Deniz Gunes - Academia.edu (original) (raw)

Papers by Deniz Gunes

Soft Matter, 2013

ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can le... more ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop-drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our set-up. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Casep* was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event, are reported. Drop-drop contact times were varied by injecting fluid using different combinations of lateral channels. Casep* shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Coalescence under flow is one of the major causes for emulsion or foam destabilization in food pr... more Coalescence under flow is one of the major causes for emulsion or foam destabilization in food processes and consequently, understanding the hydrodynamic part of the coalescence mechanism(s) is key in order to prevent it. In our experimental investigations, we use microfluidic methods to vary the parameters governing size and capillary number of head-on collisions. Drops were exposed to an initially predominantly compressional and subsequently extensional flow field by means of a flow-through device with an expansion followed by a contraction. Our results show that even when the transient compressional flow is insufficient to allow the film to drain to critical thickness for rupture, the drops will often coalesce in the following `extensional' phase. A food emulsifier was observed to apparently enhance coalescence in extension for small concentration, but provide stabilization for higher concentration, by a specific mechanism.

Journal of Polymer Science Part A: Polymer Chemistry, 2011

The synthesis of poly(p-phenylene methylene) (PPM)-based block copolymers such as poly(p-phenylen... more The synthesis of poly(p-phenylene methylene) (PPM)-based block copolymers such as poly(p-phenylene methylene)-b-poly(e-caprolactone) and poly(p-phenylene methylene)-b-polytetrahydrofuran by mechanistic transformation was described. First, precursor PPM was synthesized by acid-catalyzed polymerization of tribenzylborate at 16 C. Then, this polymer was used as macroinitiators in either ring-opening polymerization of e-caprolactone or cationic ring-opening polymerization of tetrahydrofuran to yield respective block copolymers. The structures of the prepolymer and block copolymers were characterized by GPC and 1 H NMR investigations. The composition of block copolymers as determined by 1 H NMR and TGA analysis was found to be in very good agreement. The thermal behavior and surface morphology of the copolymers were also investigated, respectively, by differential scanning calorimetry and atomic force microscopy measurements, and the contribution of the major soft segment has been observed. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4021-4026, 2011

Journal of Non-Newtonian Fluid Mechanics, 2008

The flow-induced orientation of spheroidal particles in viscoelastic fluids is studied by means o... more The flow-induced orientation of spheroidal particles in viscoelastic fluids is studied by means of rheooptical methods and flow microscopy. Using suitable model systems a wide range of rotational Péclet and Weissenberg numbers have been covered, providing a systematic and global picture of the various orientational transitions. The effects of particle size and aspect ratio have been considered separately. Increasing the shear rate gradually causes first the well-known change from a random orientation to spinning in Jeffery orbits. Upon further increasing both the Péclet and Weissenberg numbers, the period of rotation becomes larger. At this stage the orbits start to drift slowly toward a 'log-rolling' state, which leads to an orientation distribution function that is narrowly peaked around the vorticity axis. For the fluids tested here, the orbit drift rates are proportional to the shear rate. At still higher elasticities, the particles are observed to reorient again in the flow direction, with the notable exception of suspensions in Boger fluids. Some of the experimentally observed features are qualitatively in line with the results of a theory for slender bodies in second order fluids, despite both relatively small aspect ratios and a more complex rheological behavior of the suspending media. An effect of absolute particle size is also noted which might indicate interference from Brownian motion, as suggested earlier. With suitable flow histories bimodal flow-vorticity orientation distributions can be generated. Finally, specific flow-induced alignment and aggregation are observed, they are more difficult to generate than in suspensions with spheres.

Journal of Colloid and Interface Science, 2013

The effect of wettability on microfluidic EDGE emulsification was investigated at dispersed phase... more The effect of wettability on microfluidic EDGE emulsification was investigated at dispersed phase contact angles between 90°and 160°. The highest contact angle (160°) produced monodispersed emulsions with droplet size 5.0 lm and coefficient of variation <10%; however, pressure stability was very low. This was greatly enhanced at contact angles <150°; the plateaus filled completely, and the droplet generation frequencies increased up to a factor of 2.0 and 3.5 for Tween20 and Tween60 respectively at the same pressures. The emulsion became highly polydispersed at contact angles <100°due to wetting with the dispersed phase.

status: published, 2005

Associatie KULeuven. ...

