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Manipulating the Coffeering Effect DOI:10.1002/cphc.201500410 Concepts Manipulating the Coffee-Ring Effect:Interactions at Work ManosAnyfantakis *[a, b, c] and Damien Baigl *[a, b, c] In memory of Zheng Geng The evaporation of adrop of colloidal suspension pinned on particleinteractions drastically affect the morphology of the asubstrate usually results in aring of particles accumulated at deposit. General rules are established to control the CRE by the periphery of the initial drop. Intense research has been de- tuning these interactions, and guidelines for the rational physi- voted to understanding, suppressing and ultimately controlling cochemical formulation of colloidal suspensions capable of de- this so-called coffee-ring effect (CRE). Although the crucial role positingparticles in desirable patterns are provided.This of flow patterns in the CRE has been thoroughly investigated, opens perspectivesfor the reliable control of the CRE in real- the effect of interactions on this phenomenonhas been largely world formulations and creates new paradigmsfor flexible par- neglected. This Concept paper reviews recent works in this ticle patterning at all kinds of interfaces as well for the exploi- field and shows that the interactions of colloids with (and at) tation of the CRE as arobust and inexpensive diagnostic tool. liquid–solid and liquid–gas interfaces as well as bulk particle– 1. Introduction 1.1. Motivation for Studying the CRE Particle deposition from evaporating drops of liquidscontain- ing non-volatile solutes phenomenologically seems to be Understanding and controlling solute deposition from evapo- arather uncomplicated problem. However,our everyday expe- rating liquids is akey factor in various technologies, such as rience contrasts intuition. Characteristicring-shaped deposits inkjet printing,[2] DNA and protein microarrays[3,4] andmicro- are observed after the drying of spilled coffee or tea drops. patterning,[5] whichexplainsthe intense researcheffort in the Washed tablewaredisplays white stains of the same morpholo- last two decades for revealing the underlying phenomena in gy,since salt crystals accumulate at the edge of the water aseeminglycuriosity-driven research topic.[6] drops after drying. Rain drops also leave rings of dust particles From apurely scientific point of view,the CRE offers arich after drying on windows. Surprisingly,the omnipresenteffect physicochemical platform for the investigation of numerous, responsible for the formation of the above-mentioned pat- complex and usually entangled phenomena. Microscopic ef- terns, frequently referred to as the coffee-ring effect (CRE), was fects, such as particle–particle andparticle–interface interac- elucidated only less than two decades ago. Deegan et al.[1] ex- tions, macroscopic effects, such as flow patterns, and phenom- plained that, in volatile liquid droplets partially wetting asolid ena simultaneously involving different lengthscales, such as substrate, the evaporation rate has itsmaximum close to their wetting, are all integrated in achallenging physicalproblem.[7] pinned three-phase contact line. This inhomogeneous evapora- In addition, the robust and persistentcharacter of the CRE tion profile results in the development of aradial fluid flow makesitvery intriguing. The primary requirements for the CRE from the interior of the drop to the contact line, to replenish to take place—volatile liquid,finite contact angle, and pinned the liquid lost there. Solutes present in the liquid are transport- contact line—arecommonly met in the majority of practical ed by the flow and are deposited at the edge of the drop; systemsand applications,and this accounts for its ubiquitous their progressive accumulation leads to the formation of ring- nature.[1] Moreover,itoccurs in awide range of drop sizes (di- shaped patterns after drying is complete. ameters ranging from micrometers to centimeters)[8,9] and in aplethora of systemswith diverse chemical properties and var- [a] Dr.M.Anyfantakis,Prof. D. Baigl ious characteristic length scales. Examples include molecular Ecole Normale SupØrieure-PSL Research University (i.e. salt)[8] and macromolecular solutions,[10] colloidal suspen- Department of Chemistry sions (particle sizes from nanometers to micrometers)[11] and 24 rue Lhomond, 75005,Paris (France) E-mail:[email protected] biomaterial-based complex fluids (e.g. blood, protein solutions, [11–13] [email protected] and biofilms ). [b] Dr.M.Anyfantakis,Prof. D. Baigl Sorbonne UniversitØs UPMC Univ Paris 06, PASTEUR 1.2. Strategies for Controlling the CRE 75005, Paris (France) Asignificant part of research studies devoted to the CRE was [c] Dr.M.Anyfantakis,Prof. D. Baigl CNRS, UMR 8640 PASTEUR focused on uncovering the physicochemical parameters affect- 75005, Paris (France) ing evaporative particle depositionand particularly on devising ChemPhysChem 2015, 16,2726 –2734 2726 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Concepts strategies for suppressing the effect and achieving homoge- cle affinity for the LG interface is optically tuned, particle depo- nous solute deposition. Given that evaporation-driven flow sition can be dynamically controlled by using light (Sec- toward the pinned contact line is always present in avolatile tion 3.1). Moreover,wedescribe recently developed biosensing sessile drop, the vast majority of these approaches was based strategies based on dry patterns resulting from particleaggre- on affecting the hydrodynamics of drying drops. Different ad- gation induced by the attraction between probe particles and ditives, including surfactants,[10, 14] polymers,[15] cosolvents[2] and the biomarker molecule to be detected, which open the way nanoparticles,[16] were introduced to manipulate the flow pat- to low-cost and simple-to-operatepoint-of-care diagnostics terns in evaporating drops. Externalmethods that requireno (Section3.2). Conclusions and perspectivesare given in additives, such as electrowetting[17] and exposure of the drying Section4. drops to vapours of miscible, low-surface-tension liquids,[18] have also been successfully utilised. Other approaches relied on the addition or removal of deposited matter by mechanical 2. Interactions of Particles with/at Interfaces means.[19] Affecting the CRE Anumber of groups have investigated the effect of the various 1.3. Scope and Organisationofthis Paper types of interactions in adrying drop of colloidal suspension on the resulting particle deposition. In the following, we cate- The interconnection between flow patterns and final deposit gorise some of the most relevant publications based on the morphology has been recently described in detail in an in- type of interaction:particle–LS interface, particle–LG interface structive review by Larson.[7] The scope of this Concept paper and bulk particle–particle interactions. However,this classifica- is primarily to underline the generally overlooked but highly tion is not mutuallyexclusive, since it is common that in important influence of interactions on deposition patterns asingle publication, more than one of the above-mentioned emerging from evaporating drops. In particular,wefocus on types of interaction are investigated. the interactions of the suspended particles with/atthe liquid– gas (LG) and liquid–solid (LS) interfaces, as well as bulk parti- cle–particle interactions. In drying drops of most complexliq- 2.1. Interactions with the LS Interface (Substrate) uids, the morphologyofthe final deposits is determined by both the hydrodynamics (i.e.flow patterns) and the interac- Bhardwajetal.[20] studied the role of the Derjaguin–Landau– tions present in the system (Figure 1). This is expectable con- Verwey–Overbeek (DLVO) interactions on the pattern morphol- ogy from evaporating nanoliter droplets of asuspension of ti- tania nanoparticles deposited on glass. Variation of the suspen- sion pH led to modification of the DLVO particle–substrate and particle–particle interactions, which in turn alterated the shapes of the deposits. Evaporating drops at low pH values yielded arelativelyhomogeneous pattern consisting of athin uniform nanoparticle layer with athicker ring at the edge. Cal- culations showed that the particle–substrate DLVO force at low pH values was attractive. Particles in close proximity to the Figure 1. The two key elementsdictatingdeposit morphology from evapo- substrate were thusattracted to it and formed auniform de- rating sessile dropsofcolloidal suspensions:flow patterns and interactions. The latter, which are at the focus of this paper,include interactions of the posit (Figure 2a). Deposits obtained from drying drops at inter- suspended particles with the LS and LG interfaces, as well as bulk particle– mediate pH consisted of aggregates covering the whole initial- particleinteractions. ly wetted area. Near the point of particleneutral charge, the particle–substrate DLVO force was weaker than the interparti- sidering that most of interactions involving colloidsand inter- cle van der Waals attraction, and hence particle aggregation faces (e.g. Coulomb and van der Waalsinteractions, capillary occurred. At the highest pH values, drop drying led to forces, hydrophobic interactions) can typicallyexceed the ther- amarked ring pattern with almost no particles in the area en- mal energyand therefore can strongly affect particle organisa- closed by the ring. In this pH region, strong repulsive particle– tion and behaviouratinterfaces. substrate forces prevented the nanoparticles from contacting The paper is structured as follows.Selected recent publica-
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