J. Camacho - Academia.edu (original) (raw)

Papers by J. Camacho

American Journal of Botany, 2012

Soft Matter, 2011

Experimental and simulation studies of superparamagnetic colloids in strong external fields have ... more Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains an equilibrium distribution of aggregate sizes. A thermodynamic self-assembly theory supports the simulation results and it also predicts that the average aggregate size in the equilibrium state depends only on a dimensionless parameter combining the volume fraction of colloids φ0 and the magnetic coupling parameter Γ. The conditions under which this new behavior can be observed are discussed. PACS numbers: 83.10.Mj, 61.43.Hv, 82.70.Dd, 83.80.Gv Colloidal aggregation is a subject of active research for both practical (e.g. stability of many industrial products) and fundamental reasons (as a test field for statistical-mechanical theories, for example). Our interest here is in the new physics arising in the aggregation behavior of superparamagnetic colloids. These systems are a successful example of implementation of a new behavior typical of the nanoscale (superparamagnetism) in new materials with many exciting practical applications, ranging from environmental waste capture [1] to biomedicine . Superparamagnetic materials show a large magnetic dipole in presence of external field, saturation magnetization similar to that of ferromagnetic materials but no coercitivity nor remanence at the working temperature. Superparamagnetic colloids are typically made by embedding superparamagnetic nanocrystals in a non-magnetic matrix (such as polystyrene, nanoporous silica or others) .

Physical Review E, 2011

Half-lives of radionuclides span more than 50 orders of magnitude. We characterize the probabilit... more Half-lives of radionuclides span more than 50 orders of magnitude. We characterize the probability distribution of this broad-range data set at the same time that we explore a method for fitting power laws and testing goodness-of-fit. It is found that the procedure proposed recently by Clauset et al. [SIAM Rev. 51, 661 (2009)] does not perform well as it rejects the power-law hypothesis even for power-law synthetic data. In contrast, we establish the existence of a power-law exponent with a value around 1.1 for the half-life density, which can be explained by the sharp relationship between decay rate and released energy, for different disintegration types. For the case of alpha emission, this relationship constitutes an original mechanism of power-law generation.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2001

A mean-field approach for steady-state aggregation with injection is presented. It is shown that ... more A mean-field approach for steady-state aggregation with injection is presented. It is shown that for a wide variety of aggregation processes the resulting steady-size distribution obeys a power law N(m) approximately m(-alpha) with alpha=(3+beta)/2 and beta the degree of homogeneity of the coagulation kernel. The general conditions for this to happen are obtained. Some applications are studied. In particular, it predicts a potential behavior for coagulation in atmospheric aerosols with exponent alpha approximately 2, in agreement with observations. The theoretical results also agree with some animal group-size distributions and with numerical simulations in fractal aggregates.

