Micro-thermal Field-flow Fractionation Research Articles

Focusing Mechanisms in Micro-thermal Field-Flow Fractionation

Focusing Mechanisms in Micro-thermal Field-Flow Fractionation

On the effect of thermophoretic force, gravitational force, and particle–particle interactions in field-flow fractionation

ABSTRACTThe theoretical calculations confirmed that the gravitational force cannot be neglected in all field-flow fractionation techniques separating nanometer-sized colloidal particles whenever particle diameter is approximately 200 nm and larger. Particle–particle repulsive interactions, mostly electrostatic repulsions, influence substantially concentration distribution established by any effective field acting across the fractionation channel, as confirmed explicitly for thermophoretic force generated by temperature gradient in microthermal field-flow fractionation. The ionic strength of the carrier liquid causes the screening of the electrostatic double layer around the dispersed particles and thus influences the retention. The attractive particle–particle forces occur when the zeta potential of the particles approaches to 0 mV, the electrostatic repulsions are screened, and the aggregation of the particles is observed. The pH influences differently the size and zeta potential of the plain polystyrene latex particles and of the particles modified on the surface by the groups –COOH and –NH2. The role of a detergent in carrier liquid is non-negligible, as demonstrated by its presence or absence in carrier liquid.

Field-flow fractionation in microfluidic channels of low aspect ratio

ABSTRACTThe shape of the steady-state three-dimensional flow velocity profile established in carrier liquid flowing inside the rectangular cross-sectional channel for field-flow fractionation should be taken into account to optimize the separation. The central parts of this profile in the planes parallel to the main channel walls are flat with almost identical flow velocities which drop down to zero at the side walls. The separated species transported by the flow in the close-to-side walls regions move with lower average velocities compared to the species transported in the central part of the channel and are undesirably broadened. The hydrodynamic splitting of the carrier liquid at the entry of the channel where the sample is injected only into the central part of the channel eliminates the excessive zone broadening. The aspect ratio of the breadth to the thickness of the channel ratio can thus be reduced. The effect of various aspect ratios on the shape of the flow velocity profile is calculated and the results are used to optimize the aspect ratio of microfluidic channels. The experiments carried out by microthermal field-flow fractionation confirmed that the aspect ratio cannot be reduced to a value of 1, proposed by other authors.

Characterization of Diamond Nanoparticles by High-Speed Micro-Thermal Field-Flow Fractionation

Micro-thermal field-flow fractionation was used to characterize the particle size distribution of nanometer-sized diamond nanoparticles. Although the experimental conditions were chosen to perform high-speed separation, and, consequently, the resolution achieved experimentally was not very high, the application of the original correction method for the zone spreading allowed for obtaining of very good calculated particle size distribution or, explicitly, a true polydispersity index of the diamond nanoparticle sample. The future use of several samples of diamond nanoparticles of different average sizes and different surface chemistries should allow deeper insight into the effect of these particulate characteristics on the retention in micro-thermal field-flow fractionation.

Role of the shape of various bacteria in their separation by Microthermal Field-Flow Fractionation

The steady-state movement of the spherical and non-spherical particles, such as prolate or oblate rotational ellipsoids, cylinders, or parallelepipeds, suspended in a liquid and exposed to a unidirectional temperature gradient, is analyzed theoretically. The differences in the ratios of the rotational to translational diffusion coefficients of the non-spherical to spherical particles, the heterogeneity of thermal conductivity of the particle body, and the heterogeneity in surface chemical nature make possible to separate the particles according to differences in shape. Preliminary experimental separations of Gram-positive and Gram-negative, nearly spherical and rod-shaped bacteria performed by Microthermal Field-Flow Fractionation confirmed that the fractionation of the cells according to differences in shape is possible.

Relaxation of microparticles exposed to hydrodynamic forces in microfluidic conduits

The behavior of microparticles exposed to gravitational and lift forces and to the velocity gradient in flow velocity profile formed in microfluidic conduits is studied from the viewpoint of the transient period (the relaxation) between the moment at which a particle starts to be transported by the hydrodynamic flow and the time at which it reaches an equilibrium position, characterized by a balance of all active forces. The theoretical model allowing the calculation of the relaxation time is proposed. The numerical calculus based on the proposed model is compared with the experimental data obtained under different experimental conditions, namely, for different lengths of microfluidic channels, different average linear velocities of the carrier liquid, and different sizes and densities of the particles used in the study. The results are important for the optimization of microfluidic separation units such as microthermal field-flow fractionation channels in which the separation or manipulation of the microparticles of various origin, synthetic, natural, biological, etc., is performed under similar experimental conditions but by applying an additional thermodynamic force.

