The rate of infiltration of fluids into bimodal porous systems was modeled to decrease as the rat... more The rate of infiltration of fluids into bimodal porous systems was modeled to decrease as the ratio of the pore size to pore neck size was increased.
The integration of smartphones and microfluidics is nowadays the best possible route to achieve e... more The integration of smartphones and microfluidics is nowadays the best possible route to achieve effective point-of-need testing (PONT), a concept increasingly demanded in the fields of human health, agriculture, food safety, and environmental monitoring. Nevertheless, efforts are still required to integrally seize all the advantages of smartphones, as well as to share the developments in easily adoptable formats. For this purpose, here we present the free platform appuente that was designed for the easy integration of microfluidic chips, smartphones, and the cloud. It includes a mobile app for end users, which provides chip identification and tracking, guidance and control, processing, smart-imaging, result reporting and cloud and Internet of Things (IoT) integration. The platform also includes a web app for PONT developers, to easily customize their mobile apps and manage the data of administered tests. Three application examples were used to validate appuente: a dummy grayscale de...
Achieving active control on small amounts of liquids represents a substantial challenge in both s... more Achieving active control on small amounts of liquids represents a substantial challenge in both scientific and engineering aspects. Imbibition of fluids in bodies with nanoscale dimensions enables the spontaneous propelling of nano-flows because of the powerful capillarity at small-length scales. Peculiarities of nanopore imbibition at the thin film level lead to distinctive capillary transport phenomena of fluids across the nanopore matrix. This particular imbibition also impacts on the behavior of the in-contact liquid micro-volumes. These both features add versatile alternatives to the high interest in the management of femtolitre to microlitres amounts of liquids. Herein, we show a brief discussion-outlook based on recent advances in the design of versatile tools to attain programmable nano/microfluidics using mesoporous thin film platforms.
In this paper we consider coverings of the plane by circles of two different sizes. We establish ... more In this paper we consider coverings of the plane by circles of two different sizes. We establish a sufficient condition for such a covering to be solid in the sense of L. Fejes Tóth [6]. As an application of this general theorem we prove that there exist infinitely many solid coverings of this kind. * The first author wishes to thank the Austrian and Hungarian Academies of Science for supporting a visit to Budapest in May 1997.
Electrophoretic separations comprise a group of analytical techniques such as capillary zone elec... more Electrophoretic separations comprise a group of analytical techniques such as capillary zone electrophoresis, isoelectric focusing, isotachophoresis, and free flow electrophoresis. These techniques have been miniaturized in the last years and now represent one of the most important applications of the lab-on-a-chip technology. A 3D and time-dependent numerical model of electrophoresis on microfluidic devices is presented. The model is based on the set of equations that governs electrical phenomena, fluid dynamics, mass transport, and chemical reactions. The relationship between the buffer characteristics (ionic strength and pH) and surface potential of channel walls is taken into consideration. Numerical calculations were performed by using PETSc-FEM, in a Python environment, employing high performance parallel computing. The method includes a set of last generation preconditioners and solvers, especially addressed to 3D microfluidic problems, which significantly improve the numerical efficiency in comparison with typical commercial software for multiphysics. In this work, after discussing two validation examples, the numerical prototyping of a microfluidic chip for twodimensional electrophoresis is presented.
The rheological behavior of suspensions containing cross-linked polymer particles (microgels) was... more The rheological behavior of suspensions containing cross-linked polymer particles (microgels) was studied. Under flow, these particles behave as soft spheres due to the shear-induced deformability of their external polymeric layer and the suspensions are strongly shear-thinning. At concentrations where particles are densely packed, the suspensions present a solidlike behavior involving elasticity and yield stress. The rheological modeling of such suspensions was based on the concept of the effective volume fraction of particles, which depends on hydrodynamic forces. In this sense, microgel particles were assumed to have a simple core-shell configuration and to interact through a repulsive polymer-polymer potential. The viscosity equation resulting from the modeling allows: (a) appropriate correlation of the experimental data shear stress versus shear rate in steady shear flow and (b) accounting for the softness of particles, which changes with concentration and shear stress. Also the predictions of the model showed to be quite consistent with results obtained previously for the same suspensions through different techniques such as dynamic rheometry and rheoptics.
