Coffee ring effect

The coffee-ring pattern originates from the capillary flow induced by the evaporation of the drop: liquid evaporating from the edge is replenished by liquid from the interior.{{cite journal|title=Capillary flow as the cause of ring stains from dried liquid drops|journal=Nature|volume=389|pages=827–829|year=1997|doi=10.1038/39827|bibcode=1997Natur.389..827D|issue=6653|last1=Deegan|first1=Robert D.|last2=Bakajin|first2=Olgica|author-link2=Olgica Bakajin|last3=Dupont|first3=Todd F.|last4=Huber|first4=Greg|last5=Nagel|first5=Sidney R.|last6=Witten|first6=Thomas A.|s2cid =205027233}} The resulting current can carry nearly all the dispersed material to the edge. As a function of time, this process exhibits a "rush-hour" effect, that is, a rapid acceleration of the flow towards the edge at the final stage of the drying process.{{cite journal|doi=10.1080/14686996.2017.1314776|pmid=28567177|pmc=5439399|title=Suppressing the coffee-ring effect of colloidal droplets by dispersed cellulose nanofibers|journal=Science and Technology of Advanced Materials|volume=18|issue=1|pages=316–324|year=2017|last1=Ooi|first1=Yuto|last2=Hanasaki|first2=Itsuo|last3=Mizumura|first3=Daiki|last4=Matsuda|first4=Yu|bibcode=2017STAdM..18..316O}}

Evaporation induces a Marangoni flow inside a droplet. The flow, if strong, redistributes particles back to the center of the droplet. Thus, for particles to accumulate at the edges, the liquid must have a weak Marangoni flow, or something must occur to disrupt the flow.{{cite journal |title=Marangoni Effect Reverses Coffee-Ring Depositions| journal=Journal of Physical Chemistry B|volume=110|pages=7090–7094|year=2006|doi=10.1021/jp0609232 |pmid=16599468|issue=14|last1=Hu|first1=H|last2=Larson|first2=R. G.}} For example, surfactants can be added to reduce the liquid's surface tension gradient, disrupting the induced flow. Water has a weak Marangoni flow to begin with, which is then reduced significantly by natural surfactants.{{cite journal|last1=Savino|first1=R.|last2=Paterna|first2=D.|last3=Favaloro|first3=N.|title=Buoyancy and Marangoni Effects in an Evaporating Drop|journal=Journal of Thermophysics and Heat Transfer|volume=16|issue=4|year=2002|pages=562–574|issn=0887-8722|doi=10.2514/2.6716}}

Interaction of the particles suspended in a droplet with the free surface of the droplet is important in creating a coffee ring.{{cite journal| title = Alternative mechanism for coffee-ring deposition based on active role of free surface| journal =Physical Review E| volume = 94| issue = 6 | pages = 063104| year = 2016| doi = 10.1103/PhysRevE.94.063104 | pmid =28085318| last1 =Jafari Kang| first1 =Saeed| last2 =Vandadi| first2 =Vahid| last3 =Felske| first3 =James D.| last4 =Masoud| first4 =Hassan| bibcode =2016PhRvE..94f3104J| arxiv =0906.3878| s2cid =10670995}} "When the drop evaporates, the free surface collapses and traps the suspended particles ... eventually all the particles are captured by the free surface and stay there for the rest of their trip towards the edge of the drop."[http://phys.org/news/2016-12-coffee-ring-phenomenon-theory.html Coffee-ring phenomenon explained in new theory]. phys.org (December 20, 2016) This result means that surfactants can be used to manipulate the motion of the solute particles by changing the surface tension of the drop, rather than trying to control the bulk flow inside the drop. A number of unique morphologies of the deposited particles can result. For example, an enantiopure poly (isocyanate) derivative has been shown to form ordered arrays of squashed donut structures. {{cite journal| title = Spontaneous generation and patterning of chiral polymeric surface toroids| journal =Chemical Science| volume = 1| issue = 4 | pages = 469–472| year = 2010| doi = 10.1039/c0sc00159g| last1 =Carroll| first1 =Gregory| last2 =Jongejan| first2 =Mahthild| last3 =Pijper| first3 =Dirk| last4 =Feringa| first4 =Ben| url =https://pure.rug.nl/ws/files/2559399/2010ChemSciCarroll2Supp.pdf}}

File:Coffee ring suppression by cellulose.jpg particles (diameter 1.4 μm) and cellulose fibers (diameter ~20 nm, length ~1 μm). The polystyrene concentration is fixed at 0.1 wt%, and that of cellulose is 0 (left), 0.01 (center) and 0.1 wt% (right).]]