ABSTRACT PRECISION MAGNETOPHORESIS SEPARATION OF SUPERPARAMAGNETIC COLLOIDS. The manipulation of ... more ABSTRACT PRECISION MAGNETOPHORESIS SEPARATION OF SUPERPARAMAGNETIC COLLOIDS. The manipulation of magnetic particles by the use of inhomogeneous magnetic fields (magnetophoresis) has emerged as a topic of great interest in a wide range of research and technological areas [1,2]. The idea behind magnetic separation is to take advantage of the distinctive magnetic response of the particles in solution to remove them from complex mixtures by the use of applied inhomogeneous magnetic fields [3]. In the different applications, magnetic particles are typically functionalized with proper chemical groups, designed to bind to specific non-magnetic components, thus enabling the separation of non-magnetic materials by combining the use of magnetic particles and magnetic fields. Current standard magnetophoretic techniques (such as High Gradient Magnetic Separation) suffer from different difficulties, including lack of reproducibility and scalability and the loose of control over the magnetic conditions under which the magnetic particles are removed. Basically, the external magnetic field applied induces highly inhomogeneous gradients (as large as 104 T/m) in the separator [4]. The magnetic fields generated in this way are not predictable or reproducible and the magnetic force experienced by the colloids is not uniform across the system. These inhomogeneous conditions common to the HGMS approach makes difficult to develop numerical and/or analytical solutions to the problem, which would help in a better understanding of the magnetophoretic mechanisms and a higher performance; for instance, by means of a better design of separators or a better choice of the magnetic particles used in specific applications. In order to overcome these limitations and facilitate the use of magnetic colloids in biotechnological applications we have made use of a new concept of magnetic separation (the so called Precision Magnetophoresis) to effectively remove different types of superparamagnetic nanoparticles from solution. The process is based on the use of a uniform magnetic gradient, allowing better quality control and scalability, together with a better control over the experimental conditions, providing a proper framework for the development of theoretical models (analytical or numerical). Within this approach, we have studied the magnetic separation of superparamagnetic colloids by theory and experiments. Under sufficiently large magnetic fields, a fast separation process occurs in which the superparamagnetic colloids form chain-like structures (reversible aggregation) aligned parallel to the applied external magnetic field. It is observed that, under the effects of a magnetic gradient, these structures move faster than would move an individual colloid in the same conditions, resulting in a dramatic reduction (orders of magnitude) of the separation time. Thus, the reversible aggregation of colloids becomes a key aspect when one thinks to enhance the magnetic separation of magnetic colloids. The kinetics of both separation processes (cooperative and non-cooperative magnetophoresis) has been characterized theoretically and experimentally as a function of the different properties of the colloidal dispersion (magnetic response, size of particles, concentration, solvent viscosity, etc, ...) and the magnetic separator (magnetic gradient and size of the separator) [5]. Following the experimental results obtained for different combinations of magnetic colloids and magnetic fields, the required conditions for the reversible aggregation to occur has been recently discussed on the base of computer simulations and thermodynamical models [6]. In the much simpler situation in which the separation process is driven by the individual motion of the colloids, an analytical solution for the kinetics of the magnetophoresis separation process is provided [7]. The obtained solution is valid under certain restrictive but realistic conditions which are explicitly discussed here. We also show the utility of the analytical model by comparing our predictions with experimental results obtained with superparamagnetic particles of different sizes and magnetizations. We expect that the availability of a simple, analytical model will allow for a better understanding of the underlying physics of magnetic separation processes and also allow a rational design of applications. References: [1] C. T. Yavuz, A. Prakash, J. T. Mayo, V. L. Colvin, Chem. Eng. Sci., 64, 2510 (2009). [2] J. L. Corchero, A. Villaverde, Trends Biotechnol., 27, 468 (2009). [3] G. Friedman and B. Yellen, Curr. Opin. Colloid Interface Sci., 10, 158 (2005). [4] G. D. Moeser, K. A. Roach, W. H. Green, T. A. Hatton, P. E. Laibinis, AIChE J., 50, 2835 (2004). [5] G. De Las Cuevas, J. Faraudo and J. Camacho, J. Phys. Chem. C, 112, 945-950 (2008). [6] J. S. Andreu, J. Camacho, J. Faraudo, Soft Matter, 7, 2336 (2011). [7] J. S. Andreu et al. submitted to Phys. Rev. E, (2011).

Journal of Physical Chemistry C, 2008

Recent experiments (Yavuz, CT et al., Science 2006, 314, 964) show the possibility of low gradien... more Recent experiments (Yavuz, CT et al., Science 2006, 314, 964) show the possibility of low gradient magnetophoretic separation of superparamagnetic nanoparticles in aqueous solution, a process with broad potentially important applications ranging from biomedicine ...

Journal of Nanomaterials, 2012

Magnetic separation has gained much attention due to its implications in different fields, becomi... more Magnetic separation has gained much attention due to its implications in different fields, becoming feasible as an alternative to existent technologies at the industrial and lab scale. Substantial efforts are focused to improve the magnetic particles used in these applications. Here we show how a relatively simple and low-cost simulation strategy (tracer simulations) can be employed to predict the effect of various key factors in magnetic separation processes, namely, particle properties and magnetic separator designs. For concreteness, we consider here specific problems in magnetic separation. The first one is the effect of different profiles of the magnetic field in the separation of magnetic nanoparticles, and the second one is the magnetophoresis of colloidal particles in a dispersion of magnetic nanoparticles.

