Collisions of small ice particles under microgravity conditions (II): Does the chemical composition of the ice change the collisional properties?

1.9 MB·English

by  C. R. Hill

Astronomy & Astrophysics manuscript no. does_the_chemical_composition_change_the_collisional_properties_arxiv_version
©ESO2015
January6,2015
Collisions of small ice particles under microgravity conditions (II):
Does the chemical composition of the ice change the collisional properties?
C.R.Hill1,D.Heißelmann2,3,J.Blum2,H.J.Fraser1
1 TheOpenUniversity,DepartmentofPhysicalSciences,WaltonHall,MiltonKeynes,MK76AA,UK
e-mail:[email protected]
2 TechnischeUniversitätBraunschweig,InstitutfürGeophysikundextraterrestrischePhysik,Mendelssohnstraße3,38106Braun-
schweig,Germany
3 International Max-Planck Research School, Max-Planck Institute of Solar System Research, Justus-von-Liebig-Weg 3, 37077
Göttingen,Germany
5
1
Received14November2014,Accepted23December2014
0
2
n ABSTRACT
a
J Context.Understandingthecollisionalpropertiesoficeisimportantforunderstandingboththeearlystagesofplanetformationand
theevolutionofplanetaryringsystems.Simplechemicalssuchasmethanolandformicacidareknowntobepresentincoldprotostellar
5
regionsalongsidethedominantwaterice;theyarealsolikelytobeincorporatedintoplanetswhichforminprotoplanetarydisks,and
planetaryringsystems.However,theeffectofthechemicalcompositionoftheiceonitscollisionalpropertieshasnotyetbeenstudied.
]
P Aims.Collisionsof1.5cmicespherescomposedofpurecrystallinewaterice,waterwith5%methanol,andwaterwith5%formic
acidwereinvestigatedtodeterminetheeffectoftheicecompositiononthecollisionaloutcomes.
E
Methods.Thecollisionswereconductedinadedicatedexperimentalinstrument,operatedundermicrogravityconditions,atrelative
h. particleimpactvelocitiesbetween0.01and0.19m s−1,temperaturesbetween131and160Kandapressureofaround10−5mbar.
p Results.Arangeofcoefficientsofrestitutionwerefound,withnocorrelationbetweenthisandthechemicalcomposition,relative
- impactvelocity,ortemperature.
o Conclusions.Weconcludethatthechemicalcompositionoftheice(atthelevelof95%watericeand5%methanolorformicacid)
r doesnotaffectthecollisionalpropertiesatthesetemperaturesandpressuresduetotheinabilityofsurfacewettingtotakeplace.Ata
t
s levelof5%methanolorformicacid,thestructureislikelytobedominatedbycrystallinewaterice,leadingtonochangeincollisional
a properties.Thesurfaceroughnessoftheparticlesisthedominantfactorinexplainingtherangeofcoefficientsofrestitution.
[
Keywords. accretion,accretiondisks–astrochemistry–planetsandsatellites:formation–planetsandsatellites:rings–protoplan-
1 etarydisks
v
0
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81. Introduction (theso-calledbouncingbarrier)(Güttleretal.2010;Zsometal.
0 2010;Kotheetal.2013).Inrecentyears,thepresenceoficehas
0Water ice is abundant in the interstellar medium and has also been postulated as one possible solution to the bouncing bar-
.been observed towards dense clouds, cloud cores, protostellar rier; it has been shown that ice particles have a larger adhesive
1
regionsandprotoplanetarydisks(Obergetal.2011a).Recentre- androllingfrictionforce(Gundlachetal.2011),ahigherstick-
0
5searchstudyingdeuterium-to-hydrogenenrichmentsinthesolar ing threshold (Gundlach & Blum 2014) and reduced elasticity
1system has shown that water ice was present in the solar neb- (Hertzsch 2002) compared to silicate dust particles which will
:ula protoplanetary disk (Cleeves et al. 2014). Water ice is also increase the threshold velocity for sticking. In addition, recent
v
present in planetary ring systems such as the rings of Saturn model simulations have shown that ice condensation could en-
i
X(Cuzzi&Pollack1978;Cuzzietal.1980),whereitscollisional abledustgrainstogrowtodecimetresizesaroundthesnowline
propertiesplayanimportantroleinthestructureanddynamical (Ros&Johansen2013)andeventoicyplanetesimalsiftheini-
r
aevolutionoftherings.Itisnotclearwhethertheseringsareold tialicegrainsweresubmicrometre-sized(Kataokaetal.2013).
oryoung;intheformercase,theringswillberesiduesfromthe
In the context of this paper, ring systems are also of inter-
formation era. The presence of water ice across star and planet
est. Until recently, rings had exclusively been known to exist
formingregionsmaythereforeinfluencetheprocessesofplanet
aroundthefouroutermostsolarsystemplanets,thoughwiththe
formation.