The coffee-ring pattern is detrimental when uniform application of a dried deposit is required, such as in printed electronics. It can be suppressed by adding elongated particles, such as cellulose fibers, to the spherical particles that cause the coffee-ring effect. The size and weight fraction of added particles may be smaller than those of the primary ones.

It is also reported that controlling flow inside a droplet is a powerful way to generate a uniform film; for example, by harnessing solutal Marangoni flows occurring during evaporation.{{Cite journal|doi=10.1021/acs.langmuir.6b03724|title=Influence of the Particle Concentration and Marangoni Flow on the Formation of Cellulose Nanocrystal Films|year=2017|last1=Gençer|first1=Alican|last2=Schütz|first2=Christina|last3=Thielemans|first3=Wim|journal=Langmuir|volume=33|issue=1|pages=228–234|pmid=28034313|doi-access=free}}

Mixtures of low boiling point and high boiling point solvents were shown to suppress the coffee ring effect, changing the shape of a deposited solute from a ring-like to a dot-like shape.{{cite journal|last1=de Gans|first1=Berend-Jan|last2=Schubert|first2=Ulrich S.|title=Inkjet Printing of Well-Defined Polymer Dots and Arrays|journal=Langmuir|volume=20|issue=18|year=2004|pages=7789–7793|issn=0743-7463|doi=10.1021/la049469o|pmid=15323532|url=https://figshare.com/articles/Inkjet_Printing_of_Well_Defined_Polymer_Dots_and_Arrays/3326875}}

Control of the substrate temperature was shown to be an effective way to suppress the coffee ring formed by droplets of water-based PEDOT:PSS solution.{{cite journal|last1=Soltman|first1=Dan|last2=Subramanian|first2=Vivek|title=Inkjet-Printed Line Morphologies and Temperature Control of the Coffee Ring Effect|journal=Langmuir|volume=24|issue=5|year=2008|pages=2224–2231|issn=0743-7463|doi=10.1021/la7026847|pmid=18197714|doi-access=free}} On a heated hydrophilic or hydrophobic substrate, a thinner ring with an inner deposit forms, which is attributed to Marangoni convection.{{Cite journal|doi=10.1021/acs.langmuir.6b02769|title=Effects of Substrate Heating and Wettability on Evaporation Dynamics and Deposition Patterns for a Sessile Water Droplet Containing Colloidal Particles|year=2016|last1=Patil|first1=Nagesh D.|last2=Bange|first2=Prathamesh G.|last3=Bhardwaj|first3=Rajneesh|last4=Sharma|first4=Atul|journal=Langmuir|volume=32|issue=45|pages=11958–11972|pmid=27759960|arxiv=1610.06281|s2cid=46708941}}

Control of the substrate wetting properties on slippery surfaces can prevent the pinning of the drop contact line, which will, therefore, suppress the coffee ring effect by reducing the number of particles deposited at the contact line. Drops on superhydrophobic or liquid impregnated surfaces are less likely to have a pinned contact line and will suppress ring formation.{{Cite journal|last1=McBride|first1=Samantha|last2=Dash|first2=Susmita|last3=Varanasi|first3=Kripa|date=2018|title=Evaporative Crystallization in Drops on Superhydrophobic and Liquid-Impregnated Surfaces|journal=Langmuir|volume=34|issue=41|pages=12350–12358|doi=10.1021/acs.langmuir.8b00049|pmid=29609465|hdl=1721.1/129769|hdl-access=free}} Drops with an oil ring formed at the drop contact line have high mobility and can avoid the ring formation on hydrophobic surfaces.{{cite journal

| last = Tan

| first = Huanshu

| author2 = Wooh, S.

| author3 = Butt, H.-J.

| author4 = Zhang, X.

| author5 = Lohse, D.