On the Limits of High-Speed Microthermal Focusing Field-Flow Fractionation

The migration of microparticles exposed to gravitational-buoyancy force and lift force and transported by the flow of the carrier liquid inside microfluidic conduits was studied from the viewpoint of the transient period (the relaxation) between the moment at which the particles start to be transported by the hydrodynamic flow and the time at which they are focused at an equilibrium position. Relaxation times were calculated theoretically by the incremental numerical method and the results were compared with the experimental data obtained on two microfluidic channels of different lengths and for different average linear velocities of the carrier liquid. It has been found that the maximal linear velocity of the carrier liquid in a microthermal field-flow fractionation channel whose dimensions were already optimized is of the order of a few cm/s under the investigated experimental conditions. At this velocity, the contribution of the secondary relaxation time is negligible compared to the retention time of the micrometer-sized particles.

On the retention mechanisms and secondary effects in microthermal field-flow fractionation of particles

The behavior of nanometer or micrometer-sized particles, dispersed in liquid phase and exposed to temperature gradient, is a complex and not yet well understood phenomenon. Thermal field-flow fractionation (TFFF), using conventional-size channels, played an important role in the studies of this phenomenon. In addition to thermal diffusion (thermophoresis) and molecular diffusion or Brownian movement, several secondary effects such as particle–particle and/or particle–wall interactions, chemical equilibria with the components of the carrier liquid, buoyant and lift forces, etc., may contribute to the retention and complicate the understanding of the relations between the thermal diffusion and the characteristics of the retained particles. Microthermal FFF is a new high-performance technique allowing much easier manipulation and control of the operational parameters within an extended range of experimental conditions in comparison with conventional TFFF. Consequently, in combination with various other methods, it is well suited for a detailed investigation of the mentioned effects. In this work, some contradictory published results concerning the thermal diffusion of the colloidal particles, studied by TFFF but also by other methods, are analyzed and compared with our experimental findings.

Book News: Microthermal Field‐Flow Fractionation: Analysis of Synthetic, Natural, and Biological Macromolecules and Particles. Edited by Josef Janca

Journal of Separation ScienceVolume 32, Issue 5-6 p. 907-907 Book News Book News: Microthermal Field-Flow Fractionation: Analysis of Synthetic, Natural, and Biological Macromolecules and Particles. Edited by Josef Janca Francesco Dondi, Francesco DondiSearch for more papers by this author Francesco Dondi, Francesco DondiSearch for more papers by this author First published: 10 March 2009 https://doi.org/10.1002/jssc.200990021AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume32, Issue5-6Special Issue: ReviewsMarch 2009Pages 907-907 RelatedInformation

On the Concentration Effects in Micro-Thermal Field-Flow Fractionation of Polymers

Micro-Thermal Field-Flow Fractionation (Micro-TFFF) was used to study the effect of concentration of the polymer solutions injected into the separation channel. The range of the investigated concentrations was very large, becoming higher then the critical values at which the entanglement of the polymer coils leads to the formation of the macromolecular aggregates. The samples of different molar masses were used and the experiments were carried out at different temperature drops across the channel and at different temperatures of the cold wall. The multiple peaks (or oscillations) that appeared on the fractograms at a given concentration of the injected sample solution and at a given temperature drop, disappeared at lower concentration and emerged again at the same lower concentration, but at a higher temperature drop. Although the observed broadening of the fractograms can be partially caused by the formation of the macromolecular aggregates, their presence cannot explain the secondary peaks or oscillations appearing on the fractograms at the retentions corresponding to extremely high molar masses. The hydrodynamic and/or gravitational instabilities, which are caused by the formed viscosity and density gradients inside the separation channel are the most probable explanation of the origin of the mentioned secondary peaks and/or oscillations emerging at high concentrations and/or high temperature drops.

Ultra-Micro-Thermal Field-Flow Fractionation

Micro-thermal field-flow fractionation, proposed conceptually and implemented experimentally several years ago, has developed rapidly in terms of theory, instrumentation, and numerous applications for the analysis and characterization of polymers and particles of synthetic, natural, and biological origin. Although the advances have been important, achieving the ultimate limits of miniaturization imposed by the physics as well as by recent technologies represents a challenge that was explored. The result of the reported experimental study is a new separation channel for ultra-micro-thermal field-flow fractionation, which was compared, in terms of performance, with the existing compact micro-thermal field-flow fractionation unit. The limits of the miniaturization are experimentally demonstrated with the use of suspensions of colloidal particles, which represent a more difficult case of separation than polymers in solution.

Optimization of Retention in Micro-Thermal Field-Flow Fractionation for the Determination of True Particle Size Distribution

Determination of true particle size distribution of colloidal samples by field-flow fractionation requires the suppression or measurement of all contributions to the zone broadening. In order to achieve high-performance separations, experiments are often carried out at the optimal velocity of the carrier fluid at which the zone broadening due to the nonequilibrium processes is minimal. This method is traditionally used in chromatography where it was justified by classical theories. It was also applied in field-flow fractionation without raising the question on its efficiency. In this article, it is predicted theoretically and proven experimentally that high-performance separation and thus accurate determination of particle size distribution can be performed more efficiently by optimizing the experimental conditions with regard to the retention rather than to the optimal velocity of the carrier liquid.

