Spreading on the free surface of a complex fluid is ubiquitous in nature and industry, owing to the wide existence of complex fluids. Here we report on a fingering instability that develops during Marangoni spreading on a deep layer of polymer solution. In particular, the wavelength depends on molecular weight and concentration of the polymer solution. We use the Transmission Lattice Method to characterize the finger height at the micron scale. We model the evolution of spreading radius, involving viscoelastic and shear thinning effects, to suggest a more generalized law than the spreading of Newtonian fluids. We give physical explanation on the origin of the fingering instability as due to normal stresses at high shear rate generating high contact angle and deformation at the leading edge, and so selects the wavelength of the fingering instability. Understanding the spreading mechanism has particular implication in airway drug delivery, and surface coating with patterns.

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The measurement of transparent liquid surface topography is of great importance in many fields such as hydrodynamics of water-walking insects, liquid sloshing and flotation, etc. However, techniques for measuring these specular and rheological surfaces, especially for surfaces with floating objects, have been lacking. Here we report a monoscopic deformed fringe transmissometry for measuring complex transparent liquid surfaces with theoretical sensitivity up to 0.40 µm. The basic principle is to reconstruct the 3D liquid surfaces by the mathematical relations between the in-plane fringe phase-shift and the out-of-plane shape of the liquid surface. It only needs to project a single fringe pattern from the bottom of the liquid and capture a single deformed image from above. A sub-pixel sampling moiré method is proposed to process the captured fringe pattern to obtain the precise phase component and the displacement field, which is then used to reconstruct the 3D shape of the liquid surface. The discontinuous fringes in the pattern also settled by covering an extracted mask to the displacement filed. The verification test of floating 3D printed characters indicates that this new technique is robust in measuring the 3D shape of complex liquid surface. The developed technique provides a versatile tool to explore interfacial phenomena such as wetting, floating, surface wave, etc.

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