions in more complex systems.
Langmuir : the ACS journal of surfaces and colloids, Feb 21, 2017
We studied the effects of shear and its history on suspensions of carbon black (CB) in lithium io... more We studied the effects of shear and its history on suspensions of carbon black (CB) in lithium ion battery electrolyte via simultaneous rheometry and electrical impedance spectroscopy. Ketjen black (KB) suspensions showed shear thinning and rheopexy and exhibited a yield stress. Shear step experiments revealed a two time scale response. The immediate effect of decreasing the shear rate is an increase in both viscosity and electronic conductivity. In a much slower secondary response, both quantities change in the opposite direction, leading to a reversal of the initial change in the conductivity. Stepwise increases in the shear rate lead to similar responses in the opposite direction. This remarkable behavior is consistent with a picture in which agglomerating KB particles can stick directly on contact, forming open structures, and then slowly interpenetrate and densify. The fact that spherical CB particles show the opposite slow response suggests that the fractal structure of the KB...
Establishing and maintaining concentration gradients that are stable in space and time is critica... more Establishing and maintaining concentration gradients that are stable in space and time is critical for applications that require screening the adsorption behavior of organic or inorganic species onto solid surfaces for wide ranges of fluid compositions. In this work, we present a design of a simple and compact microfluidic device based on steady-state diffusion of the analyte, between two control channels where liquid is pumped through. The device generates a near-linear distribution of concentrations. We demonstrate this via experiments with dye solutions and comparison to finite-element numerical simulations. In a subsequent step, the device is combined with total internal reflection ellipsometry to study the adsorption of (cat)ions on silica surfaces from CsCl solutions at variable pH. Such a combined setup permits a fast determination of an adsorption isotherm. The measured optical thickness is compared to calculations from a triple layer model for the ion distribution, where su...
Surfactant adsorption in a three-phase system and its influence on wetting properties are relevan... more Surfactant adsorption in a three-phase system and its influence on wetting properties are relevant in various applications. Here, we report a hitherto not observed phenomenon, namely the retraction of an aqueous drop on hydrophilic solid substrates (which we refer to as 'autophobing') in ambient oil containing water-insoluble fatty acids, caused by the deposition of these fatty acids from the ambient oil onto the solid substrate. AFM measurements confirm that the surfactant is deposited on the solid by the moving contact line. This leads to a more hydrophobic substrate, the retraction of the contact line and a concomitant increase in the contact angle. The deposition process is enabled by the formation of a reaction product between deprotonated fatty acids and Ca(2+) ions at the oil/water interface. We investigate how the transition to a new equilibrium depends on the concentrations of the fatty acids, the aqueous solute, the chain lengths of the fatty acid, and the types of alkane solvent and silica or mica substrates. This phenomenon is observed on both substrates and for all explored combinations of fatty acids and solvents and thus appears to be generic. In order to capture the evolution of the contact angle, we develop a theoretical model in which the rate of adsorption at the oil-water interface governs the overall kinetics of autophobing, and transfer to the solid is determined by a mass flux balance (similar to a Langmuir Blodgett transfer).
We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow... more We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow. The experiments are carried out with a counterrotating Couette apparatus. The deformation depends on the mechanical properties of the lipid bilayer, the vesicle radius, and the viscosity of the surrounding Newtonian liquid. We show that the relevant mechanical parameter is the bending rigidity. A simple model has been developed that describes the deformation of a vesicle. This model takes thermal undulations of the bilayer into account. We have obtained a value for the bending rigidity of dimyristoyl-phosphatidylcholine bilayers and its value has been compared with literature data and with results from micropipette aspiration experiments. From the measurements we are able to discriminate between unilamellar and multilamellar vesicles. ͓S1063-651X͑97͒12212-7͔
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016
We chose water-soluble sodium hexanoate as a model organic molecule to study the role of salt ion... more We chose water-soluble sodium hexanoate as a model organic molecule to study the role of salt ions (Ca2+, Na+, Cl−) in the adsorption of carboxylates to mineral surfaces (silica, alumina, gibbsite) of variable surface charge and chemistry. Quartz crystal microbalance (QCM-D) measurements reveal a qualitatively different dependence of the adsorption behavior on the electrolyte composition for the different surfaces at near neutral pH. Overall, hexanoate adsorption is more pronounced on the positively charged alumina surfaces than on negatively charged silica surfaces. On silica, however, Ca2+ ions strongly enhance the adsorption of hexanoate, suggesting that the divalent cations act as bridges between carboxylate and deprotonated silanol surface groups. On alumina, hexanoate adsorption is found to depend only weakly on the salt composition, suggesting a direct interaction of the carboxylate group with the surface, consistent with a ligand-exchange mechanism. The adsorption behavior on partially gibbsite-covered silica surfaces is particularly rich and displays a strong non-monotonic dependence on the CaCl2 concentration. Comparison to earlier work and control experiments suggest an important role of Cl− anions, which compete with the carboxylate group for adsorption sites.
