The explosion is driven by a temperature gap, not by the oil itself
The core of the effect is a mismatch. Water reaches its boiling point long before the surrounding oil does, so as the oil heats up, the water inside is already trying to become steam while the oil is still liquid. Simulations of water-in-oil droplets heated between 300 and 600 °C identify this difference between the surrounding temperature and water's boiling point as the driving force behind the burst [2]. In plain terms, the oil is a hot jacket that keeps pumping heat into water that has nowhere to vent.
That same picture shows up in a kitchen-style setup. When a millimeter-sized water droplet sits in a heated oil bath, the heat drives rapid vaporization and the droplet explodes, and the resulting bubble behavior can be sorted into distinct types depending on how far the growing bubble sits below the oil surface [3]. The explosion is a vapor event: water turning to gas inside a liquid it cannot easily escape through.
Hotter, lower-pressure, and bigger water pockets make bigger bangs
Not every droplet bursts equally hard. In the simulations, raising the surrounding temperature and lowering the pressure both increased the probability and the intensity of the micro-explosion, and enlarging the water droplet within a certain range did the same [2]. The violence also depends on how readily the oil phase itself evaporates, which the same study found has a significant influence on explosion intensity [2].
The aftermath is dramatic. After an intense micro-explosion, the leftover oil fragments can contain less than 20 % of the oil molecules that were in the original droplet, meaning most of the oil is flung into tiny child clusters [2]. How long the whole droplet then takes to fully evaporate is set by whichever of those leftover clusters is largest and slowest to dry out [2].
Heating an oil–water drop produces several distinct breakup patterns
Heating does not produce one single behavior. Under intense radiant heating, oil–water emulsion drops show three regimes of explosive breakup, and which regime occurs depends on measurable parameters of the drop and the heating [5]. So the word "explode" covers a family of outcomes, from a relatively gentle split to a violent shattering, rather than one fixed event.
The physical makeup of the emulsion matters too. The size of the dispersed water droplets and the interfacial tension between the water and oil phases both influence whether micro-explosion occurs during heating [4]. Interfacial tension is simply how tightly the water–oil boundary holds together; a boundary that resists merging or escape can let pressure accumulate rather than leak away.
What this explanation does and does not cover
These findings come from simulations and controlled lab heating, not from a single unified theory of every droplet in every pan. The molecular-dynamics work models water mixed with polycyclic aromatic hydrocarbons, a class of oil molecules, at 300–600 °C and 0.2–1.5 MPa [2], while the bubble study focuses on a simplified millimeter-sized droplet in a heated bath [3]. The general mechanism — trapped water vaporizing faster than it can escape — is consistent across these setups, but the specific thresholds and intensities are tied to those conditions.
One related line of work looks at fuel oil/water emulsion droplets striking a hot solid surface rather than sitting under oil, and it maps out when such droplets bounce, stick, or break apart as temperature and speed change [1]. That is a different geometry from a droplet submerged in hot oil, so it informs the general picture of heat-driven droplet breakup without directly describing the underwater explosion itself.
Sources used in this answer
Regime maps of collisions of fuel oil/water emulsion droplets with solid heated surface
Fuel oil/water emulsion droplets hitting a heated solid surface show distinct collision regimes (including atomization and sticking) mapped against temperature, droplet size, and velocity.
Molecular dynamics simulation of micro-explosion of water-in-heavy oil emulsion droplets
In simulations, the temperature difference between the surroundings and water's boiling point drives micro-explosion of water-in-oil droplets, with higher temperature, lower pressure, and larger water droplets increasing its probability and intensity.
Morphology of bubble dynamics and sound in heated oil
A millimeter-sized water droplet in a heated oil bath undergoes explosive vaporization, producing bubble dynamics that fall into distinct types based on the bubble's stand-off distance from the oil surface.
Effect of physical properties on microexplosion occurrence in water-in-oil emulsion droplets
The size of dispersed water droplets and the interfacial tension between water and oil phases affect whether micro-explosion occurs during heating of water-in-oil emulsions.
Evaporation, boiling and explosive breakup of oil–water emulsion drops under intense radiant heating
Oil–water emulsion drops under intense radiant heating exhibit three regimes of explosive breakup, governed by parameters of the drop and the heating.