A number of researches have been devoted to the developments of novel separation principles and methodology, because material separation is one of the most important fundamentals in scientific and technological disciplines. In particular, flow separation methods, such as chromatography, 1,2 electrophoresis, [3][4][5][6][7] and field flow fractionation (FFF), [8][9][10] have been extensively used in various fundamental and practical fields, and have facilitated the advancements of modern science. These methods cover separation of almost all classes of materials. A number of dissolved molecules, from simple ions to macromolecules, can be separated by chromatography or electrophoresis, FFF and electrophoresis being capable of resolving even particles. The dimensions of solutes to be separated by these methods thus range from 10 -10 m to 10 -5 m. This situation may suggest that separation science and technology have been completely matured. However, studying new separation principles is still important, because they can allow the separation based on material properties that have not been utilized for conventional methods, provide higher performance and more efficient separation than existing means, and facilitate the understanding of materials through separation processes.According to chromatographic theories, the intrinsic separation performance of liquid chromatography is restricted in comparison with that of gas chromatography because of much lower solute diffusion in liquid phases. 11However, a liquid flow obviously provides versatile ways for material separation because liquids have higher density and dissolution power than gases, and thus has made possible application to various types of solutes with wide ranges in molecular weights and sizes. In the usual flow separation methods, appropriate chemical (chromatography) or physical (FFF) separation fields are created, which retain solutes to different extents and separate them. A variety of stationary phases for chromatography 2,[12][13][14] and physical forces for FFF [15][16][17][18][19] have been exploited, and have provided different capabilities and selectivity from those performed by conventional separation. In contrast, a separation method that requires neither chemical interactions nor special external fields should also be useful because of its instrumental simplicity. Hydrodynamic chromatography (HDC) is such a case, which allows separation of particles or macromolecules according to their hydrodynamic sizes just by passing them through a narrow channel. [20][21][22][23][24] Particles with larger diameter for example cannot approach the channel wall, and thus flow through the channel faster than the smaller particles, when the laminar flow profile is maintained therein. Although the applicability of this method was very restricted, recent developments of chip technology have brought about a remarkable progress. 23,24 If a capillary of radius a is used for HDC, the time for the elution of a particle of radius rp is given bywhere t0 is the time required ...

A coupled acoustic-gravity field is proposed as a novel external field for particle separation and characterization. When a standing plane ultrasound wave is generated, particles move to the node of the wave along the ultrasound force gradient. If the particles also undergo a sedimentation force, they aggregate at the equilibrium position, where these two forces are balanced. The equilibrium position, which is determined by the density and compressibility of a medium and particles, characterizes the particles. The local ultrasound energy, which is necessary for quantitative discussions, is evaluated by using a standard particle, the physical parameters of which are unambiguously determined; aluminum particles are used in the present study. The local ultrasound energy makes possible the determination of the compressibility of unknown materials. Nonporous particles of inorganic and polymeric materials, the particle sizes of which range from 3 to 100 microm, follow a derived model, suggesting that the local ultrasound energy and a derived model be valid. The proposed external field can be used for separation of particles having different acoustic natures.

A model has been derived from the PoissonÈBoltzmann equation to describe the uptake of ions by a zwitterionic micelle. Two mechanisms representing uptake selectivity are included in the model ; (1) a di †erence in the standard chemical potential of an ion between the bulk solution (partition mechanism) and ( 2) the micelles and ion-pair formation at the charged surfaces (ion-pair mechanism). In order to conÐrm the applicability of the derived model, the uptake of ions is evaluated by two independent experiments, i.e. potentiometry and chromatography, and compared with the calculation. Both the partition and the ion-pair mechanism similarly explain the ion uptake by the zwitterionic micelles. The standard chemical potential di †erences are estimated [5.0 kJ mol~1 for I~and [8.5 kJ mol~1 for SCN~when employing the partition mechanism, while the ion-pair formation constants are 0.12 M~1 for I~and 0.7 M~1 for SCN~with the ion-pair mechanism. These parameters depend strongly on the micellar sizes, but well reÑect the nature of anions irrespective of size parameters involved in the calculation.

In a coupled acoustic-gravity field, particles are aggregated at a particular position determined by their acoustic properties, such as density and compressibility. The observation of the aggregation of cation-exchange resin particles in this field has confirmed that the types of countercations strongly affect the behavior of the particles. Detailed analyses have revealed that the different behavior comes not from the density of the resins but from their compressibility as far as H+- and NBu4+-form cation-exchange resins are concerned. Since these resins are aggregated at the different positions, we can gain information of counterions from aggregation coordinates. Counterions can thus be recognized through the observation of the aggregation behavior of resin particles in the field without chemical analyses routinely employed in ion-exchange studies.

Particles accumulated in a coupled acoustic-gravity field, in which the base-material of a particle determines the equilibrium aggregation positions, are eluted from a separation channel by an orthogonal laminar flow. Elution times are controllable by changing the intensity and frequency of the ultrasonic waves.