Physical Review E, 2012

The aim of this work is the description of the chain formation phenomena observed in colloidal su... more The aim of this work is the description of the chain formation phenomena observed in colloidal suspensions of superparamagnetic nanoparticles under high magnetic fields. We propose a new methodology based on an on-the-fly Coarse-Grain (CG) model. Within this approach, the coarse grain objects of the simulation are not fixed a priori at the beginning of the simulation but rather redefined on the fly. The motion of the CG objects (single particles or aggregates) is described by an anisotropic diffusion model and the magnetic dipole-dipole interaction is replaced by an effective short range interaction between CG objects. The new methodology correctly reproduces previous results from detailed Langevin Dynamics simulations of dispersions of superparamagnetic colloids under strong fields whilst requiring an amount of CPU time orders of magnitude smaller. This substantial improvement in the computational requirements allows the simulation of problems in which the relevant phenomena extends to time scales inaccessible with previous simulation techniques. A relevant example is the waiting time dependence of the relaxation time T2 of water protons observed in Magnetic Resonance experiments containing dispersions of superparamagnetic colloids, which is correctly predicted by our simulations. Future applications may include other popular real-world applications of superparamagnetic colloids such as the magnetophoretic separation processes.

Soft Matter, 2013

In recent years, there has been a great progress in the development of superparamagnetic particle... more In recent years, there has been a great progress in the development of superparamagnetic particles targeted to a wide range of applications, including fields as diverse as biotechnology or waste removal.

Soft Matter, 2012

Recent works have demonstrated the exciting possibility of inducing a tunable magnetic behavior i... more Recent works have demonstrated the exciting possibility of inducing a tunable magnetic behavior in non-magnetic colloids by immersing them in a dispersion of superparamagnetic nanoparticles (NPs).

Physical Review E, 2011

Magnetophoresis-the motion of magnetic particles under applied magnetic gradient-is a process of ... more Magnetophoresis-the motion of magnetic particles under applied magnetic gradient-is a process of great interest in novel applications of magnetic nanoparticles and colloids. In general, there are two main different types of magnetophoresis processes: cooperative magnetophoresis (a fast process enhanced by particle-particle interactions) and noncooperative magnetophoresis (driven by the motion of individual particles in magnetic fields). In the case of noncooperative magnetophoresis, we have obtained a simple analytical solution which allows the prediction of the magnetophoresis kinetics from particle characterization data (size and magnetization). Our comparison with new experimental results shows good quantitative agreement. In addition, we show the existence of a universal curve onto which all experimental results should collapse after proper rescaling. The range of applicability of the analytical solution is discussed in light of the predictions of a magnetic aggregation model [Soft Matter 7, 2336].

Journal of Theoretical Biology, 2007

Journal of Applied Physics, 2011

ABSTRACT The transverse relaxation time T2 of protons in water suspensions of iron-oxide particle... more ABSTRACT The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied.

Colloid and Polymer Science, 2010

Superparamagnetic colloids have a great practical interest for their applications to processes ra... more Superparamagnetic colloids have a great practical interest for their applications to processes ranging from biomedicine to environmental waste and pollutants removal. A fast and efficient separation of these particles from the solvent constitutes a key step in the practical implementation of this technology. Recent experiments show fast magnetophoretic separation using relatively small magnetic gradients and high magnetic fields. The mechanism underlying this fast separation was shown to be the reversible aggregation of the magnetic beads induced by the external field. In this paper, we analyze theoretically the physicochemical conditions under which reversible aggregation can be typically achieved, the timescale at which aggregates form, and their shape. In the case of colloids stabilized electrostatically, for reasonable surface potentials (approximately −70 mV), we find that the interaction potential between two superparamagnetic particles displays a barrier with a minimum so that reversible aggregates can form. We also show that the aggregation of particles is quite fast (typically less than a second for usual concentrations) and that lateral aggregation is more energetically stable than tip-to-tip aggregation for long chains (larger than 14 microspheres). This is consistent with experimental observations and very relevant for a fast magnetophoresis since thick aggregates move faster than thin ones.

