The surface tension of liquids is an important parameter to characterize the properties of liquids. The non-contact method of measuring the surface tension coefficient of liquids has the advantages of being fast, non-destructive, simple and practical. A new non-contact method for measuring the surface tension coefficient of transparent liquid based on the theory of light propagation is developed, and a special phenomenon of "self-interference" is discovered. The "Phase Standing Wave" theory is proposed to explain the phenomenon.<br>The surface wave of the liquid is excited by a frequency-adjustable vibration source (oscillator), and it propagates dynamically in the form of oscillator and cosine wave. When the white light source or laser is incident on the liquid surface obliquely, the phase distribution (cosine-like wave) generated by the light wave carrying the surface wave is first incident on the white paper which locates at the bottom of the liquid, and the oscillator will also form the oscillator shadow on the white paper. The phase distribution of the light wave on the white paper spreads around dynamically in the form of cosine wave centering on the oscillator shadow. After diffuse reflection from the white paper, part of the diffuse reflected light in the observation direction will carry the dynamic phase generated by the surface wave for the first time back to the liquid surface and meets the surface wave again. After the light wave passes through the surface, it will carry the phase generated by the surface wave twice, that is, the superposition of two surface waves occurs in the phase of the light wave. In the phase of the light wave, the phase wave of the oscillator will propagate in the direction of the oscillator shadow, and the phase wave of the oscillator shadow will propagate in the direction of the oscillator.<br>According to the conditions of the formation of standing waves, two surface waves propagating in the opposite direction can form standing waves, so the phase standing waves can only exist within a relatively small range between the oscillator and the oscillator shadow. Thus the dark and bright "Self-interference" with stable stripes can be observed in the far field. When white light and laser are used as light sources respectively, the fringe spacing remains unchanged, indicating that the fringe spacing is independent of the wavelength of incident light, and the light wave is only the carrier of the mechanical wave phase. The light wave with different wavelength and coherence has no influence on the experimental measurement results.<br>The fringe spacing is measured automatically by analyzing and processing the image captured by camera. According to the dispersion relationship of surface waves, the surface tension coefficient of water is obtained automatically and accurately. The experimental setup is simple, easy to operate, the experimental phenomenon is intuitive, the stripes are clear and stable, and the anti-interference capability is strong, and can be observed in all directions. It can be widely applied to measure the surface tension coefficient of other transparent liquids.
