Sound Velocity Measurements Research Articles

Use of molybdenum as a pressure calibrant in a cubic press and sound velocity measurement of tungsten to 5 GPa

ABSTRACT The pressure limit of the conventional cubic press is normally below 6 GPa, and the suitable pressure calibrant of the fixed-point method is scarce. In this study, we developed a new method for pressure determination using an acoustic travel time approach. Based on the reported ultrasonic travel times of polycrystalline molybdenum under high pressure in literature, an acoustic gauge of polycrystalline molybdenum was derived. The shear wave travel times of molybdenum were measured at high pressures and utilized for the pressure calibration in a cubic press. The pressures derived from this new travel time method are in excellent agreement with those from the fixed-point method. Subsequent compressional and shear wave velocities of polycrystalline tungsten were measured up to about 5 GPa at room temperature to further verify this method. This travel time method for pressure calibration is valuable for the sound velocity measurement of other materials in an offline laboratory.

Sound velocity measurement based on laser-induced micro-flyers

The measurement of high-pressure sound velocity in solid materials is crucial for developing constitutive equations and equations of state for materials in extreme stress–strain rate conditions. In this study, we propose a novel method for high-pressure sound velocity measurement using laser-induced micro-flyer technology. By optimizing laser driving conditions and target structure design, we measure high-pressure sound velocity using the “reverse-impact geometry” approach. The well-established Photon Doppler Velocimetry system allows for high-precision, single-shot measurements of both flyer velocity and particle velocity histories. A systematic error analysis shows that the longitudinal sound velocity of aluminum obtained in this experiment is consistent with data from traditional devices, such as gas guns, within the error margin. Finally, we analyze the potential application value of this method in laser technology as well as high-pressure dynamic responses of materials, and conclude the current shortcomings and possible improvements of this method.

Role of Acoustic and Volumetric Properties in View to Estimate the Intermolecular Interactions in the Solutions of Electrolytes and l‐Isoleucine

Abstractl‐Isoleucine is subjected to density and sound velocity measurements in aqueous solutions of potassium (KCl) and sodium chlorides (NaCl) as well as water, from 288.15 to 298.15 K. Through the use of these experimental data, a variety of volumetric and acoustical parameters are calculated, including values for molar volume (Vm), available volume (Va), adiabatic compressibility (β), nonlinear parameters (B/A), Wada constant (W), Rao's constant (R), and van der Wall's constant (a). The findings shed light on many electrostatic and hydrophobic interactions within the binary system involving electrolytes, water, and amino acids. The interactions between the electrolytes and l‐isoleucine are thoroughly examined under varying electrolyte concentrations and temperatures, revealing many interactions. Positive transfer quantities demonstrate the ion‐hydrophilic and solute–solvent interaction in the aqueous medium, particularly between l‐isoleucine and NaCl and KCl. The influence of electrolyte type and temperature on these interactions is discernible. The culmination of the study's outcomes is elaborated through analyzing solute–solvent and solute–solute interactions. In particular, the interaction hierarchy is identified as l‐isoleucine + water + KCl > l‐isoleucine + water + NaCl > l‐isoleucine + swater.

Assessment of Weld Microstructure Through Sound Velocity Measurements for Power Plant Applications

This research study employed an immersion-based micro-resolution ultrasonic velocity measurement technique (MUVT) to evaluate Grade 91 (9Cr-1Mo-V) steel welds. This process utilized a 20 MHz focused ultrasonic beam with a spot size of 250 µm, a 6 µm laser vibrometer spot size for detection, and an automated 3-axis raster scanner to accurately collect ultrasonic velocity data from two Grade 91 steel welds fabricated using cold metal transfer (CMT) and flux-cored arc welding (FCAW) processes. By analyzing the MUVT data, the Poisson’s ratio (υ) and modulus of elasticity (E), which are important mechanical properties for weldments, were calculated. The correlation between mechanical properties and ultrasonic velocity was examined. The results indicated that longitudinal ultrasonic velocity could effectively identify base metal, heat-affected zone (HAZ), and weld metal areas, showing a strong correlation with hardness. In particular, the distinctive V-shaped dip in velocity and hardness data across the HAZ areas of both samples highlighted changes in the microstructure of creep strength–enhanced ferritic steel welds. Importantly, the immersion-based MUVT demonstrated high accuracy in small sections of the weld, surpassing the conventional contact ultrasonic testing method in spatial resolution detection.