Colloid and Polymer Science, 2011

We have found a missing numerical factor in Eq.(9) which defines the energy scale U 0 . The corre... more We have found a missing numerical factor in Eq.(9) which defines the energy scale U 0 . The correct definition, employed in all calculations reported in our paper [1], is:

Experimental and simulation studies of superparamagnetic colloids in strong external fields have ... more Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains

American Journal of Botany, 2012

Soft Matter, 2011

Experimental and simulation studies of superparamagnetic colloids in strong external fields have ... more Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains an equilibrium distribution of aggregate sizes. A thermodynamic self-assembly theory supports the simulation results and it also predicts that the average aggregate size in the equilibrium state depends only on a dimensionless parameter combining the volume fraction of colloids φ0 and the magnetic coupling parameter Γ. The conditions under which this new behavior can be observed are discussed. PACS numbers: 83.10.Mj, 61.43.Hv, 82.70.Dd, 83.80.Gv Colloidal aggregation is a subject of active research for both practical (e.g. stability of many industrial products) and fundamental reasons (as a test field for statistical-mechanical theories, for example). Our interest here is in the new physics arising in the aggregation behavior of superparamagnetic colloids. These systems are a successful example of implementation of a new behavior typical of the nanoscale (superparamagnetism) in new materials with many exciting practical applications, ranging from environmental waste capture [1] to biomedicine . Superparamagnetic materials show a large magnetic dipole in presence of external field, saturation magnetization similar to that of ferromagnetic materials but no coercitivity nor remanence at the working temperature. Superparamagnetic colloids are typically made by embedding superparamagnetic nanocrystals in a non-magnetic matrix (such as polystyrene, nanoporous silica or others) .

Physical Review E, 2011

Half-lives of radionuclides span more than 50 orders of magnitude. We characterize the probabilit... more Half-lives of radionuclides span more than 50 orders of magnitude. We characterize the probability distribution of this broad-range data set at the same time that we explore a method for fitting power laws and testing goodness-of-fit. It is found that the procedure proposed recently by Clauset et al. [SIAM Rev. 51, 661 (2009)] does not perform well as it rejects the power-law hypothesis even for power-law synthetic data. In contrast, we establish the existence of a power-law exponent with a value around 1.1 for the half-life density, which can be explained by the sharp relationship between decay rate and released energy, for different disintegration types. For the case of alpha emission, this relationship constitutes an original mechanism of power-law generation.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2001

A mean-field approach for steady-state aggregation with injection is presented. It is shown that ... more A mean-field approach for steady-state aggregation with injection is presented. It is shown that for a wide variety of aggregation processes the resulting steady-size distribution obeys a power law N(m) approximately m(-alpha) with alpha=(3+beta)/2 and beta the degree of homogeneity of the coagulation kernel. The general conditions for this to happen are obtained. Some applications are studied. In particular, it predicts a potential behavior for coagulation in atmospheric aerosols with exponent alpha approximately 2, in agreement with observations. The theoretical results also agree with some animal group-size distributions and with numerical simulations in fractal aggregates.

