Significance Despite strong fire prevention efforts, every year wildfires destroy millions of acres of forest. While fires are necessary for a healthy forest ecology, the vast majority are human-caused and occur in high-risk areas such as roadsides and utilities infrastructure. Yet, retardant-based treatments to prevent ignitions at the source are currently impossible with existing technologies, which are only suited for reactive fire prevention approaches. Here we develop a viscoelastic carrier fluid for existing fire retardants to enhance retention on common wildfire-prone vegetation through environmental exposure and weathering. These materials enable a prophylactic wildfire prevention strategy, where areas at high risk of wildfire can be treated and protected from ignitions throughout the peak fire season.

Abstract Polyphosphate fire retardants are a critical tactical resource for fighting fires in the wildland and in the wildland–urban interface. Yet, application of these retardants is limited to emergency suppression strategies because current formulations cannot retain fire retardants on target vegetation for extended periods of time through environmental exposure and weathering. New retardant formulations with persistent retention to target vegetation throughout the peak fire season would enable methodical, prophylactic treatment strategies of landscapes at high risk of wildfires through prolonged prevention of ignition and continual impediment to active flaming fronts. Here we develop a sprayable, environmentally benign viscoelastic fluid comprising biopolymers and colloidal silica to enhance adherence and retention of polyphosphate retardants on common wildfire-prone vegetation. These viscoelastic fluids exhibit appropriate wetting and rheological responses to enable robust retardant adherence to vegetation following spray application. Further, laboratory and pilot-scale burn studies establish that these materials drastically reduce ignition probability before and after simulated weathering events. Overall, these studies demonstrate how these materials actualize opportunities to shift the approach of retardant-based wildfire management from reactive suppression to proactive prevention at the source of ignitions.

Every year in the United States, wildfires destroy millions of acres of forest, cost billions of dollars to suppress, and destroy the lives and livelihoods of thousands of people (1⇓⇓–4). While some wildfires are critical for healthy forest ecology, human activities cause 85% of fires in the United States, accounting for 44% of the total area burned, and have tripled the length of the fire season (2). Furthermore, numerous studies indicate that beyond incident casualties and infrastructure damage, wildfires lead to dangerous levels of airborne particulates that significantly increase risk of respiratory and cardiovascular diseases among human populations (5⇓⇓⇓⇓–10).

Yet, encouragingly, these wildfires predominantly initiate at select “high-risk” locations such as roadsides and utilities infrastructure, providing targets for prophylactic treatment efforts. California exhibits one of the most severe wildfires seasons worldwide and has the highest population living in the wildland–urban interface, where wildfires pose the greatest threat to human life (11). Approximately 84% of the 300,624 wildfires occurring in California over the past 10 years were initiated at these high-risk areas (Fig. 1 and SI Appendix, Table S1). Moreover, fires initiating at these high-risk areas are Tier 2 or Tier 3 threat regions (as designated by firefighting agencies) and are more severe and burn more acres per fire on average (Fig. 1B). These data suggest that treating these high-risk landscapes with retardant formulations that provide season-long protection against ignition could greatly reduce the incidence and severity of wildfires and, as a prophylactic strategy, allow for careful consideration of local factors before application.

Fig. 1. Prophylactic treatment of landscapes at high risk of fire starts with environmentally benign polymer-particle retardant formulations. (A) Map of California displaying wildfires occurring between January 1, 2009 and December 31, 2018 with the fires initiating at high-risk locales (roadsides and utilities infrastructure) highlighted in red and fires initiating in all other locales shown in gray. Tier 2 and Tier 3 fire threat regions are highlighted in orange. (B) Bar plots exhibiting: 1) the percentage of ignitions occurring at high -risk locales throughout the entire state, and in Tier 2 and Tier 3 threat regions, and 2) the average number of acres burned per fire initiating in high-risk locales throughout the entire state, and in Tier 2 and Tier 3 threat regions. (C) Schematic of a prophylactic treatment strategy illustrating the spray delivery and adherence of a retardant-loaded viscoelastic fluid, followed by the formation of a weather-resistant, fire-retarding film.