Compressional and shear wave velocities of Fe-bearing silicate post-perovskite in Earth’s lowermost mantle

The bridgmanite (Bgm) to silicate post-perovskite (PPv) phase transition is believed to be the main cause for the distinct seismic features observed in the D'' layer, the lowermost region of the Earth’s mantle. However, the transition depth and elasticity of the PPv phase have been highly debated, as the chemical complexity within the D'' layer can largely affect the Bgm-PPv transition pressure and the associated velocity contrast. Experimental measurements of sound velocities for PPv with different chemical compositions under relevant lowermost-mantle conditions are essential but remain limited. In this study, we have reliably measured both compressional wave velocity (VP), shear wave velocity (VS), and density, for two Fe-bearing PPv compositions [(Mg0.85Fe0.15)SiO3 and (Mg0.75Fe0.25)SiO3] at lowermost mantle pressures using Impulsive Stimulated Light Scattering (ISS), Brillouin Light Scattering (BLS), and X-ray Diffraction (XRD) in diamond anvil cells. Our results indicate that the velocities of Fe-bearing PPv at 120 GPa can be described by the following relationships: VS (km/s) = 7.65–2.8XFe and VP (km/s) = 14.11–3.8XFe, where XFe represents mole fraction of the Fe content. The variations in the Fe content of PPv may provide one of the explanations for the seismic lateral variations observed at the Earth’s core mantle boundary. By comparing our results with the high-pressure velocities of Bgm, our velocity model suggests significant discontinuities across the Bgm-PPv transition, characterized by a reduction in both VP and VΦ, and an increase in VS. These findings highlight the importance of considering the influence of chemical composition, particularly Fe content which could vary significantly at the D'' region, on the seismic properties of the PPv phase. The observed velocity contrasts across the Bgm-PPv transition may contribute to the complex seismic signatures observed in the D'' layer, underscoring the potential role of this phase transition in interpreting the seismic features of the lowermost mantle region.

The Hugoniot curve and sound velocity of forsterite to 1200 GPa

The comprehension of the composition and physical state of the deep interiors of large planets, as well as the impact events pertinent to planetary formation and evolution, necessitates an understanding of the properties of planetary materials under extreme conditions. Forsterite (Mg2SiO4), a significant geological mineral, has not been fully characterized in terms of its behavior under shock compression due to a lack of consensus among previous experiments and simulations aimed at determining its Hugoniot, as well as the absence of knowledge of sound velocity at high pressures, a critical parameter indicative of phase transformation and melting.In this study, we delineated the Hugoniot curve of the mineral forsterite up to immense pressures of 1200 GPa. For the first time, we successfully constrained its sound velocity along the Hugoniot curve up to 760 GPa by combining laser-driven shock experiments with first-principles molecular dynamics simulations. The measured Hugoniot data for forsterite corroborated previous findings and suggested the occurrence of incongruent melting during shock compression. Remarkably, along their respective Hugoniot curves, the sound velocity of forsterite was observed to fall between that of the minerals bridgmanite and periclase. The remarkable agreement between the experimental results and simulation data provides reliable sound velocity measurements on the forsterite Hugoniot, which is critical for comprehensively understanding the phase transition and melting behavior of forsterite under ultra-high pressures. This knowledge sheds invaluable light on the behavior of this significant geological mineral under extreme conditions akin to those found in the interiors of planets.

Thermodynamics of micellization of octyltrimethylammonium bromide in 1,2-propanediol-water mixtures at temperatures from (293.15 to 308.15) K

The micellar properties of octyltrimethylammonium bromide (C8TAB) in both water and aqueous solutions of 1,2-propanediol have been investigated using accurate measurements of density, sound velocity, and surface tension in the temperature range between 293.15 and 308.15 K. The critical micelle concentration, CMC, the ionization degree, β, the standard thermodynamic parameters of micellization: free energy, ΔmicG°, enthalpy, ΔmicH°, entropy, ΔmicS° and the free energy of transfer, ΔtrG°, of the surfactant octyltrimethylammonium bromide in both water and 1,2 propanediol aqueous solution were evaluated from the experimental results.The CMC of the surfactant increases as the concentration of 1,2-propanediol increases, while the temperature does not exert important changes within the range considered.The value of the micellization thermodynamic parameters suggests that adding 1,2-propanediol makes the micellization process less favorable.The thermodynamic functions of transfer were calculated. The values of the free energy of transfer are small and positive and increase as the diol concentration increases confirming that the transfer of the surfactant from the bulk into the micelle is less favorable.