Proceedings of the European Polymer Congress EPF-2005, 2005

Associatie KULeuven. ...

status: published, 2007

Associatie KULeuven. ...

Journal of Colloid and Interface Science, Jun 15, 2005

We have studied the dynamics of the flocculation of poly(styrene-butadiene-acrylic acid) latex su... more We have studied the dynamics of the flocculation of poly(styrene-butadiene-acrylic acid) latex suspensions. These suspensions were flocculated by the addition of Ca 2+ ions at high concentrations of latex particles. Using diffusing wave spectroscopy and dynamic single light scattering after dilution, we have observed-depending on the pH and on the Ca 2+ concentration-several scenarios for flocculation including successive flocculation and deflocculation. This complex behavior reveals that the Ca 2+ migration within the shell of the latex is slow in acidic solvent but fast in basic solvent.

Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2016

An osmotic imbalance between the two water phases of multiple water-in-oil-in-water (W1/O/W2) emu... more An osmotic imbalance between the two water phases of multiple water-in-oil-in-water (W1/O/W2) emulsions results in either emulsion swelling or shrinking due to water migration across the oil layer. Controlled mass transport is not only of importance for emulsion stability but also allows transient emulsion thickening or the controlled release of encapsulated substances, such as nutriments or simply salt. Our prior work has shown that mass transport follows two sequential stages. In the first stage, the oil-phase structure is changed in a way that allows rapid, osmotically driven water transport in the second, osmotically dominated stage. These structural changes in the oil layer are strongly facilitated by the spontaneous formation of tiny water droplets in the oil phase, induced by the oil-soluble surfactant, i.e., polyglycerol polyricinoleate (PGPR). This study provides a simple method based on microscopy image analysis, allowing a detailed investigation of spontaneous W/O emulsif...

Langmuir, Oct 5, 2010

Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two d... more Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two different lengths, were investigated at pH 2, through surface tension measurements at water-air and water-oil interfaces and interfacial shear rheology at the water-oil interface. The applied heat treatment generates a mixed system of fibers with unconverted monomers and hydrolyzed peptides. The surface tension of this system at the water-air interface decreased more rapidly than the surface tension of native monomers, especially at short times (10(-3) to 10(2) s). This behavior was not observed when the unconverted monomers and peptides were removed by dialysis. At the water-oil interface, the adsorption kinetics was much faster than at the water-air interface, with a plateau interfacial pressure value reached after 1 h of adsorption. For all the systems, interfacial shear rheology showed the formation of a highly elastic interface, with solid-like behavior at 1-10(3) s time scales. The highest modulus was observed for the long fibers and the lowest for the native monomers. Creep-compliance curves in the linear regime could be reduced to a single master curve, showing similar spectra of relaxation times for all investigated systems. Upon large deformations, the interfaces formed with long fibers showed the most rigid and fragile behavior. This rigidity was even more pronounced in the presence of unconverted monomers.

Bulletin of the American Physical Society, Nov 24, 2013

We investigate the dissolution of a single air-in-water bubble whose surface is coated with solid... more We investigate the dissolution of a single air-in-water bubble whose surface is coated with solid particles, as an elementary model of an aging particle-stabilized foam. A microfluidic setup is used to produce a single bubble on demand, force the adsorption of particles to its interface, and hold it stationnary for long-term observation. When the gas dissolves in the surrounding liquid, the particles on the interface eventually jam, thus forming a rigid shell that encloses the bubble. As the temperature and pressure conditons are varied, this armor can either arrest the dissolution of the gas or it can buckle, which leads to the complete disapearance of the bubble. We experimentally demonstrate the existence of a threshold pressure above which the shell is not resistant enough to stabilize the bubble. This is modeled by comparing the mechanical resistance of the hollow shell with the compressive stress due to the dissolution in the liquid, which is controlled through the thermodynamic parameters. These experiments yield the first quantitative measurements of the mechanical resistance of a colloidal shell against ripening. It opens the possibility to study the behavior of more complex armors, by varying the size distribution, the shape and the chemistry of the particles.

Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two d... more Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two different lengths, were investigated at pH 2, through surface tension measurements at water-air and water-oil interfaces and interfacial shear rheology at the water-oil interface. The applied heat treatment generates a mixed system of fibers with unconverted monomers and hydrolyzed peptides. The surface tension of this system at the water-air interface decreased more rapidly than the surface tension of native monomers, especially at short times (10 -3 to 10 2 s). This behavior was not observed when the unconverted monomers and peptides were removed by dialysis. At the water-oil interface, the adsorption kinetics was much faster than at the water-air interface, with a plateau interfacial pressure value reached after 1 h of adsorption. For all the systems, interfacial shear rheology showed the formation of a highly elastic interface, with solidlike behavior at 1-10 3 s time scales. The highest modulus was observed for the long fibers and the lowest for the native monomers. Creep-compliance curves in the linear regime could be reduced to a single master curve, showing similar spectra of relaxation times for all investigated systems. Upon large deformations, the interfaces formed with long fibers showed the most rigid and fragile behavior. This rigidity was even more pronounced in the presence of unconverted monomers.

Langmuir : the ACS journal of surfaces and colloids, Jan 19, 2015

This contribution reports on the mass transport kinetics of osmotically imbalanced water-in-oil-i... more This contribution reports on the mass transport kinetics of osmotically imbalanced water-in-oil-in-water (W1/O/W2) emulsions. Although frequently studied, the control of mass transport in W1/O/W2 emulsions is still challenging. We describe a microfluidics-based method to systematically investigate the impact of various parameters, such as osmotic pressure gradient, oil phase viscosity, and temperature, on the mass transport. Combined with optical microscopy analyses, we are able to identify and decouple the various mechanisms, which control the dynamic droplet size of osmotically imbalanced W1/O/W2 emulsions. So, swelling kinetics curves with a very high accuracy are generated, giving a basis for quantifying the kinetic aspects of transport. Two sequential swelling stages, i.e., a lag stage and an osmotically dominated stage, with different mass transport mechanisms are identified. The determination and interpretation of the different stages are the prerequisite to control and trigg...

Soft Matter, 2013

ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can le... more ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop-drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our set-up. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Casep* was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event, are reported. Drop-drop contact times were varied by injecting fluid using different combinations of lateral channels. Casep* shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Soft Matter, 2013

ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can le... more ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop-drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our set-up. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Casep* was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event, are reported. Drop-drop contact times were varied by injecting fluid using different combinations of lateral channels. Casep* shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Coalescence under flow is one of the major causes for emulsion or foam destabilization in food pr... more Coalescence under flow is one of the major causes for emulsion or foam destabilization in food processes and consequently, understanding the hydrodynamic part of the coalescence mechanism(s) is key in order to prevent it. In our experimental investigations, we use microfluidic methods to vary the parameters governing size and capillary number of head-on collisions. Drops were exposed to an initially predominantly compressional and subsequently extensional flow field by means of a flow-through device with an expansion followed by a contraction. Our results show that even when the transient compressional flow is insufficient to allow the film to drain to critical thickness for rupture, the drops will often coalesce in the following `extensional' phase. A food emulsifier was observed to apparently enhance coalescence in extension for small concentration, but provide stabilization for higher concentration, by a specific mechanism.

Journal of Polymer Science Part A: Polymer Chemistry, 2011

The synthesis of poly(p-phenylene methylene) (PPM)-based block copolymers such as poly(p-phenylen... more The synthesis of poly(p-phenylene methylene) (PPM)-based block copolymers such as poly(p-phenylene methylene)-b-poly(e-caprolactone) and poly(p-phenylene methylene)-b-polytetrahydrofuran by mechanistic transformation was described. First, precursor PPM was synthesized by acid-catalyzed polymerization of tribenzylborate at 16 C. Then, this polymer was used as macroinitiators in either ring-opening polymerization of e-caprolactone or cationic ring-opening polymerization of tetrahydrofuran to yield respective block copolymers. The structures of the prepolymer and block copolymers were characterized by GPC and 1 H NMR investigations. The composition of block copolymers as determined by 1 H NMR and TGA analysis was found to be in very good agreement. The thermal behavior and surface morphology of the copolymers were also investigated, respectively, by differential scanning calorimetry and atomic force microscopy measurements, and the contribution of the major soft segment has been observed. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4021-4026, 2011

