1：Experimental Design and Method Selection:

The design is based on the principle of critical angle refractometry using total internal reflection. A custom optical coupler is designed to collect light from the smartphone screen and direct it to a sample in a flowcell, with reflected light captured by the front camera. A custom smartphone application analyzes images to detect gradient peak shifts for refractive index and Brix value measurement.

2：Sample Selection and Data Sources:

Nine water-sucrose solutions (0-22% w/w) are used for calibration, and six packaged fruit juices are tested. Samples are prepared at room temperature (25°C).

3：List of Experimental Equipment and Materials:

Smartphone (Lenovo K910L), 3D printer (Flashforge Creator X), optical coupler made from epoxy (EPO-TEK? 301-1), flowcell, peristaltic pump (Lead fluid BQ50S), silicone tubes, water-sucrose solutions, fruit juices.

4：Experimental Procedures and Operational Workflow:

The optical coupler is fabricated using 3D printing and epoxy casting. Samples are delivered into the flowcell via peristaltic pump at 100 μL/min. Images are captured every 3 seconds using the smartphone camera and analyzed in real-time with the custom app to compute refractive index and Brix value.

5：Data Analysis Methods:

Images are processed using MATLAB and Simulink. Reflected intensity profiles are differentiated to obtain gradient curves, which are filtered and fitted with a Weibull function to find peak positions. Calibration curves are established for refractive index and Brix value, and linear regression is used for quantification.