Physico-chemical analysis in ternary organic mixtures at various temperatures with an acoustical approach

Thermo-dynamical analyses of organic mixtures are essential across industries. Ultrasonic sound velocity (U) measurements in the ternary organic mixtures of Toluene, Chloroform, and Cyclohexane have been conducted between 303.15, 308.15 and 313.15 K. The experimental/derived data were utilised to compute deviations for their excess values such as adiabatic compressibility (βE), free length (LfE), free volume (VfE), impedance (ZE), internal pressure (πiE) and enthalpy (HE) to investigate and confirm the types of interactions. Using these derived parameters, excess parameters were calculated to further reaffirm the type, strength, magnitude, and potential interactions such as complex formation, dipole-dipole, and dispersive types. Moreover, both ideal and non-ideal behaviours were analysed. The ultrasonic velocity data were validated against some contemporary and well-known models like Namoto’s relation (NR), Ideal mixture relation (IMR), Impedance dependence relation (IDR), Collision factor theory (CFT) and Nutch-Kunkies (NK) theorem. From the IMR and IDR are gives well approach to experimental one. And a comparative study of these models was conducted to ascertain the possible types of interactions within the mixtures through their molecular interaction parameter, percentage deviation, standard percentage error deviation.

Reassessment of Birch's Law on hcp‐Fe From Ultrasonic Sound Velocity Measurement and Implications on the Velocity Profiles of Earth's Inner Core

AbstractWe performed in situ X‐ray diffraction and ultrasonic sound velocity measurements on hcp‐Fe up to 15 GPa, 873 K in a multi‐anvil apparatus. The elastic moduli and their pressure and temperature derivatives were determined by fitting the velocity and density data to the third‐order finite strain equations, yielding KS0 = 169.0(57) GPa, KS0′ = 5.4(6), (∂KS/∂T)P = −0.031(3) GPa/K, G0 = 104.5(27) GPa, G0′ = 1.7(2), (∂G/∂T)P = −0.060(2) GPa/K. Within the experimental P‐T range, we find significant temperature effect on the density‐velocity (ρ − VP or S) relations and caution the use of the temperature‐independent Birch's law for extrapolation to core conditions. Furthermore, temperature‐induced velocity decrease is more significant in VS than in VP, offering a possible explanation for the high Poisson's ratio in the core. Extrapolations based on our results together with previous experimental data suggest that VP of hcp‐Fe aligns with PREM at Earth's core conditions, while Vs and density are approximately 10% and 2.7% higher than PREM, respectively.

Toward an understanding of volumetric and isentropic compressibility behavior of imidazolium-based ionic liquids in aqueous polymer solutions

ABSTRACT A systematic experimental density and sound velocity measurement was carried out for imidazolium-based ionic liquids (ILs) in pure water and in aqueous solutions of 0.03 w/w of water-miscible polymers with different structures and molar masses at T= (288.15, 293.15, 298.15, 303.15 and 308.15) K and p = 84.5 kPa. The ILs under investigation are 1-butyl-3-methylimidazolium hydrogen sulphate ([C4mim][HSO4]) and 1-butyl-3-methylimidazolium bromide ([C4mim]Br). The effects of temperature, polymer type, and IL anion type on the various volumetric and compressibility properties, such as apparent molar volume, isentropic compressibility, apparent molar isentropic compressibility, and infinite dilution apparent molar properties for transfer of ILs from water to aqueous polymer solution were scrutinised. From the gathered results, ion-water interactions, ion-ion interactions at finite concentration, and water-structure breaking character for aqueous solutions of [C4mim][HSO4] are more considerable than aqueous solutions of [C4mim]Br. However, [C4mim]Br-polymer interactions are more favourable than interactions between [C4mim][HSO4] and the studied polymers.