Summary Most solid surfaces acquire a finite surface charge after exposure to aqueous environment... more Summary Most solid surfaces acquire a finite surface charge after exposure to aqueous environments caused by desorption and/or adsorption of ionic species. The resulting electrostatic forces play a crucial role in many fields of science and technology, including colloidal stability, self-assembly, wetting, and biophysics. Enhanced oil recovery (EOR) is an example of a large-scale industrial process that hinges in many respects on these phenomena. In this paper, we present a series of experiments illustrating fundamental aspects of low-salinity waterflooding in well-defined model systems. We show how pH and ion content of the water phase as well as the presence of model polar components (fatty acids) in the oil phase affect the wettability (i.e., contact-angle distribution) of oil/water/rock systems. Specifically, we discuss high-resolution atomic-force microscopy (AFM) experiments demonstrating the preferential adsorption of multivalent cations to mineral surfaces such as mica and g...
Analysis of the dynamics of colloids in shear flow can be challenging because of the superpositio... more Analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the timedependent particle coordinates. Restriction of the tracking to flow lanes and subtraction of estimated advective displacements are combined in an iterative scheme that eventually makes the spatial segmentation redundant. Tracking errors due to neglect of lateral diffusion are avoided, while drifts parallel and perpendicular to the flow are eliminated. After explaining the principles of our method, we validate it against both computer simulations and experiments. A critical overall test is provided by the Mean Square Displacement function at high Peclet numbers (up to 50). We demonstrate via simulations how the measurement accuracy depends on diffusion coefficients and flow rates, expressed in units of camera pixels and frames. Also sample-specific issues are addressed: inaccuracies in the velocity profile for dilute suspensions (Volume Fraction ≤ 0.03), and tracking errors for concentrated ones (VF ≥ 0.3). Analysis of experiments with colloidal spheres flowing through micro channels corroborates these findings, and indicates perspectives for studies on transport, mixing or rheology in microfluidic environments.
We studied both global and local effects of cylindrical confinement on the diffusive behavior of ... more We studied both global and local effects of cylindrical confinement on the diffusive behavior of hard sphere (HS) colloids.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2015
We studied the influence of salt on a commercially available carbon black (Ketjenblack 600, KB) i... more We studied the influence of salt on a commercially available carbon black (Ketjenblack 600, KB) in carbonate solvents commonly applied in rechargeable batteries. Adopting the typically used salts: lithium hexa-fluorophosphate (LiPF6), lithium bis(trifluoromethane sulfonyl) imide (LiTFSI), as well as sodium hexafluorophosphate (NaPF6) dissolved in mixtures of ethylene carbonate and propylene carbonate, we investigated both the zeta potential and the flocculation kinetics of the KB particles as a function of salt concentration between 0.01 mM and 1.0 M. Clear evidence was found for the preferential adsorption of cations. In the absence of salt, KB was found to carry a negative surface charge, but this gets neutralized by Li + at very low concentrations (~ 1 mM), and by Na + at intermediate concentrations (~ 30 mM). In the case of lithium ions, the increased adsorption at higher concentration led to a recovery of the colloidal stability around 3-30 mM, depending on the anion. At high concentrations exceeding 30-100 mM, all salts cause flocculation of the KB particles, due to a reduction of the electric double layer thickness. Since the charge neutralization of the KB by Na + takes place in the same concentration regime, no re-entrant stability is found for Na +. These findings could have implications in formulation protocols for semi-solid flow batteries, or other systems where an intermediate stable regime could assist mixing and/or structure formation at small length scales.
Analysis of the dynamics of colloids in shear flow can be challenging because of the superpositio... more Analysis of the dynamics of colloids in shear flow can be challenging because of the superposition of diffusion and advection. We present a method that separates the two motions, starting from the time-dependent particle coordinates. Restriction of the tracking to flow lanes and subtraction of estimated advective displacements are combined in an iterative scheme that eventually makes the spatial segmentation redundant. Tracking errors due to neglect of lateral diffusion are avoided, while drifts parallel and perpendicular to the flow are eliminated. After explaining the principles of our method, we validate it against both computer simulations and experiments. A critical overall test is provided by the Mean Square Displacement function at high Peclet numbers (up to 50). We demonstrate via simulations how the measurement accuracy depends on diffusion coefficients and flow rates, expressed in units of camera pixels and frames. Also sample-specific issues are addressed: inaccuracies in...
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Papers by Michel Duits