We report a complete suppression of the coffee-ring effect resulting in the formation of loosely packed two-dimensional particle monolayers by controlled evaporation of sessile drops containing soft microgel particles. These particulate deposits show gradual order-to-disorder transitions that are not abrupt like deposits of hard colloids. The areal coverage of the monolayer deposits can also be precisely controlled by tuning the particle concentration. The preferential adsorption of soft-microgel particles to the water-vapor interface, and their radial flow along the interface towards the drop edge facilitates the creation of the novel monolayer deposits. © 2017 The Royal Society of Chemistry.

Madivala Gurapa Basavaraja

Department Of Chemical Engineering

Dillip Kumar Satapathy

Department Of Physics

Muraleedharapai Mayarani

Colloids

Deposits

Drops

Gels

Order disorder transitions

Radial flow

AREAL COVERAGE

CONTROLLED EVAPORATION

MICROGEL PARTICLES

ORDER-TO-DISORDER TRANSITIONS

Particle concentrations

PARTICLE MONOLAYERS

PREFERENTIAL ADSORPTION

WATER-VAPOR INTERFACE

Monolayers

Adsorption

article

COFFEE

colloid

STAIN

WATER VAPOR

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Nanoscale

The variation in the concentration of surfactant molecules along the air-water interface of a drying sessile drop containing colloidal particle-surfactant mixtures is known to inhibit the formation of coffee stains. This also leads to the formation of particulate deposits with a region almost deprived of the particles, often called the depletion zone. The molecular size of the surface-active species used in such experiments poses limitations on the direct visualization of the build-up of surfactant molecules at the interface and how it correlates with the nucleation and growth kinetics of the depletion zone. We report a quantitative analysis of the origin and evolution of the depletion zone that forms in a drying sessile drop. By evaporating an aqueous sessile droplet containing a mixture of surface-active poly(N-isopropylacrylamide) (pNIPAM) microgels and polystyrene (PS) colloids, we establish that the concentration fluctuations of the PS particles along the air-water interface trigger a surface tension driven Marangoni flow along the interface. As a result of this, a depletion zone forms within the particulate deposit, which is analogous to the depletion zones observed for dried drops of particle- surfactant mixtures. The critical time at which the depletion zone forms correlates to the time at which surface tension-driven stress along the interface is maximum. Moreover, we show that the increase in the initial concentration of PS particles results in a decrease in the critical time. By using in situ video microscopy, we established that the rate at which the depletion zone grows is non-linear in time. The growth rate of the depletion zones for drops containing different concentrations of PS particles collapses into a master curve upon scaling with concentration and radius of the drop. © 2019 The Royal Society of Chemistry.

Soft Matter

On the origin and evolution of the depletion zone in coffee stains

We show that loosely packed homogeneous monolayers of soft colloidal particles can be fabricated via simple evaporative drying of aqueous dispersion containing colloidal particles. Owing to the inherent amphiphilicity of the soft microgels, they are preferentially adsorbed onto the water-vapor interface of the drying droplet, resulting in a uniform deposition of the particles throughout the fluid-substrate contact area. Distinct order-to-disorder transition is also identified within the dried deposits on changing spatial locations from edge to center of the circular stain. Such monolayer deposits exhibit vibrant colors on irradiating with white light due to interference. © 2019 Author(s).

AIP Conference Proceedings

Order-to-disorder transition in colored microgel monolayers

Patterns generated by controlled evaporation of droplets containing colloids are dictated by internally generated flows. This advective particle transport is crucial to the efficacy of printing and coating processes and is also an elegant route to the self-assembly of particles. We propose a novel particle transport route, which involves adsorption of particles to the interface and subsequent curvature-driven migration of the particles along the interface. This interface-mediated transport can be exploited to control the distribution of particles in the dried patterns, which we experimentally elucidate by achieving gravity-induced drop shape changes. Our experiments demonstrate that the interplay between the bulk and the interfacial transport leads to strikingly different patterns: while dried aqueous sessile drops of colloidal dispersions produce well known "coffee-rings", dried pendant drops lead to "coffee-eyes". We support our experimental findings using scaling arguments. In previous studies, the effect of gravity-induced change in drop shape on the patterns formed in drying drops has been neglected. However, we show that the structure of the patterns formed by the colloidal particles after solvent evaporation is markedly different when the drops are deformed by gravity. © 2018 American Chemical Society.

