Surface coatings and patterning technologies are essential for various physicochemical applications. In this Letter, we describe key parameters to achieve uniform particle coatings from binary solutions: First, multiple sequential Marangoni flows, set by solute and surfactant simultaneously, prevent non-uniform particle distributions and continuously mix suspended materials during droplet evaporation. Second, we show the importance of particle-surface interactions that can be established by surface-adsorbed macromolecules. To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer (0.05 wt% each) is sufficient, which offers a new physicochemical avenue for control of coatings. PACS numbers:An evaporating liquid drop, either single or multicomponent, containing solutes or particulates leaves a deposit whose form is determined by various parameters, for instance internal flow fields [1][2][3], liquid compositions [4][5][6][7][8][9][10], and interactions between suspended particles and a solid substrate [11][12][13][14], which are crucial for coating processes. In particular, control of the deposit uniformity and thickness can be important in surface patterning [15][16][17], ink-jet [4,18,19] and 3D printing technologies [20]. These processes are complex because of physicochemical dynamics that arise from Marangoni effects [2, 5-10, 12, 21, 22] and particle deposition mechanisms [11,12,14,23]. In fact, although a binary mixture is used quite often to achieve uniform particle deposition from droplets smaller than 100 µm [4,18,19], to our best knowledge such coatings have not been achieved for larger droplets. Furthermore, while the wetting and dewetting behaviors of binary mixture drops have been investigated [24,25], the relation between the deposition pattern and the evaporatively driven flow field in a binary mixture droplet is incomplete (Table S1, Supporting Information (SI)) [26].In this Letter, to achieve a uniform coating, we identify key characteristics of a multicomponent solution, which consists of a binary mixture, surface-active surfactant, and surface-adsorbed polymer. We were motivated to pursue the ideas here from examining a whisky droplet after drying on an ordinary glass where it creates a relatively uniform particle deposit (see Fig. 1), which is in contrast to the well-known 'coffee-ring stain' [1]. Based on our understanding of the drying and coating mechanisms of binary liquid droplets, whisky droplets, and more complex solution droplets, we design a model liquid that yields nearly uniform deposits by taking the approach that whisky is an ethanol-water mixture containing diverse dissolved molecules, which contribute to the * Electronic address: hastone@princeton.edu complexity of the system, the flows, and the final particle deposits.We begin with a few remarks about whisky, since it serves as a model complex mixture, where nearly uniform particle deposits are observed after drying. Whisky is an alcoholic liquid (ethanol:water, 35:65 % b...

Transport of colloids in dead-end channels is involved in widespread applications including drug delivery and underground oil and gas recovery. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, but it is, unfortunately, an extremely inefficient transport mechanism for microscale particles. Here, we explore the possibility of diffusiophoresis as a means to control the colloid transport in dead-end channels by introducing a solute gradient. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. In addition, we show that size-dependent diffusiophoretic transport of particles can be achieved by considering a finite Debye layer thickness effect, which is commonly ignored. A combination of diffusiophoresis and Brownian motion leads to a strong size-dependent focusing effect such that the larger particles tend to concentrate more and reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific delivery systems where localized targeting of particles with minimal dispersion to the nontarget area is essential.T he ability of a particle to migrate along a local solute concentration gradient, which is referred to as diffusiophoresis, has been exploited to direct transport in a variety of systems, e.g., artificial swimmers (1, 2) and collective behaviors of active colloids (3, 4). One physical mechanism for diffusiophoresis originates from surface-solute interactions, where the solute gradient sets up an osmotic pressure gradient within a narrow interaction region. This gradient leads to fluid flow along the surface of a particle, in which case propulsion occurs in the opposite direction and is referred to as chemiphoresis (5, 6). In addition, differences in diffusivities between anions and cations lead to spontaneous electrophoresis of a particle, giving an additional propulsion mechanism. A particular feature of diffusiophoresis is that the diffusiophoretic mobility, or the phoretic velocity, of a particle is independent of its size, as long as the thickness of the interaction region, e.g., the Debye screening layer when the interaction is electrostatic, is much thinner than the size of the particle (6). This feature allows the utilization of diffusiophoresis for enhancing transport of microscale particles, leading to orders of magnitude higher transport rates compared with pure diffusion (7). However, this size independence could also be a source of frustration because it precludes useful effects such as sorting or controlling transport by particle size.We anticipate that size-independent particle mobility breaks down when the thickness of the surface-solute interaction region becomes comparable to the size of the particle. Already more than a century ago this feature has been well appreciated in the field of electrokinetics as the Hückel limit (8) w...

Magnetic nanoparticles have been employed to capture pathogens for many biological applications; however, optimal particle sizes have been determined empirically in specific capturing protocols. Here, a theoretical model that simulates capture of bacteria is described and used to calculate bacterial collision frequencies and magnetophoretic properties for a range of particle sizes. The model predicts that particles with a diameter of 460 nm should produce optimal separation of bacteria in buffer flowing at 1 L h(-1) . Validating the predictive power of the model, Staphylococcus aureus is separated from buffer and blood flowing through magnetic capture devices using six different sizes of magnetic particles. Experimental magnetic separation in buffer conditions confirms that particles with a diameter closest to the predicted optimal particle size provide the most effective capture. Modeling the capturing process in plasma and blood by introducing empirical constants (ce ), which integrate the interfering effects of biological components on the binding kinetics of magnetic beads to bacteria, smaller beads with 50 nm diameters are predicted that exhibit maximum magnetic separation of bacteria from blood and experimentally validated this trend. The predictive power of the model suggests its utility for the future design of magnetic separation for diagnostic and therapeutic applications.

In this paper, we describe a method for encapsulation of biomaterials in hydrogel beads using a microfluidic droplet-merging channel. We devised a double T-junction in a microfluidic channel for alternate injection of aqueous fluids inside a droplet unit carried within immiscible oil. With this device, hydrogel beads with diameter\100 lm are produced, and various solutions containing cells, proteins and reagents for gelation could merge with the gel droplets with high efficiency in the broad range of flow rates. Mixing of reagents and reactions inside the hydrogel beads are continuously observed in a microchannel through a microscope. By enabling serial injection of each liquid with the dispersed gel droplets after they are produced from the oil-focusing channel, the device simplifies the sample preparation process, and gelbead fabrication can be coupled with further assay continuously in a single channel. Instantaneous reactions of enzyme inside hydrogel and in-situ formation of cell-containing beads with high viability are demonstrated in this report.

We present a simple method with the aid of a microfluidic droplet-generation technique to fabricate magnetic Janus particles by utilizing a solvent evaporation-induced phase separation and preferential partitioning of magnetic nanoparticles in the polymer blends. Non-aqueous emulsion droplets of the polymer blends and nanoparticles solution are produced to become Janus particles after the evaporation of the solvent. The stabilizing polymer of the nanoparticles, which is compatible only with one of the polymer blends to be phase-separated, plays a key role in the anisotropic positioning of the nanoparticles in the Janus particles. Using this phase separation-based method and microfluidics, excellent control over the size, size distribution, and morphology of the particles is achieved. Especially, we could control the morphology of the Janus particles easily by varying the volume ratio of the polymers. However, with an analysis of the shapes of resulting Janus particles, we found that non-equilibrium aspects of the evaporation-induced phase separation play a major role in determining the particle morphology. We expect that the versatility of this method in the choice of polymer blends and functional nanoparticles will enable the fabrication of colloids with various functionality and desired morphology.3 Electronic supplementary information (ESI) available: Analysis of particle shapes based on thermal equilibrium assumption, including Fig. S1 and S2. See