A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cyl... more A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cylindrical microchannels is presented. Calculations are based on constitutive models of the fluid viscosity, and take into account wall depletion effects of colloids and polymer solutions. The resulting equations allow one to predict the flow rate and electric current as functions of the simultaneously applied electric potential and pressure gradients. It is found that (i) nonlinear effects induced by the shear-dependent viscosity are limited to the pressure-driven component of the flow, and (ii) the reciprocity between electroosmosis and streaming current is complied. Thus a generalized form of the force-flux relations is proposed, which is of interest in microfluidic applications.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
The gel transition of depletion flocculated oil-in-water emulsions is analyzed as a function of t... more The gel transition of depletion flocculated oil-in-water emulsions is analyzed as a function of the strength of the droplet Á/droplet interaction. The critical volume fraction for gelation has been found to decrease exponentially with the magnitude of the interaction energy at contact, in agreement with recent proposals for aggregating colloids. The viscosity modeling of these emulsions had been discussed in a previous work (Colloids Surf. A 203 (2002) 11). The results reported here show the interplay between phase behavior and rheology of flocculated emulsions.
This work deals with the problem of deriving theoretical connections between rheology and interpa... more This work deals with the problem of deriving theoretical connections between rheology and interparticle forces in colloidal suspensions. The nature of interparticle forces de-Ž termines the colloidal structure crystalline order due to long range repulsive forces,. flocculation due to attractive forces, etc. and hence, the flow behavior of suspensions. The aim of this article is to discuss how these interactions enter the modeling of rheometric functions, in particular, the shear viscosity. In this sense, the main interactions commonly appearing in colloids are reviewed, as well as the role they play in phase transition behavior. Then, a series of approaches relating the interaction potential to viscosity is examined. The results of applying these models to experimental data are also discussed. Finally, examples of viscosity modeling for different interaction potentials are given, by using the structural model proposed previously by the authors. The possibility of relating the flow behavior of colloidal suspensions to the interaction between particles offers new perspectives for the study and technical applications of these systems.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2002
The rheological behavior of protein stabilized oil-in-water (O/W) emulsions containing free prote... more The rheological behavior of protein stabilized oil-in-water (O/W) emulsions containing free protein (sodium caseinate) in solution has been studied. The emulsions are shear-thinning due to reversible, depletion-induced flocculation. The effects of both protein and salt (sodium chloride) concentrations on the flow behavior of the emulsions have been analyzed. For this purpose, the effective potential of interaction between particles was estimated from rheometric data, using an appropriate viscosity model. Oil droplets were assumed to behave as hard spheres and to flocculate in the secondary minimum of the interaction potential curve. The rheometric results correlated quite well with theoretical predictions involving the DLVO theory plus an attractive potential due to depletion.
Main characteristics of colloidal systems that develop fluid phases with different mechanical pro... more Main characteristics of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids, are briefly reviewed both from experimental and theoretical (modelling) point of view. A non-monotonic shear stress vs. shear rate constitutive relation is presented. This relation derives from a phenomenological model of a shear ratedependent viscosity describing structural changes and involves the possibility of multivalued shear rates under a given shear stress. In the case of a stress-dependent viscosity, the same model allows one to predict vorticity banding. Predictions of this model under controlled stress are discussed, namely occurrence of a kind of top- and bottom-jumping of the shear rate in response to stress increasing-decreasing. Applying this model to evaluation of the flow curve of such colloidal systems is performed. Particular emphasis is placed on the adequate computation of the shear rate function in cylindrical Couette cells in o...
