Manos Anyfantakis

Dynamic photocontrol of the coffee-ring effect with optically tunable particle stickiness.

When a colloidal drop dries on a surface, most of the particles accumulate at the drop periphery, yielding a characteristic ring-shaped pattern. This so-called coffee-ring effect (CRE) is observed in any pinned evaporating drop containing non-volatile solutes. Here, the CRE is dynamically controlled for the first time by using light, and an unprecedented reconfigurability of the deposit profile is demonstrated. This is achieved through a new mechanism where particle stickiness is optically tuned on demand, thus offering reliable modulation of the deposition pattern. The system consists of anionic nanoparticles and photosensitive cationic surfactants dispersed in water. It is shown that light-dependent modulation of surfactant-particle interactions dictates particle attraction and trapping at the liquid-gas interface, which allows us to direct particle deposition into a wide range of patterns from rings to homogeneous disks. Patterning from single drops is photoreversible upon changing the wavelength whereas spatial control in multiple drop arrays is achieved using a photomask.

Light-Directed Particle Patterning by Evaporative Optical Marangoni Assembly

Controlled particle deposition on surfaces is crucial for both exploiting collective properties of particles and their integration into devices. Most available methods depend on intrinsic properties of either the substrate or the particles to be deposited making them difficult to apply to complex, naturally occurring or industrial formulations. Here we describe a new strategy to pattern particles from an evaporating drop, regardless of inherent particle characteristics and suspension composition. We use light to generate Marangoni surface stresses resulting in flow patterns that accumulate particles at predefined positions. Using projected images, we generate a broad variety of complex patterns, including multiple spots, lines and letters. Strikingly, this method, which we call evaporative optical Marangoni assembly (eOMA), allows us to pattern particles regardless of their size or surface properties, in model suspensions as well as in complex, real-world formulations such as commercial coffee.

Manipulating the Coffee-Ring Effect: Interactions at Work

The evaporation of a drop of colloidal suspension pinned on a substrate usually results in a ring of particles accumulated at the periphery of the initial drop. Intense research has been devoted to understanding, suppressing and ultimately controlling this so-called coffee-ring effect (CRE). Although the crucial role of flow patterns in the CRE has been thoroughly investigated, the effect of interactions on this phenomenon has been largely neglected. This Concept paper reviews recent works in this field and shows that the interactions of colloids with (and at) liquid-solid and liquid-gas interfaces as well as bulk particle-particle interactions drastically affect the morphology of the deposit. General rules are established to control the CRE by tuning these interactions, and guidelines for the rational physicochemical formulation of colloidal suspensions capable of depositing particles in desirable patterns are provided. This opens perspectives for the reliable control of the CRE in real-world formulations and creates new paradigms for flexible particle patterning at all kinds of interfaces as well for the exploitation of the CRE as a robust and inexpensive diagnostic tool.

Protein Adsorption and Reorganization on Nanoparticles Probed by the Coffee-Ring Effect: Application to Single Point Mutation Detection

The coffee-ring effect denotes the accumulation of particles at the edge of an evaporating sessile drop pinned on a substrate. Because it can be detected by simple visual inspection, this ubiquitous phenomenon can be envisioned as a robust and cost-effective diagnostic tool. Toward this direction, here we systematically analyze the deposit morphology of drying drops containing polystyrene particles of different surface properties with various proteins (bovine serum albumin (BSA) and different forms of hemoglobin). We show that deposit patterns reveal information on both the adsorption of proteins onto particles and their reorganization following adsorption. By combining pattern analysis with adsorption isotherm and zeta potential measurements, we show that the suppression of the coffee-ring effect and the formation of a disk-shaped pattern is primarily associated with particle neutralization by protein adsorption. However, our findings also suggest that protein reorganization following adsorption can dramatically invert this tendency. Exposure of hydrophobic (respectively charged) residues can lead to disk (respectively ring) deposit morphologies independently of the global particle charge. Surface tension measurements and microscopic observations of the evaporating drops show that the determinant factor of the deposit morphology is the accumulation of particles at the liquid/gas interface during evaporation. This general behavior opens the possibility to probe protein adsorption and reorganization on particles by the analysis of the deposit patterns, the formation of a disk being the robust signature of particles rendered hydrophobic by protein adsorption. We show that this method is sensitive enough to detect a single point mutation in a protein, as demonstrated here by the distinct patterns formed by human native hemoglobin h-HbA and its mutant form h-HbS, which is responsible for sickle cell anemia.

