Weird Science

Fire Wave

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (2/2024):

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Ples M., Ogni­sta fala (eng. Fire Wave), Che­mia w Szkole (eng. Che­mi­stry in school), 2 (2024), Agen­cja AS Józef Szew­czyk, pp. 40-42

Accor­ding to a rela­ti­vely stra­i­ght­for­ward defi­ni­tion, a flame is a com­plex phy­sico-che­mi­cal sys­tem that results from pyro­ly­sis and oxi­da­tion reac­tions, accom­pa­nied by the emis­sion of light. This pro­cess occurs under con­di­tions of suf­fi­cien­tly high tem­pe­ra­ture and the pre­sence of oxy­gen or ano­ther oxi­di­zing agent. Nota­bly, a flame is a region where com­bu­stion takes place in the gas phase, mea­ning both the fuel and the oxi­di­zer are in gase­ous form.

Within a flame, dif­fe­rent zones can be iden­ti­fied, each cha­rac­te­ri­zed by distinct phy­si­co­che­mi­cal pro­per­ties and pro­ces­ses. Nume­rous fac­tors influ­ence the flame’s appe­a­rance, inc­lu­ding the type of sub­stance being bur­ned and the quan­tity and ava­i­la­bi­lity of the oxi­di­zer. These fac­tors also deter­mine the inten­sity of the heat rele­a­sed and the type of elec­tro­ma­gne­tic radia­tion emit­ted — pri­ma­rily in the form of infra­red and visi­ble light.

Che­mi­sts and phy­si­ci­sts have stu­died the struc­ture and pro­per­ties of fla­mes for cen­tu­ries, hel­ping us bet­ter under­stand their com­plex nature. This know­ledge has led to appli­ca­tions in indu­stry, labo­ra­tory rese­arch, and energy pro­duc­tion. Howe­ver, we can­not yet claim full under­stan­ding — ongo­ing scien­ti­fic rese­arch con­ti­nues to unco­ver new aspects of flame beha­vior. Mastery of com­bu­stion mecha­ni­sms and flame dyna­mics is cru­cial not only in che­mi­stry and phy­sics, but also in engi­ne­e­ring, envi­ron­men­tal science, and both indu­strial and civil safety.

It is also worth noting that the flame is not only a sub­ject of scien­ti­fic inqu­iry but also a source of inspi­ra­tion for arti­sts and cre­a­tors, who seek to cap­ture its dyna­mic and mesme­ri­zing forms in their work. Thus, the flame serves as both an object of scien­ti­fic explo­ra­tion and a sym­bol of human cre­a­ti­vity [1].

By care­fully adju­sting the con­di­tions, one can pro­duce sur­pri­sing flame beha­viors and appe­a­ran­ces. One fasci­na­ting aspect of the expe­ri­ment descri­bed here is an une­xpec­ted aco­u­stic effect that accom­pa­nies it.

Expe­ri­ment

To carry out the expe­ri­ment, we need a large glass ves­sel with a nar­row neck. Based on my trials, the most sui­ta­ble con­ta­i­ner is the large glass bot­tle shown in Photo 1. It has a volume of 10 dm3 (2.6 gal­lons). Large pla­stic water bot­tles may also be used, but extreme cau­tion is requ­i­red due to the poten­tial flam­ma­bi­lity of such mate­rials.

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Photo 1 – Glass ves­sel

As the fuel, we’ll use 99% iso­pro­pyl alco­hol C3H7OH, which is com­monly used for cle­a­ning opti­cal com­po­nents and is rea­dily ava­i­la­ble.

Keep in mind that iso­pro­pa­nol can be hazar­dous. Avoid inha­ling its vapors. If con­ta­mi­na­ted or mishan­dled, it can lead to an explo­sion and the shat­te­ring of the glass ves­sel. Always wear appro­priate per­so­nal pro­tec­tive equ­ip­ment!

Pre­pa­ring the expe­ri­ment is sim­ple: pour a few mil­li­li­ters (0.1–0.3 fl oz) of iso­pro­pa­nol into the ves­sel, cover it, and allow it to eva­po­rate. The liquid is highly vola­tile, so this pro­cess sho­uld take only a few minu­tes — ten at most. The expe­ri­ment is best per­for­med at a rela­ti­vely low tem­pe­ra­ture, aro­und 15°C (59°F), because this allows the alco­hol to oxi­dize more slowly, making it easier to obse­rve the descri­bed effect.

After the alco­hol has eva­po­ra­ted, sim­ply apply a flame to the ves­sel’s open neck. Upon igni­tion (Photo 2A), a flat, cir­cu­lar flame forms and moves slowly down­ward (Pho­tos 2B–C). Simul­ta­ne­o­u­sly, a low-fre­qu­ency sound can be heard. The flame extin­gu­i­shes only upon rea­ching the bot­tom of the ves­sel (Photo 2D).

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Photo 2 – Obse­rved effect; A, B, C, D – phe­no­me­non pha­ses

The flame front is also beau­ti­fully visi­ble from below (Photo 3).

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Photo 3 – View from below

Expla­na­tion

The expla­na­tion for the obse­rved phe­no­me­non is rela­ti­vely sim­ple. Iso­pro­pa­nol is a vola­tile, flam­ma­ble alco­hol. Its vapors are hea­vier than air, so they set­tle at the bot­tom of the ves­sel, displa­cing the air and the oxy­gen it con­ta­ins.

Com­bu­stion requ­i­res both fuel and oxy­gen. These com­po­nents coe­xist only at the inter­face between the alco­hol vapor and the air, which expla­ins the flame’s flat shape. As com­bu­stion pro­gres­ses, the fuel is con­su­med, redu­cing the vapor’s volume and cau­sing the cir­cu­lar flame to descend.

Inte­re­stin­gly, a fairly regu­lar pat­tern of convec­tion cells appe­ars within the flame, cle­arly visi­ble during cer­tain pha­ses of the expe­ri­ment when vie­wed from below (Photo 4). This is an exam­ple of self-orga­ni­za­tion and resem­bles the phe­no­mena first descri­bed by Bénard [2] [3], who stu­died convec­tion in liqu­ids.

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Photo 4 – Self-orga­ni­za­tion

During com­bu­stion, gases such as car­bon dio­xide CO2 and water vapor H2O are rele­a­sed. These escape rapi­dly thro­ugh the ves­sel’s nar­row neck, gene­ra­ting vibra­tions that pro­duce sound. Once the flame goes out, a second, much shor­ter and quie­ter sound can be heard. This results from the con­trac­tion of the coo­ling gases inside the ves­sel and the inrush of air — effec­ti­vely tur­ning the sys­tem into a Helm­holtz reso­na­tor [4].

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

This text has under­gone sli­ght edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal to bet­ter suit online pre­sen­ta­tion.

Marek Ples

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