Pulling force of acoustic-vortex beams on centered elastic spheres based on the annular transducer model

To solve the difficulty of generating an ideal Bessel beam,an simplified annular transducer model is proposed to study the axial acoustic radiation force(ARF)and the corresponding negative ARF(pulling force)exerted on centered elastic spheres for acoustic-vortex(AV)beams of arbitrary orders.Based on the theory of acoustic scattering,the axial distributions of the velocity potential and the ARF for AV beams of different orders generated by the annular transducers with different physical sizes are simulated.It is proved that the pulling force can be generated by AV beams of arbitrary orders with multiple axial regions.The pulling force is more likely to exert on the sphere with a smaller k0a(product of the wave number and the radius)for the AV beam with a bigger topological charge due to the strengthened off-axis acoustic scattering.The pulling force decreases with the increase of the axial distance for the sphere with a bigger k0a.More pulling force areas with wider axial regions can be formed by AV beams using a bigger-sized annular transducer.The theoretical results demonstrate the feasibility of generating the pulling force along the axes of AV beams using the experimentally applicable circular array of planar transducers,and suggest application potentials for multi-position stable object manipulations in biomedical engineering.

Regulation mechanism of biomolecule interaction behaviors on the superlubricity of hydrophilic polymer coatings

Hydrophilic polymer coatings can improve the surface characteristics of artificial implants.However,because they are used in vivo,they inevitably come into contact with biomolecules that affect their interfacial tribological properties.In this paper,the friction behaviors of poly(vinylphosphonic acid)(PVPA)-modified Ti6Al4V and polytetrafluorethylene balls were analyzed using albumin,globulin,aggrecan,and hyaluronic acid as lubricants.The interaction properties and dynamic adsorption characteristics of the biomolecules and PVPA molecules were explored by a quartz crystal microbalance to identify the cause of the friction difference.It was found that protein molecules disturbed the superlubricity of the PVPA-phosphate-buffered saline system because of the formation of a stable adsorption film,which replaced the interfacial characteristics of the PVPA coating.Polysaccharides,with their excellent hydration properties and polymer structure,had an unstable dynamic interaction or zero adsorption with PVPA molecules,and hardly changed the superlubricity of the PVPA and phosphate-buffered-saline system.The influence mechanism of the specific friction of proteins and polysaccharides was analyzed.Interactions were observed among different biomolecules.Polysaccharides can potentially reduce protein adsorption.The result of the synergistic regulation of the friction coefficient for PVPA-modified Ti6Al4V is approximately 0.017.The results of this study will provide a theoretical basis for the use of polymer coatings in vivo.

Study on the microcosmic superlubricity mechanism of PVPA affected by metal cations

Hydrophilic polymer coatings on artificial implants generate excellent tribological properties.The friction properties of polymer coatings are affected by salt ion factors.Herein,the atomic force microscopy(AFM)was used to show that the superlubricity was achieved between poly(vinylphosphonic acid)(PVPA)-modified Ti6Al4V and polystyrene(PS)microsphere probe lubricated with monovalent salt solutions(LiCl,NaCl,KCl,and CsCl).Considering that adhesion is an important cause of friction changes,the AFM was further utilized to obtain adhesion between friction pairs in different salt solutions.The results indicated that the larger the cation radius in the lubricant,the smaller the adhesion,and the lower the friction coefficient of the PVPA coating.The electrostatic interaction between the PVPA and one-valence cations in lubricants was analyzed by the molecular dynamics(MD)simulation as it was found to be the main influencing factor of the adhesion.Combined analysis results of friction and adhesion indicated that by adjusting the size of cation radius in lubricant,the adhesion between the tribo-pairs can be changed,and eventually the magnitude of friction can be affected.This study opens up a new avenue for analyzing the friction characteristics of hydrophilic polymer coatings from the perspective of intermolecular forces.