Langmuir

Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport

Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along the three-phase contact line-a phenomena popularly known as the coffee-ring or the coffee stain effect. The formation of such rings as well as their suppression is shown to have applications in particle separation and disease diagnostics. We present an experimental study of the evaporation of sessile drops containing silica rods to elucidate the structural arrangement of particles in the ring, an effect of the addition of surfactant and salt. To this end, the evaporation of aqueous sessile drops containing model rod-like silica particles of aspect ratio ranging from ∼4 to 15 on a glass slide is studied. We first show that when the conditions such as (1) solvent evaporation, (2) nonzero contact angle, (3) contact line pinning, (4) no surface tension gradient driven flow, and (5) repulsive particle-particle/particle-substrate interactions, that are necessary for the formation of the coffee-ring are met, the suspension drops containing silica rods upon evaporation leave a ring-like deposit. A closer examination of the ring deposits reveals that several layers of silica rods close to the edge of the drop are ordered such that the major axis of the rods are oriented parallel to the contact line. After the first few layers of ordered arrangement of particles, a random arrangement of particles in the drop interior is observed indicating an order-disorder transition in the ring. We monitor the evolution of the ring width and particle velocity during evaporation to elucidate the mechanism of the order-disorder transition. Moreover, when the evaporation rate is lowered, the ordering of silica rods is observed to extend over large areas. We demonstrate that the nature of the deposit can be tuned by the addition of a small quantity of surfactant or salt. © 2014 American Chemical Society.

Journal of Physical Chemistry B

Evaporation of sessile drops containing colloidal rods: Coffee-ring and order-disorder transition

A control over the nature of deposit pattern obtained after the evaporation of solvent from a sessile drop containing dispersed materials has been demonstrated to have applications in materials engineering, separation technology, printing technology, manufacture of printed circuit boards, biology, and agriculture. In this article, we report an experimental investigation of the effect of particle shape and DLVO (Derjaguin-Landau-Verwey-Overbeek) interactions on evaporation-driven pattern formation in sessile drops. The use of a model system containing monodisperse particles where particle aspect ratio and surface charge can be adjusted reveals that a control over the nature of deposit pattern can be achieved by tuning the particle-particle and particle-substrate interactions. A clear coffee-ring formation is observed when the strength of particle-particle repulsion is higher than the particle-substrate attraction. However, complete suppression of ringlike deposits leading to a uniform film is achieved when particle-substrate and particle-particle interactions are attractive. Results illustrate that for the system of submicron ellipsoids that are hydrophilic, the nature of deposit patterns obtained after evaporation depends on the nature of interactions and not on particle shape. © 2014 American Chemical Society.

Control over coffee-ring formation in evaporating liquid drops containing ellipsoids

The recent surge to explore and exploit the effect of particle shape has led to several interesting and unique observations in various diverse fields, ranging from biology to material science. To this end, the development of novel synthesis methods has contributed immensely, and has directed this exciting research on the role of particle shape in colloidal systems and other diverse fields. Furthermore, such investigations have resulted in the development of advanced materials, which are novel and multi-functional. In this article, we review recent advances in the study of suspensions containing shape anisotropic particles. In particular, we investigate those pertaining to rod-like or ellipsoidal particles and highlight recent results in three areas-evaporation driven assembly, packing, interfacial behavior and the use of these particles in emulsion stabilization. © 2013 The Royal Society of Chemistry.

Shape anisotropic colloids: Synthesis, packing behavior, evaporation driven assembly, and their application in emulsion stabilization

Interfacial Rheology of Sterically Stabilized Colloids at Liquid Interfaces and Its Effect on the Stability of Pickering Emulsions

Chemical Society Reviews

Current status and future developments in preparation and application of colloidal crystals

Loosely packed monolayer coffee stains in dried drops of soft colloids

Journal	Data powered by TypesetNanoscale
Publisher	Data powered by TypesetRoyal Society of Chemistry
ISSN	20403364
Open Access	No