Origins of Size-Dependent Kinetics in Microdroplets

Microdroplet chemistry has broad implications for atmospheric science, chemical synthesis, and the origin of life. Central to these implications is reaction kinetics, which by textbook definition is independent of reaction system size. Yet this size independence breaks down in microscopic compartments, such as microdroplets. Here we synthesize historical developments and contemporary advances to examine how size-dependent kinetics in microdroplets arises from mass transport, phase partitioning, interfacial reactivity, and surface geometry─factors regulating apparent kinetics individually or synergistically. We highlight that the scaling of apparent kinetics in microdroplets is inherently nonunique. When similar scaling relationships arise from fundamentally different mechanisms, scaling analysis should be complemented by analysis of regime boundaries. On the experimental front, we summarize the laboratory analytical techniques used to measure apparent reaction kinetics in size-resolved droplets, with emphasis on their advantages and limitations in relation to key aspects of experimental design for studying the size-dependent kinetics in microdroplets.