Micro-thermal field-flow fractionation of bacteria

The retention of Staphylococcus epidermidis bacteria cells, achieved with the use of micro-thermal field-flow fractionation and described in this paper, represents the first experimental proof that the separation and characterization of the bio-macromolecules and biological particles is possible by exploiting Ludwig–Soret effect of thermal diffusion. The experiments were carried out under gentle experimental conditions preventing the denaturation of the bacteria. Lift forces, appearing at high linear velocities of the carrier liquid, generated the focusing mechanism of the retention which resulted in high-speed and high-performance separation performed in less than 10 min.

Micro-Thermal Field-Flow Fractionation in the Analysis of Polymers and Particles: A Review

The benefits of the miniaturization of thermal field-flow fractionation (TFFF) channel are reviewed in order to demonstrate that high-performance separations can be achieved with a micro-channel of optimized construction working under carefully chosen experimental conditions and operational variables. Micro-TFFF is highly competitive in comparison with size exclusion chromatography (SEC) of macromolecules for molar masses up to approximately one million g/mol. However, the versatility of micro-TFFF is superior to SEC for macromolecules of ultra high molar masses not only because there is not an inherent highest molar mass limit of the macromolecules that can be separated by micro-TFFF in contrast to SEC but also, as proved experimentally, due to the fact that shear degradation of the macromolecular coils, a well-known phenomenon in SEC, does not occur in micro-TFFF. Such mild conditions permit the analysis of macromolecular aggregates, micro-gels, and similar associative structures without any destruction. Moreover, colloidal submicron and micron-sized particles of synthetic, natural, or biological origin can also be separated and characterized by micro-TFFF without any modification of the separation system. The free choice of the carrier liquids affords an another advantage to micro-TFFF. It has been predicted theoretically and proven experimentally that high resolution is achieved more efficiently by increasing the temperature drop across the separation channel than with a decrease in channel thickness. This is due to the fact that the total heat flow between the hot and cold walls is substantially reduced in micro-TFFF. This makes the operation of the micro-TFFF system not only 30 times more economical from the viewpoint of electrical energy consumption but also safer. The experimental implementation and application to polymer and particle analysis confirmed the potential of micro-TFFF.

Elimination of edge effects in micro-thermal field-flow fractionation channel of low aspect ratio by splitting the carrier liquid flow into the main central stream and the thin stream layers at the side channel walls

An optimized construction of the separation channel for micro-thermal field-flow fractionation (FFF) was proposed and studied experimentally. The sample is injected in such a manner that its zone moves along the channel only in the main central stream where the flow velocity profile in the plane parallel to the main accumulation wall is practically flat. This central stream is separated from the contact with the side walls of the channel by thin flowing layers of the free carrier liquid. The retained species do not reach the thin liquid streams at the side walls where the flow rate decreases rapidly to achieve zero at the side wall according to the established 3D flow velocity profile. Such a construction of the channel allows one to reduce the aspect ratio (the ratio of the channel breadth b to its thickness w ) without increasing the zone broadening. The hydrodynamic splitting of the outlet streams allows one not only to increase the concentration of the detected species but also the determination of the sign of Soret coefficient.

Polarization, steric, and focusing micro-thermal field-flow fractionation principles, theory, instrumentation, and applications in polymers and particles analysis

The benefits of the miniaturization of the separation channel for Thermal field-flow fractionation (TFFF) are analyzed theoretically and studied experimentally. It is found that the operational variables influencing the efficiency and resolution of the separation can be optimized and that the concept of a micro-TFFF is meaningful. A miniaturized TFFF channel was proposed and constructed. It has been demonstrated experimentally that micro-TFFF allows one to carry out high-performance separations under carefully chosen experimental conditions. This new technique is highly competitive in comparison with size-exclusion chromatography (SEC) of macromolecules with molar masses up to roughly one million gram per mole. However, the versatility of micro-TFFF is superior to SEC for macromolecules of ultra-high molar masses, as well as colloidal sub-micron, and micron-sized particles of synthetic, natural, or biological origin. The free choice of the carrier liquids without any modification of the separation system affords an advantage to micro-TFFF. The experimental implementation and application to polymer and particle analysis confirmed the potential of micro-TFFF.