ABSTRACT PRECISION MAGNETOPHORESIS SEPARATION OF SUPERPARAMAGNETIC COLLOIDS. The manipulation of ... more ABSTRACT PRECISION MAGNETOPHORESIS SEPARATION OF SUPERPARAMAGNETIC COLLOIDS. The manipulation of magnetic particles by the use of inhomogeneous magnetic fields (magnetophoresis) has emerged as a topic of great interest in a wide range of research and technological areas [1,2]. The idea behind magnetic separation is to take advantage of the distinctive magnetic response of the particles in solution to remove them from complex mixtures by the use of applied inhomogeneous magnetic fields [3]. In the different applications, magnetic particles are typically functionalized with proper chemical groups, designed to bind to specific non-magnetic components, thus enabling the separation of non-magnetic materials by combining the use of magnetic particles and magnetic fields. Current standard magnetophoretic techniques (such as High Gradient Magnetic Separation) suffer from different difficulties, including lack of reproducibility and scalability and the loose of control over the magnetic conditions under which the magnetic particles are removed. Basically, the external magnetic field applied induces highly inhomogeneous gradients (as large as 104 T/m) in the separator [4]. The magnetic fields generated in this way are not predictable or reproducible and the magnetic force experienced by the colloids is not uniform across the system. These inhomogeneous conditions common to the HGMS approach makes difficult to develop numerical and/or analytical solutions to the problem, which would help in a better understanding of the magnetophoretic mechanisms and a higher performance; for instance, by means of a better design of separators or a better choice of the magnetic particles used in specific applications. In order to overcome these limitations and facilitate the use of magnetic colloids in biotechnological applications we have made use of a new concept of magnetic separation (the so called Precision Magnetophoresis) to effectively remove different types of superparamagnetic nanoparticles from solution. The process is based on the use of a uniform magnetic gradient, allowing better quality control and scalability, together with a better control over the experimental conditions, providing a proper framework for the development of theoretical models (analytical or numerical). Within this approach, we have studied the magnetic separation of superparamagnetic colloids by theory and experiments. Under sufficiently large magnetic fields, a fast separation process occurs in which the superparamagnetic colloids form chain-like structures (reversible aggregation) aligned parallel to the applied external magnetic field. It is observed that, under the effects of a magnetic gradient, these structures move faster than would move an individual colloid in the same conditions, resulting in a dramatic reduction (orders of magnitude) of the separation time. Thus, the reversible aggregation of colloids becomes a key aspect when one thinks to enhance the magnetic separation of magnetic colloids. The kinetics of both separation processes (cooperative and non-cooperative magnetophoresis) has been characterized theoretically and experimentally as a function of the different properties of the colloidal dispersion (magnetic response, size of particles, concentration, solvent viscosity, etc, ...) and the magnetic separator (magnetic gradient and size of the separator) [5]. Following the experimental results obtained for different combinations of magnetic colloids and magnetic fields, the required conditions for the reversible aggregation to occur has been recently discussed on the base of computer simulations and thermodynamical models [6]. In the much simpler situation in which the separation process is driven by the individual motion of the colloids, an analytical solution for the kinetics of the magnetophoresis separation process is provided [7]. The obtained solution is valid under certain restrictive but realistic conditions which are explicitly discussed here. We also show the utility of the analytical model by comparing our predictions with experimental results obtained with superparamagnetic particles of different sizes and magnetizations. We expect that the availability of a simple, analytical model will allow for a better understanding of the underlying physics of magnetic separation processes and also allow a rational design of applications. References: [1] C. T. Yavuz, A. Prakash, J. T. Mayo, V. L. Colvin, Chem. Eng. Sci., 64, 2510 (2009). [2] J. L. Corchero, A. Villaverde, Trends Biotechnol., 27, 468 (2009). [3] G. Friedman and B. Yellen, Curr. Opin. Colloid Interface Sci., 10, 158 (2005). [4] G. D. Moeser, K. A. Roach, W. H. Green, T. A. Hatton, P. E. Laibinis, AIChE J., 50, 2835 (2004). [5] G. De Las Cuevas, J. Faraudo and J. Camacho, J. Phys. Chem. C, 112, 945-950 (2008). [6] J. S. Andreu, J. Camacho, J. Faraudo, Soft Matter, 7, 2336 (2011). [7] J. S. Andreu et al. submitted to Phys. Rev. E, (2011).

Journal of Physical Chemistry C, 2008

Recent experiments (Yavuz, CT et al., Science 2006, 314, 964) show the possibility of low gradien... more Recent experiments (Yavuz, CT et al., Science 2006, 314, 964) show the possibility of low gradient magnetophoretic separation of superparamagnetic nanoparticles in aqueous solution, a process with broad potentially important applications ranging from biomedicine ...

Journal of Nanomaterials, 2012

Magnetic separation has gained much attention due to its implications in different fields, becomi... more Magnetic separation has gained much attention due to its implications in different fields, becoming feasible as an alternative to existent technologies at the industrial and lab scale. Substantial efforts are focused to improve the magnetic particles used in these applications. Here we show how a relatively simple and low-cost simulation strategy (tracer simulations) can be employed to predict the effect of various key factors in magnetic separation processes, namely, particle properties and magnetic separator designs. For concreteness, we consider here specific problems in magnetic separation. The first one is the effect of different profiles of the magnetic field in the separation of magnetic nanoparticles, and the second one is the magnetophoresis of colloidal particles in a dispersion of magnetic nanoparticles.