| date = 2019

| title = Porous supraparticle assembly through self-lubricating evaporating colloidal ouzo drops

| journal = Nature Communications

| volume = 10

| issue = 1

| pages = 478

| doi = 10.1038/s41467-019-08385-w

| pmid = 30696829

| pmc = 6351649

| bibcode = 2019NatCo..10..478T

| doi-access= free

}}

Alternating voltage electrowetting may suppress coffee stains without the need to add surface-active materials.{{cite journal |author1=Eral, H.B. |author2=Mampallil-Agustine, D. |author3=Duits, M.H.G. |author4=Mugele, F. | title=Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating drops using electrowetting | journal=Soft Matter| volume = 7| pages = 7090–7094| year = 2011| doi = 10.1039/C1SM05183K | issue = 10|bibcode = 2011SMat....7.4954E }} Reverse particle motion may also reduce the coffee-ring effect because of the capillary force near the contact line.{{cite journal| title = Capillary force repels coffee-ring effect| journal =Physical Review E| volume = 82| issue =1| pages = 015305(R)| year = 2010| doi = 10.1103/PhysRevE.82.015305|bibcode = 2010PhRvE..82a5305W | last1 =Weon| first1 =Byung Mook| last2 =Je| first2 =Jung Ho| pmid =20866682| url =http://oasis.postech.ac.kr/handle/2014.oak/12380}} The reversal takes place when the capillary force prevails over the outward coffee-ring flow by the geometric constraints.

The lower-limit size of a coffee ring depends on the time scale competition between the liquid evaporation and the movement of suspended particles.{{cite journal | title=Minimal Size of Coffee Ring Structure | journal=Journal of Physical Chemistry B|volume = 114| pages = 5269–5274| year = 2010| doi = 10.1021/jp912190v | pmc = 2902562 | pmid = 20353247 | issue = 16| last1=Shen| first1=X| last2=Ho| first2=C. M.| last3=Wong| first3=T. S.}} When the liquid evaporates much faster than the particle movement near a three-phase contact line, a coffee ring cannot be formed successfully. Instead, these particles will disperse uniformly on a surface upon complete liquid evaporation. For suspended particles of size 100 nm, the minimum diameter of the coffee ring structure is found to be 10 μm, or about 10 times smaller than the width of human hair. The shape of particles in the liquid is responsible for coffee ring effect.{{cite journal| title = Suppression of the coffee-ring effect by shape-dependent capillary interactions | journal =Nature| volume = 476 | pages = 308–311 | year = 2011| doi = 10.1038/nature10344| issue = 7360|bibcode = 2011Natur.476..308Y | pmid=21850105| last1 =Yunker| first1 =P. J.| last2 =Still| first2 =T| last3 =Lohr| first3 =M. A.| last4 =Yodh| first4 =A. G.| s2cid =205226009}}{{cite web |url=http://www.sciencedebate.com/science-blog/coffee-ring-effect-explained |title=Coffee-ring effect explained |publisher=ScienceDebate.com |accessdate=21 August 2011}} On porous substrates, the competition among infiltration, particle motion and evaporation of the solvent governs the final deposition morphology.{{cite journal | title=Colloidal drop deposition on porous substrates: competition among particle motion, evaporation and infiltration | journal=Langmuir|volume =31| issue=29| pages =7953–7961| year = 2015| doi = 10.1021/acs.langmuir.5b01846 | pmid=26132211| last1=Pack| first1=Min| last2=Hu| first2=Han| last3=Kim| first3=Dong-Ook| last4=Yang| first4=Xin| last5=Sun| first5=Ying|author5-link=Ying Sun (mechanical engineer)}}

The pH of the solution of the drop influences the final deposit pattern.{{cite journal | title=Self-Assembly of Colloidal Particles from Evaporating Droplets: Role of DLVO Interactions and Proposition of a Phase Diagram | journal=Langmuir| year = 2010| doi = 10.1021/la9047227 | pmid = 20337481 | volume = 26 | issue = 11 | pages = 7833–42| last1=Bhardwaj| first1=R| last2=Fang| first2=X| last3=Somasundaran| first3=P| last4=Attinger| first4=D| arxiv=1010.2564| s2cid=4789514}} The transition between these patterns is explained by considering how DLVO interactions such as the electrostatic and Van der Waals forces modify the particle deposition process.