We propose a formulation for modeling the squeeze film air damping in micro-plates typical of mic... more We propose a formulation for modeling the squeeze film air damping in micro-plates typical of micro-electromechanical devices for micro switch applications. A special finite element is developed, in which the nonlinear Reynolds equation for compressible film is used to analyze the air pressure field, whereas a standard linear elastic model is used for the displacement field. The formulation is based on a finite element discretization of both the pressure and displacement fields. The coupled equations of motion are established and, for harmonic oscillations, we show that the resulting damping matrix depends on the frequency. The typical dimensions and properties of the MEMS device are in the order of hundred micrometers length and some micrometers (3-8 um) thick, with a separation from the substrate of also some micrometers (e.g. 3-5 um). For these dimensions, the influence of damping owing to the surrounding air cannot be neglected, having an important contribution to the quality fa...
This work deals with the evaluation of the flow curve of colloidal systems that develop fluid pha... more This work deals with the evaluation of the flow curve of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids. The problem involved is that, as different fluid phases coexist in the flow domain of the rheometric cell, measured data cannot be directly converted into rheometric functions. In order to handle this problem, a shear stress vs. shear rate constitutive relation is introduced to interpret the steady state flow curves. The relation derives from a phenomenological description of structural changes, and involves the possibility of multivalued shear rates under a given shear stress. Numerical predictions satisfactorily match up to experimental data of wormlike micellar solutions. A crucial aspect is the adequate computation of the shear rate function from raw data measured in the rheometric cell.
The fluid front dynamics resulting from the coexisting infiltration and evaporation phenomena in ... more The fluid front dynamics resulting from the coexisting infiltration and evaporation phenomena in nanofluidic systems has been recently studied by using well-characterized mesoporous thin films. A series of striking effects have been observed, namely (i) the infiltration rate is abnormally slow and decreases with increasing pore diameter, (ii) the fluid front manifest an optically invisible roughening, and (iii) under certain conditions, the infiltration-evaporation front presents backwards-forwards oscillations. The present work describes a simple model accounting for the mechanisms behind some of these effects, and presents perspectives on the study of the more complex behaviors, with particular emphasis on the searchig for novel applications. Key-Words: nanofluidics, capillary infiltration, evaporation, front-broadening, complex dynamics Introduction: The versatility of mesoporous films is opening up many new uses in microfluidic devices, for instance in biocatalysis [1]. Neverthe...
Los dispositivos de microfluídica denominados laboratorios en chips se han consolidado como la nu... more Los dispositivos de microfluídica denominados laboratorios en chips se han consolidado como la nueva tecnología para ensayos de laboratorio y sus aplicaciones continúan expandiéndose. En este ámbito de permanente innovación, el papel ha emergido recientemente como un sustrato de gran interés, particularmente para fabricar dispositivos portables, autónomos y de bajo costo, con aplicaciones en diagnóstico médico, cuidado personal, control de calidad de alimentos y monitoreo ambiental. Los microdispositivos se acoplan además a teléfonos móviles para realizar el análisis y la transmisión de datos, lo cual potencia enormemente su desempeño. En este trabajo se realiza una breve revisión el tema, destacando algunos avances recientes en la investigación, y se discuten las perspectivas para desarrollar la tecnología en nuestra región.
Main characteristics of colloidal systems that develop fluid phases with different mechanical pro... more Main characteristics of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids, are briefly reviewed both from experimental and theoretical (modelling) point of view. A non-monotonic shear stress vs. shear rate constitutive relation is presented. This relation derives from a phenomenological model of a shear ratedependent viscosity describing structural changes and involves the possibility of multivalued shear rates under a given shear stress. In the case of a stress-dependent viscosity, the same model allows one to predict vorticity banding. Predictions of this model under controlled stress are discussed, namely occurrence of a kind of top- and bottom-jumping of the shear rate in response to stress increasing-decreasing. Applying this model to evaluation of the flow curve of such colloidal systems is performed. Particular emphasis is placed on the adequate computation of the shear rate function in cylindrical Couette cells in o...
The rate of infiltration of fluids into bimodal porous systems was modeled to decrease as the rat... more The rate of infiltration of fluids into bimodal porous systems was modeled to decrease as the ratio of the pore size to pore neck size was increased.
The integration of smartphones and microfluidics is nowadays the best possible route to achieve e... more The integration of smartphones and microfluidics is nowadays the best possible route to achieve effective point-of-need testing (PONT), a concept increasingly demanded in the fields of human health, agriculture, food safety, and environmental monitoring. Nevertheless, efforts are still required to integrally seize all the advantages of smartphones, as well as to share the developments in easily adoptable formats. For this purpose, here we present the free platform appuente that was designed for the easy integration of microfluidic chips, smartphones, and the cloud. It includes a mobile app for end users, which provides chip identification and tracking, guidance and control, processing, smart-imaging, result reporting and cloud and Internet of Things (IoT) integration. The platform also includes a web app for PONT developers, to easily customize their mobile apps and manage the data of administered tests. Three application examples were used to validate appuente: a dummy grayscale de...
Achieving active control on small amounts of liquids represents a substantial challenge in both s... more Achieving active control on small amounts of liquids represents a substantial challenge in both scientific and engineering aspects. Imbibition of fluids in bodies with nanoscale dimensions enables the spontaneous propelling of nano-flows because of the powerful capillarity at small-length scales. Peculiarities of nanopore imbibition at the thin film level lead to distinctive capillary transport phenomena of fluids across the nanopore matrix. This particular imbibition also impacts on the behavior of the in-contact liquid micro-volumes. These both features add versatile alternatives to the high interest in the management of femtolitre to microlitres amounts of liquids. Herein, we show a brief discussion-outlook based on recent advances in the design of versatile tools to attain programmable nano/microfluidics using mesoporous thin film platforms.
In this paper we consider coverings of the plane by circles of two different sizes. We establish ... more In this paper we consider coverings of the plane by circles of two different sizes. We establish a sufficient condition for such a covering to be solid in the sense of L. Fejes Tóth [6]. As an application of this general theorem we prove that there exist infinitely many solid coverings of this kind. * The first author wishes to thank the Austrian and Hungarian Academies of Science for supporting a visit to Budapest in May 1997.
Electrophoretic separations comprise a group of analytical techniques such as capillary zone elec... more Electrophoretic separations comprise a group of analytical techniques such as capillary zone electrophoresis, isoelectric focusing, isotachophoresis, and free flow electrophoresis. These techniques have been miniaturized in the last years and now represent one of the most important applications of the lab-on-a-chip technology. A 3D and time-dependent numerical model of electrophoresis on microfluidic devices is presented. The model is based on the set of equations that governs electrical phenomena, fluid dynamics, mass transport, and chemical reactions. The relationship between the buffer characteristics (ionic strength and pH) and surface potential of channel walls is taken into consideration. Numerical calculations were performed by using PETSc-FEM, in a Python environment, employing high performance parallel computing. The method includes a set of last generation preconditioners and solvers, especially addressed to 3D microfluidic problems, which significantly improve the numerical efficiency in comparison with typical commercial software for multiphysics. In this work, after discussing two validation examples, the numerical prototyping of a microfluidic chip for twodimensional electrophoresis is presented.
The rheological behavior of suspensions containing cross-linked polymer particles (microgels) was... more The rheological behavior of suspensions containing cross-linked polymer particles (microgels) was studied. Under flow, these particles behave as soft spheres due to the shear-induced deformability of their external polymeric layer and the suspensions are strongly shear-thinning. At concentrations where particles are densely packed, the suspensions present a solidlike behavior involving elasticity and yield stress. The rheological modeling of such suspensions was based on the concept of the effective volume fraction of particles, which depends on hydrodynamic forces. In this sense, microgel particles were assumed to have a simple core-shell configuration and to interact through a repulsive polymer-polymer potential. The viscosity equation resulting from the modeling allows: (a) appropriate correlation of the experimental data shear stress versus shear rate in steady shear flow and (b) accounting for the softness of particles, which changes with concentration and shear stress. Also the predictions of the model showed to be quite consistent with results obtained previously for the same suspensions through different techniques such as dynamic rheometry and rheoptics.