Journal of Non-Newtonian Fluid Mechanics, 2008

The flow-induced orientation of spheroidal particles in viscoelastic fluids is studied by means o... more The flow-induced orientation of spheroidal particles in viscoelastic fluids is studied by means of rheooptical methods and flow microscopy. Using suitable model systems a wide range of rotational Péclet and Weissenberg numbers have been covered, providing a systematic and global picture of the various orientational transitions. The effects of particle size and aspect ratio have been considered separately. Increasing the shear rate gradually causes first the well-known change from a random orientation to spinning in Jeffery orbits. Upon further increasing both the Péclet and Weissenberg numbers, the period of rotation becomes larger. At this stage the orbits start to drift slowly toward a 'log-rolling' state, which leads to an orientation distribution function that is narrowly peaked around the vorticity axis. For the fluids tested here, the orbit drift rates are proportional to the shear rate. At still higher elasticities, the particles are observed to reorient again in the flow direction, with the notable exception of suspensions in Boger fluids. Some of the experimentally observed features are qualitatively in line with the results of a theory for slender bodies in second order fluids, despite both relatively small aspect ratios and a more complex rheological behavior of the suspending media. An effect of absolute particle size is also noted which might indicate interference from Brownian motion, as suggested earlier. With suitable flow histories bimodal flow-vorticity orientation distributions can be generated. Finally, specific flow-induced alignment and aggregation are observed, they are more difficult to generate than in suspensions with spheres.

Journal of Colloid and Interface Science, 2013

The effect of wettability on microfluidic EDGE emulsification was investigated at dispersed phase... more The effect of wettability on microfluidic EDGE emulsification was investigated at dispersed phase contact angles between 90°and 160°. The highest contact angle (160°) produced monodispersed emulsions with droplet size 5.0 lm and coefficient of variation <10%; however, pressure stability was very low. This was greatly enhanced at contact angles <150°; the plateaus filled completely, and the droplet generation frequencies increased up to a factor of 2.0 and 3.5 for Tween20 and Tween60 respectively at the same pressures. The emulsion became highly polydispersed at contact angles <100°due to wetting with the dispersed phase.

status: published, 2005

Associatie KULeuven. ...

Proceedings of the European Polymer Congress EPF-2005, 2005

Associatie KULeuven. ...

status: published, 2007

Associatie KULeuven. ...

Journal of Colloid and Interface Science, Jun 15, 2005

We have studied the dynamics of the flocculation of poly(styrene-butadiene-acrylic acid) latex su... more We have studied the dynamics of the flocculation of poly(styrene-butadiene-acrylic acid) latex suspensions. These suspensions were flocculated by the addition of Ca 2+ ions at high concentrations of latex particles. Using diffusing wave spectroscopy and dynamic single light scattering after dilution, we have observed-depending on the pH and on the Ca 2+ concentration-several scenarios for flocculation including successive flocculation and deflocculation. This complex behavior reveals that the Ca 2+ migration within the shell of the latex is slow in acidic solvent but fast in basic solvent.

Langmuir : the ACS journal of surfaces and colloids, Jan 14, 2016

An osmotic imbalance between the two water phases of multiple water-in-oil-in-water (W1/O/W2) emu... more An osmotic imbalance between the two water phases of multiple water-in-oil-in-water (W1/O/W2) emulsions results in either emulsion swelling or shrinking due to water migration across the oil layer. Controlled mass transport is not only of importance for emulsion stability but also allows transient emulsion thickening or the controlled release of encapsulated substances, such as nutriments or simply salt. Our prior work has shown that mass transport follows two sequential stages. In the first stage, the oil-phase structure is changed in a way that allows rapid, osmotically driven water transport in the second, osmotically dominated stage. These structural changes in the oil layer are strongly facilitated by the spontaneous formation of tiny water droplets in the oil phase, induced by the oil-soluble surfactant, i.e., polyglycerol polyricinoleate (PGPR). This study provides a simple method based on microscopy image analysis, allowing a detailed investigation of spontaneous W/O emulsif...