detection of two small dense rings around the Centaur (10199)
It has been proposed that planets form from the dust in a Chariklo(Braga-Ribasetal.2014;Duffardetal.2014),itseems
protoplanetary disk by a process of dust aggregation (Weiden- thatringsystemsarenotexclusivelypresentaroundgiantplan-
schilling1977,1980).Whilethistheoryhasgainedwidespread ets.TheringsaroundSaturnarethemostwellstudiedandcon-
acceptance,thereremainsaprobleminthatgrowthbetweencen- sist predominantly of small icy bodies (Cuzzi et al. 2010), be-
timetreandkilometresizeshasnotbeendemonstrated,asthere tween1cmand10mdiameter,dominatedbywaterice(Zebker
isacriticalvelocity,reducingwithparticlesize(Weidenschilling etal.1985).Theringdynamicsarecomplex,influencedbyper-
1977), beyond which particles tend to bounce rather than stick turbations by resonances between the ring particles and nearby
Articlenumber,page1of8
A&Aproofs:manuscriptno.does_the_chemical_composition_change_the_collisional_properties_arxiv_version
moons, which are counteracted by frequent inelastic collisions tutionwasthoughttobeduetothedifferenceinvelocityranges
at low relative velocities. Typical collision velocities in the un- betweenthetwostudies.
perturbed rings are well below 1 cm s−1 (Esposito 2002; Col- The range of coefficients of restitution and lack of depen-
well et al. 2009), but gravitational perturbations may raise the denceonimpactvelocityandnormalisedimpactparameterisin
averagecollisionspeedconsiderably.Thecollisionsdissipateki- contrast to previous studies of ice collisions which show a de-
neticenergyfromtheringsandtherebydeterminethestabilityof creaseincoefficientofrestitutionwithincreasingimpactveloc-
the rings (Salo et al. 2001; Schmidt et al. 2001); consequently, ity(Bridgesetal.(1984),Hatzesetal.(1988))andverylittleloss
data pertaining to collisional properties and outcomes are vital ofkineticenergyforglancingcollisions(i.e.b/R∼1)(Supulver
tomodeleffortswhichareusedtounderstandringinstabilities, et al. 1995). The reason for this was thought to be the differ-
wakesandoverallstructure,includingtheverystronglyconfined enceinsurfaceroughness;whiletheicesurfacesintheworkof
heights observed in Saturnian and other ring systems (Colwell Bridges et al. (1984), Hatzes et al. (1988) and Supulver et al.
etal.2006,2007;Hedmanetal.2007;Thomsonetal.2007). (1995) were smooth, the ice surfaces in the studies of Heißel-
While water is abundant in planet forming regions, many mannetal.(2010)andPaperIwereroughandanisotropic.This
othermoleculeshavealsobeendetectedinprotostellarregions. explains the ranges of coefficient of restitution and the lack of
Consequently, the icy mantles on dust grains in protoplanetary dependenceonimpactvelocityandnormalisedimpactparame-
disks are likely to be composed of water dominated ices which ter and is in accordance with the work of Hatzes et al. (1988)
mayalsocontainaplethoraof"contaminants",includingmany where both frost and roughened surfaces were found to reduce
simple molecules such as CO, CO , CH OH, CH , NH and thecoefficientofrestitution.Forafullcomparisonoftheexper-
2 3 4 3
HCOOH (e.g. Pontoppidan et al. (2004), Aikawa et al. (2012), imentalconditionsinthesestudies,wereferthereadertoPaper
Obergetal.(2011b),Nobleetal.(2013)).Recentworkhassug- I.
gestedthatthemostabundantvolatilespeciesinplanetsofsolar Aninherentlimitationinthestudiesoficecollisionstodate
composition are H O, CO, CO , CH OH and NH (Marboeuf (as related to planet formation and planetary ring systems) is
2 2 3 3
etal.2014).Itisalsopossiblethatsomeofthesespeciesmaybe theirfocusonpurewaterice.Itispossiblethatthepresenceof
presentinplanetaryringssuchasthoseofSaturn.WhileSaturn’s otherchemicalsmightchangethecollisionalproperties;Bridges
rings are composed of mainly water ice, there is evidence that etal.(1996)studiedthestickingforcesbetweenfrostcoatedwa-
other species are present (Poulet et al. 2003; Cuzzi et al. 2009) ter ice at atmospheric pressure and temperatures between 110-
anditisnotunreasonabletosupposethatsomeofthosemayin- 150Kanddiscoveredthatmethanolfrostshadstrongersticking
clude the above molecules. This provides the main motivations forcesthanwaterfrosts.Themeltingpointofmethanolis176K
forthispaper:firstlytostudytheeffectsofminorchemicalpol- atatmosphericpressuresoitispossiblethattheincreasedstick-
lutantsoncollisionsbetweenicyparticlesatlowvelocities,and ing forces are due to surface melting. This is the same mecha-
secondly to understand how the outcomes of these collisional nismthatleadstoiceaggregationinatmosphericclouds(Hobbs
processes affect processes of planet formation and ring system 1965),leadingtoincreasedaggregationaroundthemeltingpoint
evolution. of water (Hobbs et al. 1974). In protoplanetary disks and plan-
The work on this paper builds on our previous publication etary rings, the temperatures and pressures are too low for this
(Hill et al. (2015), henceforth Paper I) in which the collisions processtodominate.Bothmethanolandformicacidhavebeen
of millimetre-sized ice particles (both spheres and irregularly detectedinprotostellarregionsinthesolidphase(Pontoppidan
shaped fragments, diameters 4.7-10.8 mm) were studied under etal.(2004),Obergetal.(2011b),Aikawaetal.(2012),Schutte
microgravityconditionsonaparabolicflight.Themainparam- et al. (1999), Zasowski et al. (2009)) at the 1-30% abundance
eters studied were the coefficient of restitution, ε, and the im- levelrelativetowatericeformethanoland1-5%levelforformic
pactparameter,b.Thecoefficientofrestitutionistheratioofthe acid (Boogert et al. 2008). We have therefore chosen to focus
relativevelocitiesoftheparticlesafterandbeforethecollision, thisstudyonicyparticlesformedbyfreezingeitherpurewater
whichisrelatedtothetranslationalkineticenergylostinthecol- or5%methanolorformicacidinwater,toinvestigatetheeffects
lision: ofthechemicalcompositionoficeonitscollisionalproperties.