Materials used for wildfire management are either categorized as fire suppressants or fire retardants, with many suppressants commonly used as short-term retardants. Fire suppressants are used for direct application onto an ongoing fire and include perfluorinated surfactant-based foams and “water-enhancing gels” based on superabsorbent polymers (12⇓⇓⇓⇓⇓⇓⇓–20). Perfluorinated surfactant-based foams are highly effective at suppressing actively burning fires; however, they are classified as high-risk environmental contaminants because of their long-term environmental persistence, potential for bioaccumulation, and toxicity (21⇓–23). Conversely, in addition to direct suppression of fires, water-enhancing gels have been used as short-term retardants on buildings in the path of encroaching fires (13⇓⇓⇓⇓–18). These gels are only effective when wet and do not stop fires once the water has evaporated, which often occurs in under an hour during normal wildland fire conditions (16, 24⇓–26). As a result, these gels cannot be used for long-term preventative treatment of wildland fuels.

On the other hand, fire retardants deemed “long-term retardants” use water primarily as a carrier medium for fire-retarding chemicals that maintain their efficacy even after drying (24⇓–26). The “long-term” designation refers simply to the ability to maintain efficacy after drying and not the duration of their efficacy. The most widely deployed commercial wildland fire-retardant formulations use ammonium polyphosphate (APP) or ammonium phosphate as the active fire-retarding component mixed in aqueous formulations containing polymeric viscosity modifiers (i.e., guar gum and clay particles). In particular, Phos-Chek LC95A (PC) is the primary long-term retardant formulation used on natural wildland fuels (26, 27). Formulations such as PC are a primary tactical resource in fighting wildfires by reducing combustion efficiency and intumescing on the surface of vegetation to form a barrier against further fuel combustion (28, 29). More than 100 million gallons of these retardants are deployed annually to slow advancing flame fronts and to support crews in firebreak development (26, 27). Although the performance-enhancing additives in PC are useful for improving spread and reducing drift when dropped from aircraft during suppression efforts, they do not retain the retardants on target vegetation for extended periods of time, or through environmental exposure or weathering (e.g., rain or wind). As such, these materials cannot be used as preventative treatments to provide season-long protection against ignitions in natural wildland fuels.

Ultimately, existing fire retardants and suppressants are used only in emergency response efforts to mitigate the impact of ongoing wildfires and have failed to realistically provide a season-long preventative treatment due to unsuitable materials properties and/or environmental and health concerns (12⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓–23).

Here, we report an environmentally benign cellulose-based viscoelastic fluid as a carrier for APP that improves adherence and retention on target vegetation and enables prolonged prevention of ignition in the wildland. These materials are formed through dynamic and multivalent polymer–particle (PP) interactions, whereby cellulose derivatives such as hydroxyethylcellulose (HEC) and methylcellulose (MC) adsorb onto colloidal silica particles (CSPs) in a multivalent, noncovalent manner (SI Appendix, Fig. S1) (30). Manufacturing of these materials is straightforward and inexpensive at large scales as they contain solely nontoxic starting materials widely used in food, drug, cosmetic, and agricultural formulations (30⇓⇓–33). Due to the noncovalent PP interactions, the viscoelastic fluids are shear-thinning and exhibit low thixotropy, allowing them to be deployed with standard equipment for pumping or spraying used frequently in agricultural applications (Movie S1). We previously showed on the analytical scale that these materials can be used as carriers for APP fire retardants and may provide functional improvements over standard formulations (30). Here we demonstrate the prevention and suppression of wildfires on “light, flashy vegetation” and “1-h” vegetation at laboratory and pilot scale using PP materials loaded with APP. These PP materials do not have inherent fire-retarding effects and are solely used to enhance adherence and retention of APP (SI Appendix, Fig. S1). Wetting and rheological behavior are used to describe how PP materials enhance adherence of APP onto target vegetation during spray application and dry-film experiments demonstrate retention of APP through weathering. Lastly, we show that this combination of adherence and retention enables a preventative treatment strategy on landscapes at high risk for ignitions to reduce the incidence and severity of fire starts.