Quantitative Prediction of the Development and Opening Sequence of Fractures in an Ultradeep Carbonate Reservoir: A Case Study of the Middle Ordovician in the Shunnan Area, Tarim Basin, China

Summary Quantitative prediction of reservoir tectonic fracture development characteristics, opening pressures, and opening sequences is critical in the exploration and development of oil- and gas-bearing reservoirs and thus has received widespread attention. Using numerical simulations of the paleostress field during the formation of tectonic fractures and the rock fracture criterion, we predict the development and occurrence of fractures in the Middle Ordovician Yijianfang Formation in the Shunnan region of the Tarim Basin, China. The local paleostress fields reflected by the mechanical properties and occurrence of tectonic fractures obtained from core descriptions, acoustic emission (AE) experiments, paleomagnetic experiments, sound velocity measurements, and borehole breakouts were used to determine the regional paleostress and in-situ stress. We established a geomechanical model by combining the mechanical parameters of the rocks with the finite element method (FEM), optimizing the boundary conditions with a self-adaptive constraint algorithm, and conducting numerical simulations of the in-situ stresses. Fracture occurrence and numerical simulation results of the in-situ stress field were used to determine the opening pressure (Pk) and opening sequence of the fractures. The level of fracture development decreases away from the strike-slip fault in the study area. Fracture development is positively correlated with the Young’s modulus, paleostress difference, and paleostress difference coefficient of the rock. The direction of the maximum horizontal principal stress is from north-northeast (NNE) to northeast (NE). Initially, shear fractures and tensional fractures oriented NNE 30°–35° and NE 40°–45°, respectively, open during the water injection process. Pk is positively correlated with the horizontal stress difference coefficient and the angle between the fracture strike and the maximum horizontal principal stress. At the structural highs (burial depths shallower than 6450 m) and the structural lows (burial depths deeper than 6450 m), the burial depth correlates negatively and positively with Pk, respectively. This investigation of the development, occurrence, Pk, and opening sequence of tectonic fractures and their principal controlling factors will have a positive impact on the future exploration and production opportunities of similar fractured reservoirs.

Research on Measuring System of Sodium Hydroxide Concentration in Impregnation Solution Based on Ultrasonic Velocity Method

A sound velocity measurement system based on FPGA was designed. The time of flight was calculated by using the wave crest of ultrasonic signal to achieve high precision measurement of sound velocity. The high-speed data communication between field programmable gate array (FPGA) and host computer system is realized by using AN9238, DPRAM and 485 communication port. On the basis of FPGA hardware, high-speed AD chip is used for sampling to complete data storage and processing. The experimental results show that compared with the standard system, the time-of-flight measurement accuracy of the designed system is 0.1㎲, and the measurement accuracy of the solution sound velocity is better than 2 m/s, which verifies the reliability of the scheme.

On the Natyre of the Influence of the Architecture of Molecules on the Thermophisical Properties of Alkane Isomers

Purpose. Performing measurements of sound velocity and density in the liquid phase of hexane isomers on the saturation line in a wide range of state parameters, including the critical region.Methods. Using a precision pulse-phase method for measuring the speed of sound in the liquid phase of hexane isomers and their density with a pycnometer at atmospheric pressure. The paper discusses the results of precision measurements of sound velocity and density in five hexane isomers. The speed of sound was measured on the equilibrium curve in the liquid phase of isomers by the pulse-phase method in the range from -30 to their critical point. The measurement error of the speed of sound did not exceed 1 m/s. Density measurements were performed using a pycnometer at atmospheric pressure in the range from -30 to their normal boiling point with an error not exceeding 0.05%. The results of sound velocity and density measurements were used to study the character of intermolecular forces. The necessity of taking into account the non-covalent chemical interaction of molecules of the studied substances is shown.Results. Sound velocity measurements were performed on the saturation line in the liquid phase of all five hexane isomers in the temperature range from -30 C to the critical temperature of all 5 isomers. The obtained results are used to study the features of the dependence of the energy of intermolecular forces on the parameters of the state in the field of research.Conclusion. It is shown that the energy of intermolecular forces in marginal hydrocarbons and other simple substances is the sum of 3 terms representing: 1) the energy of the dispersive attractive forces proportional to the square of the density; 2) the energy of the repulsive forces proportional to the biquadrate of the density and 3) the energy of weak chemical non-covalent binding forces proportional to the cubic root of the density of matter.

Thermo-acoustical investigation of monosodium glutamate food preservative in an aqueous solution of poly-ethylene glycols (400 and 600) by using ultrasonic technique

This research presents a thermodynamic and acoustic investigation of the monosodium glutamate (MSG) food preservative in an aqueous solution. The addition of polyethylene glycol (PEG 400) and (PEG 600) as a solute is employed to create a ternary solution. The acoustic and volumetric parameters for the liquid mixture (water + monosodium glutamate + polyethylene glycol) at various temperatures (288.15 K, 298.15 K, 308.15 K, and 318.15 K) and constant pressure of 0.1 MPa. were determined using a range of monosodium glutamate concentrations [(0.01), (0.02), and (0.03) mol.kg−1].In the experiment, a density and sound velocity measurement, the Anton Paar DSA 5000 M, was used. Many distinct thermodynamic variables, including partial molar transfer parameters and apparent and partial molar characteristics, are calculated from the experimental data. According to the McMillan-Mayer theory, the estimated derived parameters are then used to produce the pair-triplet coefficients and the first derivatives of the partial molar expansibilities. The results provide valuable insights into the MSG-PEG system's molecular interactions and thermodynamic behavior, offering potential applications for enhanced food preservation techniques in the food industry.

Quadcopter Sound Characterization using a UAV Test Stand

Unmanned Aerial Vehicles (UAVs) generate a complex acoustic field that is not entirely understood. Many components and their interactions contribute to the overall noise level and behavior of the acoustic field. A UAV test stand configured to emulate a small quadcopter with a wheelbase of 61 cm was used to determine the acoustic response of a multicopter. Microflown Technologies Scan & Paint 3D solution was utilized to obtain sound pressure, particle velocity, and sound intensity measurements close to the source, along with propagation planes for three-dimensional visualizations containing both amplitude and direction. Draw away measurements using a PU probe were also completed to analyze the sound field characteristics further from the source. The number of propellers in operation was varied to determine the impact of adding additional sources. Aeroacoustic sources, near and far field behavior, interaction noise, directivity, and the sound field propagation of each case is discussed.

Analysis of the variation of in situ seafloor sediments acoustic characteristics with porosity based EDFM

Numerous factors influence the acoustic characteristics of seafloor sediments, necessitating a comprehensive study that combines theoretical analysis, laboratory measurements and in situ measurements to support acoustic prediction and inversion. In this study, a porosity-based effective density fluid model (P-EDFM) is established to analyze the variation of acoustic properties with the porosity of seafloor sediments. On the biases of P-EDFM, the attribute of measured sound velocity and acoustic attenuation coefficient of seafloor sediment in Series 9B of the SAX99 was well interpreted within the frequency range of 25-100 kHz. The in situ measured sound velocity ratio was well predicated by the P-EDFM in the East China Sea and Yellow Sea. It reveals that the in situ sound velocity ratio decreases with increasing bulk porosity and with decreasing bulk density. The scattering and differences in the acoustic attenuation coefficient measured in situ in seafloor sediments are found to be greater than those observed for sound velocity. After considering the influence of temperature in the P-EDFM, the prediction of in situ sound velocity aligns well with the measured dataset. While, the acoustic attenuation coefficient exhibits an inflection point, increasing initially and then decreasing with changes in porosity, similar to the observed pattern in Hamilton’s observation and estimation. By incorporating temperature and frequency influences, the in situ measurements of sound velocity of seafloor sediments are corrected into laboratory sound velocities by using the P-EDFM. The result reveals the sediment samples’ sampling and transmitting process has a much greater impact on the sound velocity of sandy sediment in the East China Sea compared to muddy sediment. Overall, P-EDFM can predict the in situ sound velocity and sound attenuation coefficient under different temperatures and frequencies, with a lower prediction error for sound velocity compared to sound attenuation coefficient.

