Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

A combination is presented of the inclusive deep inelastic cross sections measured by the H1 and ZEUS Collaborations in neutral and charged current unpolarised e ± p scattering at HERA during the period 1994-2000. The data span six orders of magnitude in negative four-momentum-transfer squared, Q 2 , and in Bjorken x. The combination method used takes the correlations of systematic uncertainties into account, resulting in an improved accuracy. The combined data are the sole input in a NLO QCD analysis which determines a new set of parton distributions, HERAPDF1.0, with small experimental uncertainties. This set includes an estimate of the model and parametrisation uncertainties of the fit result.

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high-temperature rubbing with a soft-doping method based on spin-coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3-alkylthiophene)s and poly(2,5-bis(3dodecylthiophen-2-yl)thieno[3,2-b]thiophene) with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F 4 TCNQ) yields highly oriented conducting polymer films that display polarized UV-visible-near-infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV-vis-NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F 4 TCNQ − anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm −1 and S = 60 ± 2 µV K −1 ). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.

When a granular material such as sand is mixed with a certain amount of liquid, the surface tension of the latter bestows considerable stiffness to the material, which enables, for example, sand castles to be sculpted. The geometry of the liquid interface within the granular pile is of extraordinary complexity and strongly varies with the liquid content. Surprisingly, the mechanical properties of the pile are largely independent of the amount of liquid over a wide range. We resolve this puzzle with the help of X-ray microtomography, showing that the remarkable insensitivity of the mechanical properties to the liquid content is due to the particular organization of the liquid in the pile into open structures. For spherical grains, a simple geometric rule is established, which relates the macroscopic properties to the internal liquid morphologies. We present evidence that this concept is also valid for systems with non-spherical grains. Hence, our results provide new insight towards understanding the complex physics of a large variety of wet granular systems including land slides, as well as mixing and agglomeration problems.

Regioregular head‐to‐tail (HT)‐coupled poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) with a weight‐average molecular weight (Mw) in the 7.3–69.6 kDa range is crystallized by directional epitaxial solidification in 1,3,5‐trichlorobenzene (TCB) to yield highly oriented thin films. An oriented and periodic lamellar structure consisting of crystalline lamellae separated by amorphous interlamellar zones is evidenced by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both the overall crystallinity as well as the orientation of the crystalline lamellae decrease significantly with increasing Mw. The total lamellar periodicity is close to the length of “fully extended” chains for Mw = 7.3 kDa (polystyrene‐equivalent molecular weight, eq. PS) and it saturates to a value of ca. (25–28) ± 2 nm for Mw ≥ 18.8 kDa (eq. PS). This behavior is attributed to a transition from an oligomeric‐like system, for which P3HT chains are essentially in a fully extended all‐trans conformation and do not fold, to a semicrystalline system that involves a periodic alternation of crystalline lamellae separated by extended amorphous interlamellar zones, which harbor chain folds, chain ends, and tie molecules. For P3HT with Mw of ca. 7.3 kDa (eq. PS), epitaxial crystallization on TCB allows for the growth of both “edge‐on” and “flat‐on” oriented crystalline lamellae on the TCB substrate. The orientation of the lamellae is attributed to 1D epitaxy. Because of the large size of the “flat‐on” crystalline lamellae, a characteristic single‐crystal electron diffraction pattern corresponding to the [001] zone was obtained by selected area electron diffraction (SAED), indicating that P3HT crystallizes in a monoclinic unit cell with a = 16.0 Å, b = 7.8 Å, c = 7.8 Å, and γ = 93.5°.

The wetting of microstructured surfaces is studied both experimentally and theoretically. Even relatively simple surface topographies such as grooves with rectangular cross section exhibit a large variety of different wetting morphologies as observed by atomic force microscopy. This polymorphism arises from liquid wedge formation along the groove corners and from contact line pinning along the groove edges. A global morphology diagram is derived that depends only on two system parameters: (i) the aspect ratio of the groove geometry and (ii) The contact angle of the underlying substrate material. For microfluidics, the most interesting shape regimes involve extended liquid filaments, which can grow and shrink in length while their cross section stays essentially constant. Thus, any method by which one can vary the contact angle can be used to switch the length of the filament, as is demonstrated in the context of electrowetting.surface topography ͉ wetting phenomena ͉ microfluidics R apid and efficient handling of relatively small amounts of liquids is a crucial requirement in molecular biology or biomedicine, e.g., for decoding the human genome or for analysis of small blood samples. To do this, one would like to construct labs-on-a-chip on the micrometer scale (see, e.g., ref. 1). An obvious prerequisite for such a lab is appropriate compartments for the confinement of very small amounts of liquids. These microcompartments should have some basic properties: they should have a well defined geometry by which one can measure the precise amount of liquid contained in them; they should be able to confine variable amounts of liquid; and they should be accessible in such a way that one can add and extract liquid in a convenient manner.A variety of concepts has been developed for the construction of such microfluidic systems. In most cases, a solid matrix is used that surrounds micropipes, reservoirs, etc. Here, we explore an alternative system design, namely open microfluidic systems, which contain free liquid͞vapor (or liquid͞liquid) interfaces. One advantage of these open structures is that they are directly accessible and easy to clean.There are two general strategies to construct open microfluidic systems. The first one is to chemically pattern planar substrates and to prepare distinct surface domains that differ in their wettability (2-4). The second strategy, explored here, is to use nonplanar surface topographies that can be fabricated by available photolithographic methods. We find that even relatively simple topographies such as grooves with rectangular cross sections already exhibit a large variety of different liquid morphologies such as droplets, filaments, and wedges. A systematic comparison of experimental observations and theoretical calculations reveals, however, that this polymorphism is primarily determined by only two parameters: (i) the aspect ratio X of the groove geometry, i.e., the ratio of the groove depth to the groove width; and (ii) the contact angle of the underlying substrate material. For...

Highly oriented and crystalline regioregular poly(3‐hexylthiophene) (P3HT) films have been obtained by directional solidification. Oriented P3HT crystallizes from a solution in 1,3,5‐trichlorobenzene, which plays the dual role of solvent and orienting substrate. Periodic alternation of crystalline lamellae separated by amorphous zones, the typical fingerprint of the semicrystalline structure of P3HT, are observed (see figure).

The wetting and dewetting of chemically structured substrates with striped surface domains is studied theoretically. The lyophilic stripes and the lyophobic substrate are characterized by different contact angles ␥ and ␦ , respectively. We determine the complete bifurcation diagram for the wetting morphologies ͑i͒ on a single lyophilic stripe and ͑ii͒ on two neighboring stripes separated by a lyophobic one. We find that long channels can only be formed on the lyophilic stripes if the contact angle ␥ is smaller than a certain threshold value ch (V) which depends only weakly on the volume V and attains the finite value ch (ϱ) in the limit of large V. This asymptotic value is equal to ch (ϱ)ϭarccos(/4)Ӎ38°for all lyophobic substrates with ␦ у/2. For a given value of ␥ Ͻ ch (ϱ), the extended channels spread onto the lyophilic stripes with essentially constant cross section.