Conclusion Overall, we have demonstrated that HEC/MC/CSP viscoelastic fluids can be engineered to exhibit viscoelastic fluid-phase and film-phase materials properties that support uniform application, adherence, and retention of polyphosphate fire retardants onto target wildland vegetation. Crucially, these materials are created from biodegradable and nontoxic starting materials through a facile and scalable manufacturing process (SI Appendix, Fig. S13). This combination of material properties allows for prevention of seasonal attrition of fire-retardant coverage induced by weathering or premature microbial degradation and enable a prophylactic treatment strategy to prevent wildfires on landscapes at high risk for fire starts. We propose that the utilization of such a strategy will reduce the incidence and severity of wildfire to protect critical infrastructure and the lives and livelihoods of people in wildfire-prone regions.

Methods Materials. HEC (molecular weight ∼ 1,300 kDa) and MC (molecular weight ∼ 90 kDa) were obtained from Sigma-Aldrich. CSPs (Ludox TM-50; diameter ∼ 15 nm) were obtained from Sigma-Aldrich. APP was obtained from Sigma-Aldrich or from Parchem. PC was provided by Phos-Chek. California Wildfire Map. Map and associated fire ignition data were gathered from California ignition data from January 1, 2009 through December 31, 2018 available through the Fire and Resource Assessment Program (FRAP) database. The number of total wildfires excludes structure fires. Tier 2 threat regions represent areas with elevated risk of impact on people and property from a wildfire and total 37,023,418 acres in the state of California. Tier 3 threat regions represent areas with extreme risk of impact on people and property from a wildfire and total 7,988,148 acres in the state of California. Complete data are presented in SI Appendix, Table S1. Polymer-Particle Viscoelastic Fluid Formation. Polymer-particle formulations were prepared according to previously described methods (30). The concentrations used were 0.1 or 0.2 wt % HEC/MC (0.85/0.15) with 0.5, 1, or 2 wt % CSP, and 13.5 wt % APP. Dynamic and Flow Rheometry. All rheometry experiments were performed on a torque-controlled Discover HR2 Rheometer (TA Instruments) using a 60-mm cone plate (2.007°) geometry. Frequency sweeps were conducted in the linear viscoelastic regime from 0.1 to 100 rad/s. Steady-shear experiments were performed from 0.1 to 100 s−1. Step-shear experiments were performed alternating between 100 and 0.2 s−1. Low shear-rate steady-shear experiments were performed from 1 to 10−5 s−1 and dynamic yield stresses were calculated using the Herschel–Bulkley equation for points up to 10−2⋅s−1. Biodegradability Studies. The COD and BOD of the HEC/MC and HEC/MC/CSP mixtures were determined according to standard methods (39). Laboratory Water Drop Test. One mL of PC or of each PP material formulated with ammonium polyphosphate was pipetted onto a glass slide and allowed to dry overnight. These glass slides were then placed at an ∼50° incline and water was dripped onto the dried sample from a nozzle ∼1.3 cm above the slide in a controlled manner using a syringe pump. The syringe pump was set at a flow rate of 5 mL/min and a total of 20 mL of water was applied. A Canon EOS REBEL T5i/EOS 700D DSLR camera was used to take videos and images. Laboratory Spray Experiments. Each formulation (100 mL) was loaded into a backpack sprayer (Field King) and sprayed onto a layer of grass taped to a wood slab. The nozzle was placed ∼30 cm away from the grass and sprayed in bursts. The videos were captured using a Canon EOS REBEL T5i/EOS 700D DSLR camera. Laboratory Treatment Retention Experiments. Grass (150 g) was spread out and spray-treated with 1 or PC (200 g). The mass of the runoff was measured. The treated grass was then dried to a consistent weight. The final weight of the grass was measured and compared to the untreated control to quantify the amount of treatment adhered on the vegetation. Treated vegetation (20 g) was then weathered with either 0, 0.25, or 0.5 inch (0, 445, or 889 mL) of simulated rainfall and then dried. The grass samples were then homogenized by grinding, dissolved in piranha solution (3:1 sulfuric acid: hydrogen peroxide), and the phosphorus content was determined using ICP-OES. Laboratory-Scale Grass Burn Experiments. Grass burn chambers (SI Appendix, Fig. S7A; n = 4) were loaded with treated, weathered, and dried grass (30 g). The chamber ignitor was heated to 250 °C and the thermocouple temperatures were monitored over time. At the end of the burn, samples were allowed to cool to ambient temperature and the total mass of remaining sample was recorded. Laboratory-Scale Chamise Chip Burn Experiments. Chamise chip burn chambers (SI Appendix, Fig. S7B; n = 4) were loaded with treated, weathered, and dried chips (1 kg) placed into the top section of each burn chamber, with untreated chips (500 g) placed in the bottom. The untreated vegetation was ignited and the thermocouple temperatures were monitored over time. Pilot-Scale Grass Burn Experiments. Grass plots (3 m × 3 m) that were either mowed or unmowed to simulate roadside conditions were treated (leaving a center circle untreated), allowed to dry, weathered, and dried again. The center of each plot was ignited with a hand torch and the burn area was monitored with a drone (DJI; Phantom 3 Professional). Pilot-Scale Chamise Burn Experiments. Chamise was treated, allowed to dry, weathered, and dried again. Chamise piles (1 m × 1 m) were ignited from a starter bundle and the burn was monitored using both a normal camera and an infrared camera (FLIR; Vue Pro-336).