Effect of channel width on the retention of colloidal particles in polarization, steric, and focusing micro-thermal field-flow fractionation

The effect of the channel width on the performance of separation by micro-thermal field-flow fractionation (micro-TFFF) of the carboxylated polystyrene latex particles was studied by using the particles in diameter range from 100 nm to 3800 nm. It has been shown that the retention order follows the anticipated polarization, steric, and focusing mechanism in the corresponding size range and under the specific conditions, appropriate to each channel thickness. However, the attractive interactions of the particles with the accumulation wall can complicate the separation as has been proven by the experiments carried out by using the carrier liquids of different ionic strengths. Three channel thicknesses (0.025, 0.100, and 0.250 mm) were tested thus imposing the volumes of micro-channels of roughly 9, 37, and 92 μl. Such an experimental investigation has never been performed with respect to the applicability of the TFFF within an extended range of molar masses or particle sizes. The advantages and drawbacks of different channel widths are discussed with respect to the performance of separation of micro-TFFF but also by taking into account the practical requirements of the construction of the micro-TFFF channel. The principal finding is that very thin channel (w = 0.025 mm) substantially reduces the range of particle sizes or polymer molar masses that can effectively be separated due to the mixed separation mechanism, steric exclusion being effective from smaller particle size. The found dependence of the resolution on the imposed experimental conditions including the channel width has allowed the elucidation of some peculiar results published in the literature, which were contradictory with regard to the known theoretical and experimental findings.

Poly(aniline- co-pyrrole): powders, films, and colloids. Thermophoretic mobility of colloidal particles

Copolymers of aniline and pyrrole have been prepared by the oxidation of monomer mixtures of various compositions with ammonium peroxydisulfate. The copolymers have a lower conductivity than the corresponding parent homopolymers or are even non-conducting. This means that sequences of constitutional units, responsible for the conduction, are disrupted by the incorporation of the second comonomer. The ability of both comonomers to copolymerise has been proved on the basis of FTIR spectra. Thin copolymer films grow during copolymerization on glass supports immersed in the reaction mixture. Their thickness of ∼130 nm, estimated by optical absorption, is little dependent on the monomer mixture composition. Colloidal copolymer dispersions are produced when the polymerization is carried out in the presence of a steric stabilizer, poly( N-vinylpyrrolidone). The particle size 200–500 nm, determined by dynamic light scattering (DLS), does not significantly depend on the content of aniline in the reaction mixture. The thermophoretic mobility observed in micro-thermal field-flow fractionation (micro-TFFF) experiments abruptly changes with the composition of copolymer colloidal particles. Recent lack of information on the thermal conductivity of the studied copolymers does not allow one to interpret quantitatively the results of micro-TFFF, however, it is interesting to note a strong coherence of the amplitude of thermal diffusion coefficients and the electrical conductivity of the studied copolymer colloidal particles.

Effect of channel width on the retention of colloidal particles in polarization, steric, and focusing micro-thermal field-flow fractionation

The effect of the channel width on the performance of separation by micro-thermal field-flow fractionation (micro-TFFF) of the carboxylated polystyrene latex particles was studied by using the particles in diameter range from 100 nm to 3800 nm. It has been shown that the retention order follows the anticipated polarization, steric, and focusing mechanism in the corresponding size range and under the specific conditions, appropriate to each channel thickness. However, the attractive interactions of the particles with the accumulation wall can complicate the separation as has been proven by the experiments carried out by using the carrier liquids of different ionic strengths. Three channel thicknesses (0.025, 0.100, and 0.250 mm) were tested thus imposing the volumes of micro-channels of roughly 9, 37, and 92 μl. Such an experimental investigation has never been performed with respect to the applicability of the TFFF within an extended range of molar masses or particle sizes. The advantages and drawbacks of different channel widths are discussed with respect to the performance of separation of micro-TFFF but also by taking into account the practical requirements of the construction of the micro-TFFF channel. The principal finding is that very thin channel ( w = 0.025 mm) substantially reduces the range of particle sizes or polymer molar masses that can effectively be separated due to the mixed separation mechanism, steric exclusion being effective from smaller particle size. The found dependence of the resolution on the imposed experimental conditions including the channel width has allowed the elucidation of some peculiar results published in the literature, which were contradictory with regard to the known theoretical and experimental findings.

High-Speed Micro-Thermal Focusing Field-Flow Fractionation of Micron-Size Particles

Micron-size polystyrene-based latex particles were separated by using new micro-thermal field-flow fractionation (micro-TFFF). The order of retention from the largest to the smallest particles that appears at high field strength and high flow rate corresponds to the focusing mechanism which itself is a consequence of the lift forces acting on the particles. The mechanism of steric exclusion can only be effective at low flow rates of the carrier liquid. Whenever high-speed separation was performed, the focusing effect clearly dominated the FFF mechanism. This application of micro-TFFF in focusing mode to the separation of the particles is the first one published. As a result, micro-TFFF thus became a very universal technique for the separation of synthetic and natural macromolecules and of particles of various origin and size up to large (micron-size) diameter.