Transmission-speckle correlation for measuring dynamic deformation of liquid surface

• We report a transmission-speckle correlation method.
• Liquid surface deformation is calculated from transmission-speckles’ movement.
• Newton iteration algorithm is used for height measurement of liquid surface.
• The sensitivity of height measurement of liquid surface is superior to 1.0 μm.

In this paper, a method based on transmission-speckle correlation is proposed for measuring static and dynamic deformation of liquid surface. In the method, a high-speed camera placed vertically above the tested liquid surface is used to observe and record a special speckle pattern put in advance at the bottom of liquid. According to the Snell׳s law, the deformation of liquid surface will lead to the movement of the transmission-speckles. In terms of this, the quantitative relationship between height changes of liquid surface and the in-plane displacement of transmission-speckles can be deduced. Combining with multi-directional Newton iteration algorithm, the dynamic deformation field of liquid surface can be calculated from the in-plane displacement vector field of transmission-speckle images in different moment using speckle correlation method. The sensitivity of the method in measuring height changes of liquid surface is discussed. In this paper, a validation test to measure the surface morphology of a plano-convex lens demonstrates the feasibility and the effectiveness of the method. In addition, the dynamic deformation and propagation process of ripples in the water surface caused by a droplet were investigated.