A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cyl... more A theoretical description of the electrokinetic flow of non-Newtonian fluids through slit and cylindrical microchannels is presented. Calculations are based on constitutive models of the fluid viscosity, and take into account wall depletion effects of colloids and polymer solutions. The resulting equations allow one to predict the flow rate and electric current as functions of the simultaneously applied electric potential and pressure gradients. It is found that (i) nonlinear effects induced by the shear-dependent viscosity are limited to the pressure-driven component of the flow, and (ii) the reciprocity between electroosmosis and streaming current is complied. Thus a generalized form of the force-flux relations is proposed, which is of interest in microfluidic applications.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
The gel transition of depletion flocculated oil-in-water emulsions is analyzed as a function of t... more The gel transition of depletion flocculated oil-in-water emulsions is analyzed as a function of the strength of the droplet Á/droplet interaction. The critical volume fraction for gelation has been found to decrease exponentially with the magnitude of the interaction energy at contact, in agreement with recent proposals for aggregating colloids. The viscosity modeling of these emulsions had been discussed in a previous work (Colloids Surf. A 203 (2002) 11). The results reported here show the interplay between phase behavior and rheology of flocculated emulsions.
This work deals with the problem of deriving theoretical connections between rheology and interpa... more This work deals with the problem of deriving theoretical connections between rheology and interparticle forces in colloidal suspensions. The nature of interparticle forces de-Ž termines the colloidal structure crystalline order due to long range repulsive forces,. flocculation due to attractive forces, etc. and hence, the flow behavior of suspensions. The aim of this article is to discuss how these interactions enter the modeling of rheometric functions, in particular, the shear viscosity. In this sense, the main interactions commonly appearing in colloids are reviewed, as well as the role they play in phase transition behavior. Then, a series of approaches relating the interaction potential to viscosity is examined. The results of applying these models to experimental data are also discussed. Finally, examples of viscosity modeling for different interaction potentials are given, by using the structural model proposed previously by the authors. The possibility of relating the flow behavior of colloidal suspensions to the interaction between particles offers new perspectives for the study and technical applications of these systems.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2002
The rheological behavior of protein stabilized oil-in-water (O/W) emulsions containing free prote... more The rheological behavior of protein stabilized oil-in-water (O/W) emulsions containing free protein (sodium caseinate) in solution has been studied. The emulsions are shear-thinning due to reversible, depletion-induced flocculation. The effects of both protein and salt (sodium chloride) concentrations on the flow behavior of the emulsions have been analyzed. For this purpose, the effective potential of interaction between particles was estimated from rheometric data, using an appropriate viscosity model. Oil droplets were assumed to behave as hard spheres and to flocculate in the secondary minimum of the interaction potential curve. The rheometric results correlated quite well with theoretical predictions involving the DLVO theory plus an attractive potential due to depletion.
Main characteristics of colloidal systems that develop fluid phases with different mechanical pro... more Main characteristics of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids, are briefly reviewed both from experimental and theoretical (modelling) point of view. A non-monotonic shear stress vs. shear rate constitutive relation is presented. This relation derives from a phenomenological model of a shear ratedependent viscosity describing structural changes and involves the possibility of multivalued shear rates under a given shear stress. In the case of a stress-dependent viscosity, the same model allows one to predict vorticity banding. Predictions of this model under controlled stress are discussed, namely occurrence of a kind of top- and bottom-jumping of the shear rate in response to stress increasing-decreasing. Applying this model to evaluation of the flow curve of such colloidal systems is performed. Particular emphasis is placed on the adequate computation of the shear rate function in cylindrical Couette cells in o...
