The instability of a floating object is the main factor preventing near-field acoustic levitation (NAFL) from being widely used in the manufacture of micro-electro-mechanical systems. Therefore, investigating the restoring force due to the generation mechanisms of NAFL is necessary to ensure the stable levitation of the floating object. This study presents a theoretical analysis to evaluate the restoring force based on the gas-film-lubrication theory. The gas-film pressure between the reflector and the radiator is expressed in the form of the dimensionless Reynolds equation in a cylindrical coordinate system, which is solved by an eight-point discrete grid method due to the discontinuous gas-film distribution. An experimental rig is constructed to measure the restoring force at various eccentricities, which can be used to support the developed numerical model. The theoretical results show that the restoring force increases with an increment in eccentricity, which agrees with experimental results. Furthermore, theoretical prediction results indicate that the restoring force increases when the amplitude of the radiator and weight of the levitator increases, which indicates higher system stability. The results of the radiator vibration mode on the restoring force show that the restoring force is the largest in the first-order mode.

The influence of the ambient pressure on the load capacity of near-field acoustic levitation is studied. Theoretical models based on the acoustic radiation pressure and hydrodynamic lubrication theory are built, taking into account the changes of the ambient pressure. Both simulation and measured results show that the levitation force is positively proportional to the ambient pressure. The levitation force increased by 4–5 times at an ambient pressure of 0.5 MPa. A maximum load capacity of 0.309 MPa is obtained.

Previous research has experimentally demonstrated the feasibility of near-field acoustic levitation (NFAL) in water. This paper proposes an analytical model of underwater NFAL to study its levitation performance. At first, the fluid’s compressibility must be determined because it is significant to the model’s accuracy. Therefore, the Reynolds equations describing the film pressure distribution are deduced based on fluid film lubrication theory for incompressible and compressible fluids. The calculated squeeze film forces for each case are compared with experimental observations from the literature to confirm the fluid’s compressibility. The comparison results show that the theoretical numerical results under the compressible condition are closer than the incompressible condition to the experimental data. Further numerical results show that increasing the water temperature decreases the mean squeeze film force when the water temperature is higher than 20 °C. In addition, the mean squeeze film force is increased by increasing the vibration amplitude.