Whiletheeventualaimistoinvestigatetheeffectsofthechem-
v
ε= a (1) icalcompositiononplanetformation,theformoficeusedhere
v
b (solidicespheres)ismorerelevanttoplanetaryringsystemsthan
whereaandbdenoteafterandbeforethecollisionrespectively. protoplanetarydisks,wheretheiceislikelytobeintheformof
The impact parameter is the distance of closest approach of porousagglomerates(e.g.Kataokaetal.(2013)).Thereforethe
the two particles perpendicular to their relative velocity vector. results will be more immediately applicable to planetary rings,
Henceforthweusethenormalisedimpactparameterb/R,which butitishopedthattheywillbeastartingpointfordiscussionof
istheimpactparameter,b,normalisedtoR,thedistancebetween iceinprotoplanetarydisksaswell.
thecentreofmassesofthetwoparticlesatthepointofcollision,
i.e.thesumofthetworadiiforsphericalparticles.
2. Experimentaldetails
The relative collision velocities ranged from 0.26 to
0.51 m s−1. The coefficients of restitution were spread evenly The experimental setup has previously been discussed in detail
between 0.08 and 0.65 with no dependence of this property (Salter et al. (2009) and Paper I) and is summarised again here
on either impact velocity or normalised impact parameter. The forcompleteness(seeFig.1).Theexperimentwasdesignedand
spreadofcoefficientsofrestitutionwasattributedtothesurface built to carry out low velocity particle collisions on parabolic
roughness of the particles. This corroborates previous work by flights. Prior to take off, the particles are loaded into a pre-
Heißelmannetal.(2010),wherecollisionsof1.5cm(diameter) cooled(toaround77K)rotatingdoublehelixparticlereservoir,
ice spheres were studied under microgravity conditions. In this orcolosseum(1).Thisisrotated(2)duringtheflighttolineup
case,theimpactvelocitieswerebetween0.06and0.22m s−1and two diametrically opposed ice particles with motorised pistons
thecoefficientofrestitutionwasspreadevenlybetween0.06and (3) which accelerate the particles to a pre-defined constant ve-
0.84.Thedifferencebetweentherangesofcoefficientsofresti- locitydirectedtowardsthecentreofthecollisionvolume,where
Articlenumber,page2of8
C.R.Hill,D.Heißelmann,J.Blum,H.J.Fraser:Collisionsofsmalliceparticlesundermicrogravityconditions(II):
surfaces.Thetargetwascomposedofpurecrystallinewaterice
andwasplacedinthecentreofthechamberimmediatelypriorto
evacuation.Thetargetwasplacedinadifferentorientationwith
respecttothechamberforeachflight,givingaccesstodifferent
normalisedimpactparameters. Thenormalisedimpact parame-
ter is related to the angle of the target to the direction of travel
(θ)bythefollowingequation:
b/R=cos(θ) (2)
On the first flight of this campaign, the target was at 90◦ to the
direction of travel of the particles, giving b/R = 0 - completely
headoncollisions.Thesecondflighthadthetargetat60◦tothe
directionoftravel,givingb/R=0.5.Thethirdflighthadthetar-
√
getmountedat30◦tothedirectionoftravel,givingb/R= 3/2.
Fig. 1. Computer aided design schematic of the experimental setup Technicalproblemspreventedcollectionofdatainthisconfigu-
(adapted from Salter et al. (2009). The particles are stored in an alu- rationandsoparticle-particlecollisionswereperformedinstead.
minium colosseum (1) which is rotated (2) to line up two colosseum Itwasthereforenecessarytoopenthechamberduringtheflight
holeswithtwodiametricallyopposedmotorisedpistons(3)whichcol- to remove the target. The particles remained within the sam-
lidestheparticleseitherwitha52mmdiametericetarget(4)placedat plereservoirandhencetheirexposuretotheatmosphereofthe
the centre of the collision volume or with each other when the target
planewasminimal.Itisunlikelythatsignificantfrostformation
isremoved.Thesystemispassivelycooledbypassingliquidnitrogen
could occur on the particle surfaces under these conditions and
throughacoolingring(5)priortotakeoff.Theparticlesarekeptwithin
comparisonwithpreviousparticle-particlecollisionsconducted
the colosseum and protected from radiative heating by an aluminium
under similar conditions (but without the opening of the cham-
shield (6), and a 45 kg copper block (7) acts as a thermal reservoir,
keepingthesystemcoolduringtheflight.Theentiresetupisenclosed ber)showsthatthisassumptioniscorrect(seeSection4.1fora
withinavacuumchamber(8). comparison of our data with that of Heißelmann et al. (2010)).
