For cellulose impinging on a polished silicon surface, the solid particle transitions to a liquid droplet, after which the liquid droplet reacts to primarily form gases and vapors. The reactive evaporation rate, which correlates with the rate of heat transfer into the droplet14, is defined as the change in mass of the cellulose droplet from the time it is completely liquid until it fully reacts divided by that time interval†. For low temperatures (500–650 °C), intermediate liquid evaporation rate increases nearly linearly with temperature (Fig. 1a). In this regime, liquid intermediate cellulose wets the surface, providing rapid solid-liquid heat transfer (Fig. 1b). Receding contact angles for wetting intermediate liquid cellulose were 56–62 degrees†. However, between 675 °C and 750 °C, the evaporation rate decreases with increasing temperature. At these temperatures, the rate of vapor and gas production is sufficient such that film boiling was observed, with a gas layer greatly reducing heat transfer between the surface and the particle. Above 750 °C, the intermediate liquid fully de-wets the surface and evaporation rate increases with further increases in temperature. For these conditions, intermediate cellulose liquid moved erratically on polished silicon surfaces.

Figure 1 Reactive Liftoff of Crystalline Cellulose Particles on Polished Silicon. a. The rate of evaporation of initially crystalline cellulose particles (average 220 μm) varies by an order of magnitude as the intermediate droplet transitions from low temperature wetting (blue) to film boiling (black) and into the Leidenfrost regime (green). Error bars represent 95% confidence. b. Initially microcrystalline cellulose forms a melt (160 ms), wets polished silicon at 625 °C and completely evaporates by 250 ms. See extended data for video. c. Microcrystalline cellulose particles form a melt on polished silicon at 750 °C which lifts off the surface and moves out of frame (179 ms). Scale bars = 100 μm. Full size image

The heat transfer curve measured in Fig. 1a closely resembles those measured in conventional liquid evaporation Leidenfrost curves16. Observed heat transfer rates vary by nearly an order of magnitude as the particle is subject to transitioning modes of heat transfer. At low temperatures, solid-liquid conduction via wetting provides rapid thermal flux into the particle. With the onset of film boiling, convection through the gas layer becomes the dominant mode of heat transfer. This result indicates that, above 650 °C, the lifetime of a reacting cellulose particle is determined by the rate of heat transfer into the particle. Reacting cellulose intermediate liquid differs from conventional Leidenfrost droplet volatile liquids in that the heat flux supplied to the droplet is balanced by both heat of vaporization as well as heat of reaction for cellulose17.

The uniqueness of a Leidenfrost cellulose particle undergoing reaction derives from the balance of surface heat transfer and biopolymer reaction rates. Comparison of the timescale for reaction, based on cellulose pyrolysis kinetics18,19, with the timescale for heat transfer to the droplet via a dimensionless “Reacting Leidenfrost” quantity, ϕ RL = τ conv/ τ rxn = ρC p L c k rxn /h, provides two extreme conditions†. At small ϕ RL , negligible reaction implies the heating of solid unreacting particles, whereas at large ϕ RL , the particle fully reacts to a slowly evaporating liquid and may exhibit Leidenfrost behavior consistent with conventional liquids (e.g. methanol or water). However, cellulose in the range of 500–700 °C as depicted in Fig. 1 exhibits comparable heat transfer and reaction rates†, 10−1 < ϕ RL < 10+1, such that reaction, evaporation and particle liftoff occur simultaneously.

The complexity of particle conversion results in dramatic variation in surface interaction. The apparent cross-sectional cellulose particle area as viewed from above and normalized to the original particle area was traced with time between 500 °C and 800 °C and plotted in Fig. 2a. At 500 °C and 600 °C, the particle area first increases, as the intermediate liquid cellulose spreads to wet the polished silicon surface. This is followed by rapid decrease in area as the intermediate liquid reacts, evaporates and shrinks. At 800 °C – in the Leidenfrost regime – the particle does not spread to wet the surface and shows significantly reduced heat transfer as the particle slowly reacts. Even at 800 °C, the particle exhibits an overall lifetime similar to that observed below 600 °C. Figure 2b,c depict cellulose on a polished surface at 625 °C and 750 °C respectively, showing the qualitative differences between droplet spreading and wetting at lower temperatures and de-wetting in the Leidenfrost regime above 750 °C. Previous studies have shown that the vapor layer underneath a Leidenfrost droplet is very thin (particle radius >> vapor film height) for the majority of the droplet lifetime20,21. As observed in Fig. 2c, the droplet height is on the same order of magnitude as droplet radius, indicating a substantial rate of gas and vapor production.

Figure 2 Individual Cellulose Particles on Polished Silicon. a. Cross sectional area of cellulose particles (initially ~300 μm) normalized to initial values with reaction time (0–800 ms) for 500–800 °C on polished silicon. b. Profile images of particles at lower temperature (625 °C), which liquefy and wet with increased contact area before rapidly evaporating. See extended data for full video. c. Profile images of particles at higher temperatures (750 °C), where crystalline cellulose liquefies and off gases at sufficient rate to lift molten cellulose droplets above the surface. Full size image

Deviation from the Leidenfrost curve in Fig. 1a was observed when cellulose was pyrolyzed on porous pressed silica and alumina surfaces. Under these conditions, there was no observable transition to Leidenfrost behavior with increased temperature. As a result, the rate shown in Fig. 3a is nearly linear with temperature across the measured range (500–750 °C). Additionally, at temperatures below 675 °C, the observed heat transfer rates are lower compared with those from the polished silicon surface, indicating a lower solid-liquid heat transfer coefficient. However, at 775 °C, the heat transfer rate for the porous surfaces is higher than that observed for the polished surface. These results indicate that particle liftoff from vapor generation is completely inhibited across this temperature range, which agrees with previous work that suggests that vapors and gases from an evaporating liquid droplet penetrate into surface features, such as channels and macropores, thereby suppressing the Leidenfrost effect11,12,22,23,24,25,26. In Fig. 3b, a snapshot of a pyrolyzing cellulose particle on porous alumina at 750 °C, an expected Leidenfrost regime, shows no particle liftoff.

Figure 3 Structured Surfaces for Suppression of Cellulose Particle Liftoff. a. Cellulose particles (~220 μm) liquefy and evaporate at increasing rate as temperature increases on porous silica and alumina, with no measurable transition to film boiling. Error bars represent 95% confidence. b,c. Droplet of molten cellulose on porous alumina (image and diagram). See extended data for full video. d. Position of cellulose particles on polished silicon (green) and porous alumina (blue) indicate suppressed liftoff and motion (skittering) on porous materials. e. 3D profilometry of porous alumina indicates minimal surface roughness. f. Scanning electron micrograph reveals 1–5 μm macropores for sweeping product vapors away from particles. Full size image

A common characteristic of droplets exhibiting the Leidenfrost effect is the ‘skittering’ or dancing motion across the surface, as is commonly observed with a superheated water droplet on metal surfaces. Traces of individual particle motion from high speed imaging for cellulose and polished and porous alumina surfaces are shown in Fig. 3d, with each trace starting at the plot origin. Significant particle motion was observed for the polished surface, with droplets moving in a generally consistent direction once initiated. However, in the case of the porous surfaces, almost no particle motion was observed (Figure S7†). Optical surface profilometry in Fig. 3e depicts flat surface topography, with peak to valley height of only 10–20 μm, indicating that the surface of porous alumina was relatively flat compared to the length scale of cellulose particles. SEM micrographs in Fig. 3f indicate visible macropores between alumina particles, giving support to the mechanism of gas flow through surface macropores for suppression of Leidenfrost behavior.