Investigation of Secondary Radiation Forces Near Boundaries: Interaction Dynamics with Primary Radiation Forces

Understanding the behaviour and characteristics of Secondary Radiation Force (SRF) near boundaries is crucial for advancing ultrasonic manipulation. Investigating the development and interaction of SRF near boundaries offers valuable insights into particle dynamics within confined environments. This article examines SRF generation through an induced boundary, which perturbs the Primary Radiation Force (PRF) in a mid-focused standing wave field. The experiments are first conducted on an expanded polystyrene sphere in the air, without a boundary. The simulated pressure field is perturbed at various frequencies using a stepped-sine method to identify the natural frequency of a trapped object. In this case, the object's movement is treated like a damped oscillator. Next, a boundary is introduced to observe its influence on the natural frequency of the particle and hence the restoring force coefficient. The inclusion of the boundary leads to a significant increase in natural frequency, and the presence of the SRF is quantified by comparing the natural frequency measured with and without the boundary. Notably, as the particle nears the boundary, it encounters an attractive SRF force. To explore this, boundaries of different dimensions are tested, revealing that the attraction increases with the boundary size and the attraction distance slightly varies. These findings not only enhance theoretical models of ultrasonic manipulation but also enable more precise control in practical applications such as ultrasonic levitation, medical ultrasound, and microfluidics. The results provide a deeper understanding of SRF behaviour near boundaries, contributing to the advancement of both scientific knowledge and technological applications.