Fig.2showsanexamplecollisionofanicespherecontaining5%
formicacidwiththeicetargetat90◦tothedirectionoftravel.
theycollideinperfectfreefallconditionseitherwithatarget(4) Table1showsabreakdownofallthecollisionssuccessfully
or with each other. The system is cryogenically cooled to tem- performedduringthiscampaign.Thenumbersofcollisionssuit-
peraturesaround77Kbypassingliquidnitrogenthroughacool- ableforanalysisandunsuitableforanalysisaregiven.Residual
ing ring (5) prior to take off and is kept cool during flight with acceleration of the plane (when the quality of microgravity is
the use of a U-shaped aluminium heat shield (6) and a 45 kg poorandthecameramoveshorizontallywithrespecttothepar-
copperblock(7)onwhichtheparticlereservoirsits.Thecopper ticlesasaresult)wasaparticularproblemontheseconddayof
actsasbothathermalreservoirandacryo-pumpforanyresidual thecampaign(targetat60◦)andhenceonly8collisionssuitable
gas. The entire set up is encased inside a vacuum chamber (8), foranalysiswereobtained.Poorqualityhamperedimageanaly-
with a typical residual gas pressure of 10−5 mbar. The pistons siseffortsinafewcasesandtherewasonecaseoffragmentation.
enter the collision volume through two differentially pumped Thisisthoughttoberelatedtothewayinwhichtheicespheres
feedthroughs.Acollisionisinitiatedbyacceleratingthepistons areformedratherthanarealcollisionaloutcome.Table2shows
(which are controlled by Labview software) to a maximum ve- abreakdownofthecollisionssuitableforanalysis,theresultsof
locityofbetween0.01and0.1m s−1.Thepistonsmakecontact whicharepresentedinSection4.1.
withtheparticles,arebroughttoasharpstopafewmillimetres
later, and then are immediately retracted to their starting posi- Table 1. A break down of the collisional outcomes observed in this
tions. The piston tips remain within the cooled region at rest, study.Theanglesrefertotheanglebetweenthetargetandthedirection
meaning that they are at the same temperature as the particles oftraveloftheparticles.Thecollisionsthatweresuitableforanalysis
are detailed in Section 4.1. The other collisions were excluded from
anddonotinduceparticleheatingorsurfacemeltingduringcol-
theanalysisduetoresidualacceleration(poorqualityofmicrogravity),
lision.Ahighspeedcamerawithaframerateof107framesper
poorimagequalityandfragmentation.
secondwascombinedwithprismopticstocapturetwoviewsof
thecollision48.8◦apart.
Collisionaloutcome Numberofcollisions
Theparticlesusedinthisexperimentwereicespheresof1.5
Targetat90◦ Suitableforanalysis 35
cmdiameter.Somewerecomposedofpure,distilledwater,and
Residualacceleration 1
others were composed of a 5% solution of methanol or formic
Poorquality 4
acidindistilledwater.Theparticleswereproducedusingspher-
Fragmentation 1
ical moulds and a standard kitchen freezer. Making ice parti-
Targetat60◦ Suitableforanalysis 8
clesinthiswayproducescrystallinehexagonalicewitharough,
Residualacceleration 41
anisotropicsurface(Heißelmannetal.2010).Theparticleswere
Poorquality 5
removed from the freezer and placed directly into a container
Fragmentation 0
of liquid nitrogen situated within the chamber. Once the parti-
Notarget Suitableforanalysis 15
cles had reached 77 K, they were individually loaded into the
Residualacceleration 3
pre-cooledsamplereservoir.Surfacefrostingwasminimisedby
Poorquality 7
pre-coolingtheparticlereservoirto∼77Kpriortoloadingand
Fragmentation 0
evacuatingthechambertoabasepressureofaround10−5 mbar
Total 120
immediately after loading while the ice particles were still out-
gassingN ,therebypreventingfrostfromformingontheparticle
2
Articlenumber,page3of8
A&Aproofs:manuscriptno.does_the_chemical_composition_change_the_collisional_properties_arxiv_version
11..44
Pure water Target 0°
5% methanol Target 60°
11..22 5% formic acid No target
ε Pure water (previous data) Previous data (no target)
n,
o 11..00
uti
stit 00..88
e
of r
nt 00..66
e
ci
effi 00..44
o
C
00..22
00..00
00..0000 00..0055 00..1100 00..1155 00..2200 00..2255
Relative impact velocity (m s−1)
Fig. 3. Coefficient of restitution as a function of impact velocity for
collisions of 1.5 cm (diameter) ice spheres with a target at 90◦ to the
Fig.2.Imagesequenceofanicespherecontaining5%formicacidcol- direction of travel (b/R = 0, shown by crosses), a target at 60◦ to the
lidingwithapureicetarget(indicated)at90◦tothedirectionoftravelat directionoftravel(b/R=0.5,shownbysquares)andnotarget(boththis
avelocityof0.08m s−1.Theimageswerecapturedusingbeamsplitter data (shown by circles) and data previously reported by Heißelmann
opticswiththeviewontheleftseparatedfromtheviewshownonthe etal.(2010)(shownbystars)).Thechemicalcompositionofthespheres
rightby48.8◦.Theimagesaretaken4/107sapart. areindicatedwithcolours(blackforwater,redformethanol,bluefor
formicacidandgreyforthepreviousdata).Therectangles(blackfor
Table2.Abreakdownofanalysedcollisionsintermsofchemicalcom- purewater,redfor5%methanolinwaterandbluefor5%formicacid
position.Theanglesrefertotheanglebetweenthetargetandthedirec- inwater)showthedataencompassedbyonestandarddeviationofthe
tionoftraveloftheparticles.Theicesphereswerecomposedofpure mean.