Langmuir, Oct 5, 2010

Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two d... more Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two different lengths, were investigated at pH 2, through surface tension measurements at water-air and water-oil interfaces and interfacial shear rheology at the water-oil interface. The applied heat treatment generates a mixed system of fibers with unconverted monomers and hydrolyzed peptides. The surface tension of this system at the water-air interface decreased more rapidly than the surface tension of native monomers, especially at short times (10(-3) to 10(2) s). This behavior was not observed when the unconverted monomers and peptides were removed by dialysis. At the water-oil interface, the adsorption kinetics was much faster than at the water-air interface, with a plateau interfacial pressure value reached after 1 h of adsorption. For all the systems, interfacial shear rheology showed the formation of a highly elastic interface, with solid-like behavior at 1-10(3) s time scales. The highest modulus was observed for the long fibers and the lowest for the native monomers. Creep-compliance curves in the linear regime could be reduced to a single master curve, showing similar spectra of relaxation times for all investigated systems. Upon large deformations, the interfaces formed with long fibers showed the most rigid and fragile behavior. This rigidity was even more pronounced in the presence of unconverted monomers.

Bulletin of the American Physical Society, Nov 24, 2013

We investigate the dissolution of a single air-in-water bubble whose surface is coated with solid... more We investigate the dissolution of a single air-in-water bubble whose surface is coated with solid particles, as an elementary model of an aging particle-stabilized foam. A microfluidic setup is used to produce a single bubble on demand, force the adsorption of particles to its interface, and hold it stationnary for long-term observation. When the gas dissolves in the surrounding liquid, the particles on the interface eventually jam, thus forming a rigid shell that encloses the bubble. As the temperature and pressure conditons are varied, this armor can either arrest the dissolution of the gas or it can buckle, which leads to the complete disapearance of the bubble. We experimentally demonstrate the existence of a threshold pressure above which the shell is not resistant enough to stabilize the bubble. This is modeled by comparing the mechanical resistance of the hollow shell with the compressive stress due to the dissolution in the liquid, which is controlled through the thermodynamic parameters. These experiments yield the first quantitative measurements of the mechanical resistance of a colloidal shell against ripening. It opens the possibility to study the behavior of more complex armors, by varying the size distribution, the shape and the chemistry of the particles.

Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two d... more Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two different lengths, were investigated at pH 2, through surface tension measurements at water-air and water-oil interfaces and interfacial shear rheology at the water-oil interface. The applied heat treatment generates a mixed system of fibers with unconverted monomers and hydrolyzed peptides. The surface tension of this system at the water-air interface decreased more rapidly than the surface tension of native monomers, especially at short times (10 -3 to 10 2 s). This behavior was not observed when the unconverted monomers and peptides were removed by dialysis. At the water-oil interface, the adsorption kinetics was much faster than at the water-air interface, with a plateau interfacial pressure value reached after 1 h of adsorption. For all the systems, interfacial shear rheology showed the formation of a highly elastic interface, with solidlike behavior at 1-10 3 s time scales. The highest modulus was observed for the long fibers and the lowest for the native monomers. Creep-compliance curves in the linear regime could be reduced to a single master curve, showing similar spectra of relaxation times for all investigated systems. Upon large deformations, the interfaces formed with long fibers showed the most rigid and fragile behavior. This rigidity was even more pronounced in the presence of unconverted monomers.

Langmuir : the ACS journal of surfaces and colloids, Jan 19, 2015

This contribution reports on the mass transport kinetics of osmotically imbalanced water-in-oil-i... more This contribution reports on the mass transport kinetics of osmotically imbalanced water-in-oil-in-water (W1/O/W2) emulsions. Although frequently studied, the control of mass transport in W1/O/W2 emulsions is still challenging. We describe a microfluidics-based method to systematically investigate the impact of various parameters, such as osmotic pressure gradient, oil phase viscosity, and temperature, on the mass transport. Combined with optical microscopy analyses, we are able to identify and decouple the various mechanisms, which control the dynamic droplet size of osmotically imbalanced W1/O/W2 emulsions. So, swelling kinetics curves with a very high accuracy are generated, giving a basis for quantifying the kinetic aspects of transport. Two sequential swelling stages, i.e., a lag stage and an osmotically dominated stage, with different mass transport mechanisms are identified. The determination and interpretation of the different stages are the prerequisite to control and trigg...

Soft Matter, 2013

ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can le... more ABSTRACT The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop-drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our set-up. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Casep* was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event, are reported. Drop-drop contact times were varied by injecting fluid using different combinations of lateral channels. Casep* shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.