Acknowledgments We thank Alan Peters (Division Chief) and David Fowler (Fire Captain, prefire engineering) from Cal Fire San Luis Obispo, and Daniel Turner from the San Luis Obispo County Fire Safe Council for helpful discussions and providing safety support during large-scale burns. We thank Ashley Fisher, Hannah K. Panno, and David Fowler from Cal Fire San Luis Obispo for help gathering and analyzing fire ignition data for the state of California from the FRAP. We thank David Kempken (Support Shop Manager) for technical support in burn chamber development. We would also like to thank FLIR for providing access to the Vue Pro 336 infrared camera. Part of this work was performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation (NSF) under Award ECCS-1542152. This work was supported by a Kodak Fellowship (A.C.Y.), NSF Alliances for Graduate Education and the Professoriate (AGEP) Fellowship (H.L.H.), and a Realizing Environmental Innovation Program Grant from the Stanford Woods Institute for the Environment.

Footnotes Author contributions: A.C.Y., H.L.H., A.H.K., L.M.S., C.S.C., J.D.A., and E.A.A. designed research; A.C.Y., H.L.H., A.H.K., L.M.S., R.J.B., E.T.M., C.P.B., D.C., J.D.A., and E.A.A. performed research; G.D.M. and A.J.W. contributed new reagents/analytic tools; A.C.Y., H.L.H., A.H.K., L.M.S., R.J.B., E.T.M., C.P.B., G.D.M., A.J.W., D.C., and E.A.A. analyzed data; and A.C.Y., H.L.H., and E.A.A. wrote the paper.

Conflict of interest statement: A.C.Y. and E.A.A. are listed as inventors on a patent describing the technology reported in this manuscript. On 25 October 2018, J.D.A. and E.A.A. founded a company called LaderaTECH, which has licensed the technology from Stanford University. All other authors declare no competing interests.

This article is a PNAS Direct Submission.

Data deposition: Data that support the results of this study are available within the paper and the Supporting Information. Additional relevant data are available upon request from the corresponding author.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1907855116/-/DCSupplemental.