hexagonalcrystallinewaterice,5%methanolinwaterand5%formic
acidinwater.
resultsofHeißelmannetal.(2010)whichwereforcollisionsof
Collisiontype Purewater 5% 5% Total purewatericespheresofidenticalsize.Apartfromtwooutliers
methanol formicacid around 0.01 m s−1, the data in the current study lies virtually
Targetat90◦ 7 10 18 35 within the range of the previous data. The two distinct veloc-
Targetat60◦ 8 0 0 8 ity groupings in our data are because of the different relative
Notarget 2 8 5 15 velocities obtained by colliding a particle with a stationary tar-
Total 58 get(thelowervelocityregion)andcollidingaparticlewithan-
othermovingparticle(thehighervelocityregion).Thevelocity
range is similar for both studies (0.05-0.19 m s−1 for the cur-
3. Analysismethodology rent study (disregarding the two outliers around 0.01 m s−1))
Thedataanalysiswasperformedasfollows.Theparticleswere and 0.06-0.22 m s−1 for the previous study). The values of ε
manually tracked frame by frame in both views of the colli- arespreadbetween0.08and0.81forourdataandbetween0.02
sion; the co-ordinates from the two views were combined us- and 0.84 for the previous data; the range is virtually identical
ing a transformation algorithm to give orthogonal (x, y, z) co- forthecurrentandpreviousresults.Thereisnoapparentdiffer-
ordinates. Linear fits in each dimension yielded particle trajec- encebetweenthecollisionalpropertiesofthedifferentchemical
tories from which velocities before and after the collision and compositions. The similarity of the target data with the previ-
hence coefficients of restitution were calculated. The velocities ousparticle-particlecollisiondatashowsthatcollidingasphere
wereresolvedintocomponentsnormalandtangentialtothecol- withalargerbodyhasthesameeffectascollidingitwithanother
lidingsurfacestogivenormalandtangentialcoefficientsofresti- sphere, aresult whichis notsurprising consideringthe similar-
tution.Fortheparticle-particlecollisions,normalisedimpactpa- ities in contact area during collision in both cases. Finally, the
rameters (b/R) were also calculated from particle trajectories. similarity of these particle-particle collisions with the previous
Where errors are shown, they were calculated using standard work reiterates the fact that the brief opening of the chamber
methodsforthepropagationoferrors. duringtheflighttoremovethetargetdidnotaffecttheresults;if
significant frosting of the particle surfaces had occurred during
this time, the coefficients of restitution would be lower (Hatzes
4. Resultsanddiscussion etal.1988).
Where there was sufficient data, rectangles have been plot-
4.1. Coefficientofrestitution
tedtoshowhowmanypointslaywithinonestandarddeviation
Bouncing was the only collisional outcome observed in this ofthemean(ignoringanyoutliers).Theserectangleshavebeen
study, corroborating previous results in Paper I and also those plottedindependentlyforeachexperimentalsetup.Forthetarget
of Heißelmann et al. (2010). If ice particles have a critical ve- at90◦andforparticle-particlecollisions,thereisagoodoverlap
locityfortheonsetofbouncing,itwillbebelow0.01m s−1(the betweentherectangles,demonstratingthatthereisnostatistical
lowestvelocitycollisioninthecurrentstudy)forparticlesofthis differencebetweenthedatasets.Wethereforeconcludethatthe
size.Fig.3showsthecoefficientsofrestitution(ε)asafunction presenceofmethanolandformicacidintheiceatalevelof5%
ofrelativeimpactvelocityforthisstudycomparedtheprevious does not change the coefficient of restitution. For the target at
Articlenumber,page4of8
C.R.Hill,D.Heißelmann,J.Blum,H.J.Fraser:Collisionsofsmalliceparticlesundermicrogravityconditions(II):
60◦, technical difficulties prevented the capture of useful colli- targetat90◦(b/R=0)andthedataforparticle-particlecollisions
sions for any other material than pure water, meaning that the (b/R=0.00-0.33)bothhaveasmallcomponentoftangentialim-
effectofchemicalcompositioncannotbetestedforthisflight. pactvelocitycomparedtothenormalcomponent(Fig.4a,b,e,
At this point it is instructive to consider why the inclusion f),whichistobeexpectedforheadon(ornearlyheadon)col-
ofmethanol andformicaciddoes notnoticeablyaffectthe col- lisions.Thetangentialcoefficientsofrestitutionarespreadfrom
lisional properties of the ice spheres. As mentioned in the in- around0.1to1.3andfromaround0.2to1.1forparticle-particle
troduction, methanol frosts are much stickier than water frosts collisions. The spread for the normal coefficients of restitution
between 110 and 150 K at atmospheric pressure (Bridges et al. is much less, from around 0 to 0.6 for the target at 90◦, and
1996); this is most likely because of surface wetting near the fromaround0to0.7forparticle-particlecollisions.Thisshows
melting point. However, surface wetting is not possible at the thatheadoncollisionscanresultinbothscatteringandrebound,
lowpressureswithinourchamberforeithermethanolorformic withmoreenergygoingintoscatteringthanreboundonaverage.