We propose a formulation for modeling the squeeze film air damping in micro-plates typical of mic... more We propose a formulation for modeling the squeeze film air damping in micro-plates typical of micro-electromechanical devices for micro switch applications. A special finite element is developed, in which the nonlinear Reynolds equation for compressible film is used to analyze the air pressure field, whereas a standard linear elastic model is used for the displacement field. The formulation is based on a finite element discretization of both the pressure and displacement fields. The coupled equations of motion are established and, for harmonic oscillations, we show that the resulting damping matrix depends on the frequency. The typical dimensions and properties of the MEMS device are in the order of hundred micrometers length and some micrometers (3-8 um) thick, with a separation from the substrate of also some micrometers (e.g. 3-5 um). For these dimensions, the influence of damping owing to the surrounding air cannot be neglected, having an important contribution to the quality fa...
This work deals with the evaluation of the flow curve of colloidal systems that develop fluid pha... more This work deals with the evaluation of the flow curve of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids. The problem involved is that, as different fluid phases coexist in the flow domain of the rheometric cell, measured data cannot be directly converted into rheometric functions. In order to handle this problem, a shear stress vs. shear rate constitutive relation is introduced to interpret the steady state flow curves. The relation derives from a phenomenological description of structural changes, and involves the possibility of multivalued shear rates under a given shear stress. Numerical predictions satisfactorily match up to experimental data of wormlike micellar solutions. A crucial aspect is the adequate computation of the shear rate function from raw data measured in the rheometric cell.
The fluid front dynamics resulting from the coexisting infiltration and evaporation phenomena in ... more The fluid front dynamics resulting from the coexisting infiltration and evaporation phenomena in nanofluidic systems has been recently studied by using well-characterized mesoporous thin films. A series of striking effects have been observed, namely (i) the infiltration rate is abnormally slow and decreases with increasing pore diameter, (ii) the fluid front manifest an optically invisible roughening, and (iii) under certain conditions, the infiltration-evaporation front presents backwards-forwards oscillations. The present work describes a simple model accounting for the mechanisms behind some of these effects, and presents perspectives on the study of the more complex behaviors, with particular emphasis on the searchig for novel applications. Key-Words: nanofluidics, capillary infiltration, evaporation, front-broadening, complex dynamics Introduction: The versatility of mesoporous films is opening up many new uses in microfluidic devices, for instance in biocatalysis [1]. Neverthe...
Los dispositivos de microfluídica denominados laboratorios en chips se han consolidado como la nu... more Los dispositivos de microfluídica denominados laboratorios en chips se han consolidado como la nueva tecnología para ensayos de laboratorio y sus aplicaciones continúan expandiéndose. En este ámbito de permanente innovación, el papel ha emergido recientemente como un sustrato de gran interés, particularmente para fabricar dispositivos portables, autónomos y de bajo costo, con aplicaciones en diagnóstico médico, cuidado personal, control de calidad de alimentos y monitoreo ambiental. Los microdispositivos se acoplan además a teléfonos móviles para realizar el análisis y la transmisión de datos, lo cual potencia enormemente su desempeño. En este trabajo se realiza una breve revisión el tema, destacando algunos avances recientes en la investigación, y se discuten las perspectivas para desarrollar la tecnología en nuestra región.
Main characteristics of colloidal systems that develop fluid phases with different mechanical pro... more Main characteristics of colloidal systems that develop fluid phases with different mechanical properties, namely shear-banding fluids, are briefly reviewed both from experimental and theoretical (modelling) point of view. A non-monotonic shear stress vs. shear rate constitutive relation is presented. This relation derives from a phenomenological model of a shear ratedependent viscosity describing structural changes and involves the possibility of multivalued shear rates under a given shear stress. In the case of a stress-dependent viscosity, the same model allows one to predict vorticity banding. Predictions of this model under controlled stress are discussed, namely occurrence of a kind of top- and bottom-jumping of the shear rate in response to stress increasing-decreasing. Applying this model to evaluation of the flow curve of such colloidal systems is performed. Particular emphasis is placed on the adequate computation of the shear rate function in cylindrical Couette cells in o...
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Papers by Claudio Berli