Evaporation of Drops Containing Silica Nanoparticles of Varying Hydrophobicities: Exploiting Particle–Particle Interactions for Additive-Free Tunable Deposit Morphology

We describe the systematic and quantitative investigation of a large number of patterns that emerge after the evaporation of aqueous drops containing fumed silica nanoparticles (NPs) of varying wettabilities for an extended particle concentration range. We show that for a chosen system, the dry pattern morphology is mainly determined by particle-particle interactions (Coulomb repulsion and hydrophobic attraction) in the bulk. These depend on both particle hydrophobicity and particle concentration within the drop. For high and intermediate particle concentrations, interparticle hydrophobic attraction is the dominant factor defining the deposit morphology. With increasing particle hydrophobicity, patterns ranging from rings to domes are observed, arising from the time needed for the drop to gel compared with the total evaporation time. On the contrary, drops of dilute suspensions maintain a finite viscosity during most of the drop lifetime, resulting in dry patterns that are predominantly rings for all particle hydrophobicities. In all investigated systems, the NP concentration corresponded to a large excess of NPs in the bulk compared with the maximal amount that could be adsorbed at available interfaces, making particle-interface interactions such as adsorption of hydrophobic NPs at the air-water interface a negligible contribution over bulk particle-particle interactions. This work emphasizes the advantage of particle surface chemistry in tuning both particle-particle interactions and particle deposition onto solid substrates in a robust manner, without the need for any additive such as a surfactant.

Magnetic Actuation of Drops and Liquid Marbles Using a Deformable Paramagnetic Liquid Substrate

Abstract The magnetic actuation of deposited drops has mainly relied on volume forces exerted on the liquid to be transported, which is poorly efficient with conventional diamagnetic liquids such as water and oil, unless magnetosensitive particles are added. Herein, we describe a new and additive‐free way to magnetically control the motion of discrete liquid entities. Our strategy consists of using a paramagnetic liquid as a deformable substrate to direct, using a magnet, the motion of various floating liquid entities, ranging from naked drops to liquid marbles. A broad variety of liquids, including diamagnetic (water, oil) and nonmagnetic ones, can be efficiently transported using the moderate magnetic field (ca. 50 mT) produced by a small permanent magnet. Complex trajectories can be achieved in a reliable manner and multiplexing potential is demonstrated through on‐demand drop fusion. Our paramagnetofluidic method advantageously works without any complex equipment or electric power, in phase with the necessary development of robust and low‐cost analytical and diagnostic fluidic devices.

Evaporative Optical Marangoni Assembly: Tailoring the Three-Dimensional Morphology of Individual Deposits of Nanoparticles from Sessile Drops

We have recently devised the evaporative optical Marangoni assembly (eOMA), a novel and versatile interfacial flow-based method for directing the deposition of colloidal nanoparticles (NPs) on solid substrates from evaporating sessile drops along desired patterns using shaped UV light. Here, we focus on a fixed UV spot irradiation resulting in a cylinder-like deposit of assembled particles and show how the geometrical features of the single deposit can be tailored in three dimensions by simply adjusting the optical conditions or the sample composition, in a quantitative and reproducible manner. Sessile drops containing cationic NPs and a photosensitive surfactant at various concentrations are allowed to evaporate under a single UV beam with a diameter much smaller than that of the drop. After complete evaporation, the geometrical characteristics of the NP deposits are precisely assessed using optical profilometry. We show that both the volume and the radial size of the light-directed NP deposit can be adjusted by varying the diameter or the intensity of the UV beam or alternatively by changing the concentration of the photosensitive surfactant. Notably, in all these cases, the deposits display an almost constant median height corresponding to a few layers of particles. Moreover, both the radial and the axial extent of the patterns are tuned by changing the NP concentration. These results are explained by the correlation among the strength of Marangoni flow, the particle trapping efficiency, and the volume of the deposit, and by the role of evaporation-driven flow in strongly controlling the deposit height. Finally, we extend the versatility of eOMA by demonstrating that NPs down to 30 nm in diameter can be effectively patterned on glass or polymeric substrates.