Two-Step Correction Based on In-Situ Sound Speed Measurements for USBL Precise Real-Time Positioning

The ultra-short baseline (USBL) positioning system has been widely used for autonomous and remotely operated vehicle (ARV) positioning in marine resource surveying and ocean engineering fields due to its flexible installation and portable operation. Errors related to the sound speed are a critical factor limiting the positioning performance. The conventional strategy adopts a fixed sound velocity profile (SVP) to correct the spatial variation, especially in the vertical direction. However, SVP is actually time-varying, and ignoring this kind of variation will lead to a worse estimation of ARVs’coordinates. In this contribution, we propose a two-step sound speed correction method, where, firstly, the deviation due to the acoustic ray bending effect is corrected by the depth-based ray-tracing policy with the fixed SVP. Then, the temporal variation of SVP is considered, and the fixed SVP is adaptively adjusted according to the in situ sound velocity (SV) measurements provided by the conductivity–temperature–depth (CTD) sensor equipped at the ARV. The proposed method is verified by semi-physical simulation and sea-trail dataset in the South China Sea. When compared to the fixed-SVP method, average positioning accuracy with the resilient SVP be improved by 8%, 21%, and 26% in the east, north, and up directions, respectively. The results demonstrate that the proposed method can efficiently improve the adaptability of sound speed observations and deliver better performance in USBL real-time positioning.

Sound velocities of natural clinohumite at high pressures and implications for seismic velocities of subducted slabs in the upper mantle

Clinohumite is one of the major transformation products of serpentine at high pressures and an important H2O and F carrier in the upper mantle at subduction zones. Sound velocity measurements on clinohumite under high P-T conditions are crucial for modeling slab structure, arc magmatism, seismicity, and deep H2O cycling process. The compressional (P-) and shear (S-) wave velocities of a natural F-bearing clinohumite [Mg8.17Ti0.19Fe0.01(SiO4)4O0.38(OH)1.17,F0.74] were investigated using ultrasonic interferometry technique up to 14.6 GPa at room temperature in a multi-anvil apparatus. The variations of both the P- and S-wave velocities of clinohumite with pressure are the smallest when compared to the common anhydrous and other dense hydrous magnesium silicate (DHMS) phases in the upper mantle. Finite strain analysis of the acoustic velocities and density data yields the adiabatic bulk (KS) and shear (G) moduli and their pressure derivatives: KS0 = 121.0(5), G0 = 76.5(2), KS’ = 3.9(1), G' = 1.1(1). Furthermore, the density and seismic velocity profiles of subducted harzburgite assemblages with various degrees of hydration were calculated up to 14 GPa along an isotherm at 1073 K. Our results indicate that clinohumite has a larger effect on the density of peridotite than phase A and 23 Å phase under the same degree of hydration of peridotite in the upper mantle. Moreover, the VP and VS velocity contrasts between dry harzburgite and harzburgite with 2 wt% H2O from clinohumite are about ∼1.6% and ∼ 1.0%.

Field induced phase transition and magnetic phase diagram of multiferroic Ca2CoSi2O7

Magnetic phase transitions are important for the application of individual materials toward modern spintronics and sensing. Using high-resolution sound velocity and pyrocurrent measurements, we have obtained a detailed magnetic phase diagram of multiferroic Ca2CoSi2O7 for magnetic fields applied along the crystallographic [100]- and [001]-directions. The magneto-acoustic measurements were conducted for both longitudinal and transverse modes in both static and pulsed magnetic fields up to 17 and 68 T, respectively. Distinct anomalies with significant changes in sound velocity and sound attenuation were observed at the onset of the structural and magnetic phase transitions. Interestingly, results obtained for fields parallel to the [100]-direction reveal a hysteresis and step-like decrements in ultrasound indicating a transition into a different metastable structural distortion below TN and at magnetic fields between 4 and 10 T that has not been previously reported. We believe that this field-induced distorted structure phase may be the cause of the subsequent phase transition at 11 T. At low temperatures, a softening of the lattice within the ordered phases is also observed, which may be due to residual spin fluctuations.

Structural Behavior of Amphiphilic Triblock Copolymer P104/Water System.

A detailed study of the different structural transitions of the triblock copolymer PEO27-PPO61-PEO27 (P104) in water, in the dilute and semi-dilute regions, is addressed here as a function of temperature and P104 concentration (CP104) by mean of complimentary methods: viscosimetry, densimetry, dynamic light scattering, turbidimetry, polarized microscopy, and rheometry. The hydration profile was calculated through density and sound velocity measurements. It was possible to identify the regions where monomers exist, spherical micelle formation, elongated cylindrical micelles formation, clouding points, and liquid crystalline behavior. We report a partial phase diagram including information for P104 concentrations from 1 × 10-4 to 90 wt.% and temperatures from 20 to 75 °C that will be helpful for further interaction studies with hydrophobic molecules or active principles for drug delivery.