acid.Itisalsousefultoconsiderwherethemethanolislocated For the target at 60◦ (b/R=0.5), there is a much larger tangen-
within our ice spheres. Considering the methanol-water phase tialcomponenttotheimpactvelocitythanforthetargetat90◦,
diagram (Takaizumi & Wakabayashi (1997), Deschamps et al. although the normal component to the impact velocity is still
(2010)),at255K(thetemperatureofastandardkitchenfreezer), larger(Fig.4candd).Thespreadsoftangentialandnormalco-
thewaterwillhavefrozenbutthemethanolwillremainasaliq- efficientsofrestitutionareverysimilarinthiscase,botharound
uid.Asthefreezingprocesswillfreezefromtheoutsidein,itis 0.3-0.7. This demonstrates that for b/R=0.5, the distribution of
possiblethatallofthemethanolisatthecentreoftheicesphere energyintoreboundandscatteringisroughlyequal.Inallcases,
due to exclusion from the water ice as it freezes, meaning that thechemicalcompositionoftheicedoesnotaffectthedistribu-
thesurfaceisessentiallypurewatericewhichwouldexplainthe tionoftranslationalkineticenergy.
similarity in collisional properties. Formic acid freezes at 282 Considering the highest value of ε obtained in this study, a
K, which is higher than the freezing point of water, although it minimum of 29% of the particles’ translational kinetic energy
is likely that the presence of the water will depress the freez- (1−ε2 with ε = 0.84) is lost in the collision. From the image
ingpointofformicacid.Ataconcentrationof5%,wespeculate sequences of the collisions, it is clear that some of the energy
thattheformicacidwillbedistributedthroughouttheicesphere isconvertedintorotationalenergy.Abreakdownoftheparticle
due to the similarity in freezing points. Regardless of where in rotation is shown in Table 3. The overall values are given but
thespheresthemethanolandformicacidispresent,atalevelof dividing the spheres by chemical composition does not signif-
5%,theoverallstructureislikelytobedominatedbycrystalline icantly alter the proportions. Due to the spherical shape of the
waterice.Itislikelyforthisreasonthatthecollisionalproperties particles(andhencethelackofdistinguishingmarks),itwasnot
shownodependenceonthecompositionoftheice. possibletoextractmorequantitativerotationalinformation.
To determine the strength of any correlation between the
variables, the linear Pearson correlation coefficient was com- Table3.Thepercentageofparticlesthatrotateanddonotrotatebefore
puted: andafterthecollision.
(cid:80)n (y −y¯)(x −x¯) Rotates(%) Doesnotrotate(%) Unclear(%)
r= (cid:112)(cid:80)n (iy=0−iy¯)2(cid:112)(cid:80)ni (x −x¯)2 (3) Before 4 89 7
i=0 i i=0 i After 71 10 19
The number of points is given by n and the mean values of x
andyaregivenby x¯andy¯ respectively.Thisgivesameasureof The majority of the particles were fired from the pistons
thestrengthofcorrelationbetweentwovariables,xandy,witha withoutanyobservablerotation(89%).Themajorityofthepar-
valueof1/-1indicatingperfectpositive/negativecorrelationand ticles did rotate after the collision (71%) but 10% did not. It is
avalueof0indicatingnocorrelation.Thevaluereturnedisonly clearfromthisandpreviousresultsinPaperIthatrotationcan-
significantwherethereisasignificantnumberofdatapointsand notaccountforalloftheparticles’energyloss.Acomputational
soonlycasesthatmeetthisconditionwillbediscussed.Forthe study by Zamankhan (2010) showed that most translational ki-
targetat90◦,thedataforpurewater,methanolandformicacid netic energy in icy grain collisions is dissipated due to surface
shows no correlation between coefficient of restitution and im- fracturing. We cannot confirm or refute this here because such
pact velocity (values of 0.21, 0.06 and 0.05 respectively). The surface fracturing would not be visible in our images. It is also
sameistrueforpurewaterwiththetargetat60◦(correlationco- possiblethatenergyisconvertedintoheatleadingtodesorption
efficientof0.13)andformethanolwithnotarget(correlationco- ofsurfacematerial,whichagainisnotdetectableinourexperi-
efficientof0.47);therestofthedatasetscontainaninsufficient mentandisbeyondthescopeofthispaper.
number of points for any correlation to be statistically signifi-
cant.Thesevaluesindicatethatthecoefficientofrestitutiondoes
4.2. Effectoftemperature
not depend significantly on impact velocity which corroborates
ourpreviousworkinPaperIandtheworkofHeißelmannetal. Duetoparabolicflightrestrictions,itisnotpossibletocoolthe
(2010).Here,asbefore,thesurfaceroughnessislikelytobethe chamberwithliquidnitrogenduringflight.Theflowofnitrogen
most important factor in explaining the range of coefficients of mustbestoppedpriortotakeoff,relyingonthecopperintheex-
restitution. perimenttoactasaheatsink.Thechambertemperatureslowly
Fig.4showsthecoefficientsofrestitutiontangentialandnor- increasesduringtheflight,givingusanopportunitytostudythe
maltothecollidingsurfacesasafunctionoftangentialandnor- effect of temperature on coefficient of restitution. The data for
malimpactvelocityrespectively.Calculatingthesegivesfurther the third flight of this campaign have been excluded due to the
information about the distribution of energy after the collision. possibilityofadditionalwarmingwhenthechamberwasopened
Thenormalcoefficientofrestitutiongivesinformationaboutthe toremovethetarget.Thetemperaturerangedfrom131to160K,
reboundoftheparticlesandthetangentialcoefficientofrestitu- increasingatarateof22.5Kh−1 forthefirst20minutesandat
tiongivesinformationaboutparticlescattering.Thedataforthe arateof7.0Kh−1 fortheremainderoftheflight.Fig.5shows
Articlenumber,page5of8
A&Aproofs:manuscriptno.does_the_chemical_composition_change_the_collisional_properties_arxiv_version
11..44 11..44
εII Pure water Pure water
ential coefficient of restitution, 0001100011..........4680246802 55%% mfoermthica naoclid εmal coefficient of restitution, ⊥ 0001100011..........4680246802 55%% mfoermthica naoclid
ang 00..22 Nor 00..22
T
00..00 00..00
00..0000 00..0022 00..0044 00..0066 00..0088 00..1100 00..1122 00..0000 00..0022 00..0044 00..0066 00..0088 00..1100 00..1122
Tangential relative impact velocity (m s−1) Normal relative impact velocity (m s−1)
11..44 11..44
εII Pure water Pure water
ential coefficient of restitution, 0001100011..........4680246802 εmal coefficient of restitution ,⊥ 0001100011..........4680246802
ang 00..22 Nor 00..22
T
00..00 00..00
00..0000 00..0022 00..0044 00..0066 00..0088 00..1100 00..1122 00..0000 00..0022 00..0044 00..0066 00..0088 00..1100 00..1122
Tangential relative impact velocity (m s−1) Normal relative impact velocity (m s−1)
11..44 11..44
εII Pure water Pure water
ential coefficient of restitution, 0001100011..........4680246802 55%% mfoermthica naoclid εmal coefficient of restitution, ⊥ 0001100011..........4680246802 55%% mfoermthica naoclid
ang 00..22 Nor 00..22
T
00..00 00..00
00..0000 00..0055 00..1100 00..1155 00..2200 00..0000 00..0055 00..1100 00..1155 00..2200
Tangential relative impact velocity (m s−1) Normal relative impact velocity (m s−1)
Fig. 4. Coefficients of restitution normal and tangential to the colliding surfaces as a function of the normal and tangential impact velocity
respectively.Thechemicalcompositionsareindicatedbydifferentcoloursandsymbols:blackplussymbolsforpurewaterice,redcrossesfor
5%methanolinwater,andbluecirclesfor5%formicacidinwater.a.Tangentialcoefficientofrestitutionforthetargetat90◦tothedirectionof
travel(b/R=0).b.Normalcoefficientofrestitutionforthetargetat90◦tothedirectionoftravel(b/R=0).c.Tangentialcoefficientofrestitution
forthetargetat60◦tothedirectionoftravel(b/R=0.5).d.Normalcoefficientofrestitutionforthetargetat60◦tothedirectionoftravel(b/R=
0.5).e.Tangentialcoefficientofrestitutionforparticle-particlecollisions(b/R=0.00-0.33).f.Normalcoefficientofrestitutionforparticle-particle
collisions(b/R=0.00-0.33).
the coefficient of restitution as a function of temperature. The would be required (around 250 K and a few mbar, considering
dataforthesecondflight(targetat60◦)isatlowertemperatures the phase diagram of water (Ehrenfreund et al. 2003)). In this
thanthedataforthefirstflight(targetat90◦)becausetheavail- temperature and pressure regime, we would expect to see a re-
ablecollisionsfromthisflightalltookplaceearlierintheflight duction in coefficient of restitution with increasing temperature
thanthoseonthefirstflight.Thereisnocorrelationbetweenthe owingtogreaterstickingforcesathighertemperatures,asseen
parameters(acorrelationcoefficientof-0.06)andhencewecon- bySupulveretal.(1997).
cludethattemperaturehasnoeffectoncoefficientofrestitution
overthistemperaturerange.Thiscorroboratesourpreviouswork
in Paper I and again we conclude that this is due to the lack of 5. Astrophysicalimplications
surfacemeltingatthesetemperaturesandpressures.Inorderfor
It appears that inclusion of methanol and formic acid at a level
surfacemeltingtotakeplace,highertemperaturesandpressures
of 5% has no impact on the collisional properties. However,
Articlenumber,page6of8
C.R.Hill,D.Heißelmann,J.Blum,H.J.Fraser:Collisionsofsmalliceparticlesundermicrogravityconditions(II):
1. Sticking does not occur as a collisional outcome. Universal
11..44 bouncing was observed, demonstrating that the critical ve-
Pure water Target 0° locity for the onset of bouncing of ice particles of this size
ε 11..22 55%% mfoermthica naoclid Target 60° (1.5cmdiameter)islessthan0.01m s−1.
on, 11..00 2. Nodifferencewasobservedbetweenthecoefficientsofresti-
uti tutionforpurecrystallinewaterice,waterwith5%methanol
estit 00..88 and water with 5% formic acid. The presence of methanol
of r and formic acid in the ice does not affect the collisional
ent 00..66 behaviour in our experiments. We postulate that this is be-
ci
effi 00..44 causeallsampleshavesurfacestructuresthataredominated
Co bycrystallinewaterice.Asinpreviousstudies(PaperIand
00..22
Heißelmannetal.(2010)),thesurfaceroughnessisthedom-
00..00 inantfeatureoftheparticles,nottheirchemicalcomposition.
113300 113355 114400 114455 115500 3. A broad range of coefficients of restitution was found with
Temperature (K)
no correlation between this and impact velocity, replicating
Fig.5.Coefficientofrestitutionasafunctionoftemperature.Thereis theresultsofpreviousstudies(PaperIandHeißelmannetal.
nocorrelationbetweenthetwoparameters.Thechemicalcompositions (2010)). As before, this is thought to be due to the rough,
areindicatedbydifferentcolours:blackforpurewaterice,redfor5% anisotropicsurfacesoftheiceparticles.
methanolinwater,andbluefor5%formicacidinwater.Thesymbols 4. Atleast29%oftheparticles’initialtranslationalkineticen-
indicatetheorientationofthetarget;aplussymbolforthetargetat90◦to ergyislostinthecollision.Someofthisenergyisconverted
thedirectionoftravel,andasquareforthetargetat60◦tothedirection
into rotational energy but this cannot account for all of the
oftravel.
energyloss.
5. Temperaturedidnotaffectcoefficientofrestitutionoverthe
rangemeasured(131to160K).
methanolfreezeoutinplanetaryringsandprotoplanetarydisks
willhappenatlowertemperaturesthanthoseatwhichourfreez-
Acknowledgements. We thank the Deutsches Zentrum für Luft- und Raum-
ingprocesstookplace,whichcouldleadtoatrulymixedice.In
fahrt (DLR) for providing us with the parabolic flights and for financial aid
thisscenario,methanolmayhaveagreatereffectoncollisional throughgrantnos.50WM0936and50WM1236.WethanktheEuropeanSpace
properties, particularly if it is present in higher concentrations Agency(ESA)forprovidinguswiththeICAPShigh-speedcamerasystemdur-
neartheicesurface.However,formicacidhasnotbeendetected ingtheparabolicflightcampaign.D.HeißelmannwassupportedbytheDeutsche
Forschungsgemeinschaft(DFG)throughgrantno.BL298/11-1.Wewouldlike
abovethe5%levelandislikelytobeevenlydistributedthrough-
tothanktheEuropeanCommunity’sSeventhFrameworkProgrammeLASSIE
outouricesample,leadingustoconcludethattheinclusionof FP7/2007-2013(LaboratoryAstrochemicalSurfaceScienceinEurope)forfund-
formicacidwillnotaffectthecollisionalpropertiesoftheices. ingH.J.FraserandC.R.Hill’sparticipationinthedataanalysisandinterpre-
Therefore, it seems unlikely that the inclusion of other species tationfromthiswork,undergrantagreementno.238258.C.R.HillthanksThe
onthelevelof5%willaffectthecollisionaloutcomesoficypar- OpenUniversityforaPhDstudentship;H.J.FraserthanksSUPA(ScottishUni-
versitiesPhysicsAlliance)andTheUniversityofStrathclydeforsupportingthe
ticlesinringsystemsorprotoplanetarydisks. originalexperimentalwork.Last,butnotleast,wethankallparabonauts(Katha-
Boththepureicespheresandtheicespherescontaining5% rinaBairlein,EikeBeitz,RenéWeidlingandMartinUhrin)whoparticipatedin
methanol or formic acid yielded a broad range of coefficients theparabolic-flightcampaignfortheirhelpinexecutingtheexperimentsandBob
Dawsonforhisassistanceduringthecampaign.
ofrestitution.ThiscorroboratespreviousresultsbyHeißelmann
etal.(2010)andPaperI.Historically,thecoefficientsofrestitu-
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