The coffee ring effect is utilized in convective deposition by researchers wanting to order particles on a substrate using capillary-driven assembly, replacing a stationary droplet with an advancing meniscus drawn across the substrate.{{cite journal| title = Controlled rapid deposition of structured coatings from micro-and nanoparticle suspensions| journal =Langmuir| volume = 20| pages = 2099–2107| year = 2004| doi = 10.1021/la035295j| pmid =15835658| issue = 6 | last1 =Prevo| first1 =Brian G.| last2 =Velev| first2 =Orlin D.}}{{cite journal | title = Investigation of the Deposition of Microsphere Monolayers for Fabrication of Microlens Arrays | journal =Langmuir| volume = 24| pages = 12150–12157| year = 2008| doi = 10.1021/la801100g| pmid =18533633| issue = 21 | last1 =Kumnorkaew| first1 =Pisist| last2 =Ee| first2 =Yik-Khoon| last3 =Tansu| first3 =Nelson| last4 =Gilchrist| first4 =James F.}}{{cite journal| title = Steady-state unidirectional convective assembling of fine particles into two-dimensional arrays| journal =Chemical Physics Letters| volume = 243| pages = 462–468| year = 1995| doi = 10.1016/0009-2614(95)00837-T |bibcode = 1995CPL...243..462D| issue = 5–6 | last1 =Dimitrov| first1 =Antony S.| last2 =Nagayama| first2 =Kuniaki}} This process differs from dip-coating in that evaporation drives flow along the substrate as opposed to gravity.

Convective deposition can control particle orientation, resulting in the formation of crystalline monolayer films from nonspherical particles such as hemispherical,{{Cite journal|last1=Hosein|first1=Ian D.|last2=Liddell|first2=Chekesha M.|date=2007-08-01|title=Convectively Assembled Nonspherical Mushroom Cap-Based Colloidal Crystals|journal=Langmuir|volume=23|issue=17|pages=8810–8814|doi=10.1021/la700865t|pmid=17630788}} dimer,{{Cite journal|last1=Hosein|first1=Ian D.|last2=John|first2=Bettina S.|last3=Lee|first3=Stephanie H.|last4=Escobedo|first4=Fernando A.|last5=Liddell|first5=Chekesha M.|date=2008-12-24|title=Rotator and crystalline films viaself-assembly of short-bond-length colloidal dimers|journal=Journal of Materials Chemistry|volume=19|issue=3|doi=10.1039/B818613H|pages=344–349}} and dumbbell{{Cite journal|last1=Hosein|first1=Ian D.|last2=Liddell|first2=Chekesha M.|date=2007-10-01|title=Convectively Assembled Asymmetric Dimer-Based Colloidal Crystals|journal=Langmuir|volume=23|issue=21|pages=10479–10485|doi=10.1021/la7007254|pmid=17629310}} shaped particles. Orientation is afforded by the system trying to reach a state of maximum packing of the particles in the thin meniscus layer over which evaporation occurs. They showed that tuning the volume fraction of particles in solution will control the specific location along the varying meniscus thickness at which assembly occurs. Particles will align with their long axis in- or out-of-plane depending on whether or not their longer dimension of the particle was equal to the thickness of the wetting layer at the meniscus location. Such thickness transitions were established with spherical particles as well.{{Cite journal|last1=Meng|first1=Linli|last2=Wei|first2=Hong|last3=Nagel|first3=Anthony|last4=Wiley|first4=Benjamin J.|last5=Scriven|first5=L. E.|last6=Norris|first6=David J.|date=2006-10-01|title=The Role of Thickness Transitions in Convective Assembly|journal=Nano Letters|volume=6|issue=10|pages=2249–2253|doi=10.1021/nl061626b|pmid=17034092|bibcode=2006NanoL...6.2249M}} It was later shown that convective assembly could control particle orientation in assembling multi-layers, resulting in long-range 3D colloidal crystals from dumbbell shaped particles.{{Cite journal|last1=Hosein|first1=Ian D.|last2=Lee|first2=Stephanie H.|last3=Liddell|first3=Chekesha M.|date=2010-09-23|title=Dimer-Based Three-Dimensional Photonic Crystals|journal=Advanced Functional Materials|volume=20|issue=18|pages=3085–3091|doi=10.1002/adfm.201000134|s2cid=136970162 }} These finds were attractive for the self-assembled of colloidal crystal films for applications such as photonics. Recent advances have increased the application of coffee-ring assembly from colloidal particles to organized patterns of inorganic crystals.

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Category:Phase transitions

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