AQUA_MEM: Multi-Dimensional Aqueous State Vector Encoding for Digital Information Storage in Water Droplets

We present AQUA_MEM, a framework for encoding digital information into the measurable physicochemical properties of aqueous solutions. Each water droplet serves as a multi-dimensional state vector whose independent physical channels (pH, ionic conductivity, and optical chromaticity) encode discrete bit values. We demonstrate a proof-of-concept system encoding 8 bits per 50 μL droplet across three channels with 8, 8, and 4 discrete levels, achieving 100% decode accuracy over multiple trials with persistence exceeding 14 days in sealed glass containers. The full system uses commodity sensors totaling under €50 and an ESP32 microcontroller. We analyze the scaling path toward picoliter droplet volumes: with five encoding channels, information densities on the order of 25 MB/L become reachable using existing microfluidic infrastructure. This work constitutes the first systematic framework for intentional multi-channel digital encoding in aqueous solutions.