Physical Review E, 2012

The aim of this work is the description of the chain formation phenomena observed in colloidal su... more The aim of this work is the description of the chain formation phenomena observed in colloidal suspensions of superparamagnetic nanoparticles under high magnetic fields. We propose a new methodology based on an on-the-fly Coarse-Grain (CG) model. Within this approach, the coarse grain objects of the simulation are not fixed a priori at the beginning of the simulation but rather redefined on the fly. The motion of the CG objects (single particles or aggregates) is described by an anisotropic diffusion model and the magnetic dipole-dipole interaction is replaced by an effective short range interaction between CG objects. The new methodology correctly reproduces previous results from detailed Langevin Dynamics simulations of dispersions of superparamagnetic colloids under strong fields whilst requiring an amount of CPU time orders of magnitude smaller. This substantial improvement in the computational requirements allows the simulation of problems in which the relevant phenomena extends to time scales inaccessible with previous simulation techniques. A relevant example is the waiting time dependence of the relaxation time T2 of water protons observed in Magnetic Resonance experiments containing dispersions of superparamagnetic colloids, which is correctly predicted by our simulations. Future applications may include other popular real-world applications of superparamagnetic colloids such as the magnetophoretic separation processes.

Soft Matter, 2013

In recent years, there has been a great progress in the development of superparamagnetic particle... more In recent years, there has been a great progress in the development of superparamagnetic particles targeted to a wide range of applications, including fields as diverse as biotechnology or waste removal.

Soft Matter, 2012

Recent works have demonstrated the exciting possibility of inducing a tunable magnetic behavior i... more Recent works have demonstrated the exciting possibility of inducing a tunable magnetic behavior in non-magnetic colloids by immersing them in a dispersion of superparamagnetic nanoparticles (NPs).

Physical Review E, 2011

Magnetophoresis-the motion of magnetic particles under applied magnetic gradient-is a process of ... more Magnetophoresis-the motion of magnetic particles under applied magnetic gradient-is a process of great interest in novel applications of magnetic nanoparticles and colloids. In general, there are two main different types of magnetophoresis processes: cooperative magnetophoresis (a fast process enhanced by particle-particle interactions) and noncooperative magnetophoresis (driven by the motion of individual particles in magnetic fields). In the case of noncooperative magnetophoresis, we have obtained a simple analytical solution which allows the prediction of the magnetophoresis kinetics from particle characterization data (size and magnetization). Our comparison with new experimental results shows good quantitative agreement. In addition, we show the existence of a universal curve onto which all experimental results should collapse after proper rescaling. The range of applicability of the analytical solution is discussed in light of the predictions of a magnetic aggregation model [Soft Matter 7, 2336].

Journal of Theoretical Biology, 2007

Journal of Applied Physics, 2011

ABSTRACT The transverse relaxation time T2 of protons in water suspensions of iron-oxide particle... more ABSTRACT The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied.

Colloid and Polymer Science, 2010

Superparamagnetic colloids have a great practical interest for their applications to processes ra... more Superparamagnetic colloids have a great practical interest for their applications to processes ranging from biomedicine to environmental waste and pollutants removal. A fast and efficient separation of these particles from the solvent constitutes a key step in the practical implementation of this technology. Recent experiments show fast magnetophoretic separation using relatively small magnetic gradients and high magnetic fields. The mechanism underlying this fast separation was shown to be the reversible aggregation of the magnetic beads induced by the external field. In this paper, we analyze theoretically the physicochemical conditions under which reversible aggregation can be typically achieved, the timescale at which aggregates form, and their shape. In the case of colloids stabilized electrostatically, for reasonable surface potentials (approximately −70 mV), we find that the interaction potential between two superparamagnetic particles displays a barrier with a minimum so that reversible aggregates can form. We also show that the aggregation of particles is quite fast (typically less than a second for usual concentrations) and that lateral aggregation is more energetically stable than tip-to-tip aggregation for long chains (larger than 14 microspheres). This is consistent with experimental observations and very relevant for a fast magnetophoresis since thick aggregates move faster than thin ones.

Colloid and Polymer Science, 2011

We have found a missing numerical factor in Eq.(9) which defines the energy scale U 0 . The corre... more We have found a missing numerical factor in Eq.(9) which defines the energy scale U 0 . The correct definition, employed in all calculations reported in our paper [1], is:

Experimental and simulation studies of superparamagnetic colloids in strong external fields have ... more Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains