Search

When a thin sheet is confined to a small volume, it crumples into a complex and stiff state. Remarkably, adding cuts to the sheet does not significantly change the stiffness even though the cuts alter the structure within the crumpled matter.

Capillary origami takes advantage of the surface forces of a liquid drop to assemble thin film structures. After a structure is assembled, the drop then evaporates away. The transient nature of the liquid drop means that the creation of dry and stable structures is impossible. Work presented in this paper shows that adhesion is, in fact, a key tool that enables the creation of stable, complex, capillary assembled origami structures, rather than a problem to be avoided. Here, polydimethylsiloxane thin films were used in several simple experiments designed to identify the balance between substrate–film adhesion and film–film adhesion in the context of capillary assembly. We then demonstrate how directional adhesion can be used to direct film peeling in order to create non-trivial patterned folds after a fluid drop is deposited. A minimal complex structure, a “double-fold” was created to demonstrate how adhesion uniquely facilitates multiple-step capillary assembly. Finally, a familiar “origami airplane” was created with these methods, demonstrating that adhesion aided capillary origami can be used to assemble complex, functional structures.

In this work, we revisit experimentally and theoretically the mechanics of a tape loop. Using primarily elastic materials (polydimethylsiloxane, PDMS, or polycarbonate, PC) and confocal microscopy, we monitor the shape as well as the applied forces during an entire cycle of compression and retraction of a half-loop compressed between parallel glass plates. We observe distinct differences in film shape during the cycle; points of equal applied force or equal plate separation differ in shape upon compression or retraction. To model the adhesion cycle in its entirety, we adapt the 'Sticky Elastica' of [T. J. W. Wagner et al., Soft Matter, 2013, 9, 1025-1030] to the tape loop geometry, which allows a complete analytical description of both the force balance and the film shape. We show that under compression the system is generally not sensitive to interfacial interactions, whereas in the limit of large separation of the confining parallel plates during retraction the system is well described by the peel model. Ultimately, we apply this understanding to the measurement of the energy release rate of a wide range of different cross-linker ratio PDMS elastomer half-loops in contact with glass. Finally, we show how the model illuminates an incredibly simple adhesion measurement technique, which only requires a ruler to perform.

In recent years, there has been a considerable interest in the mechanics of soft objects meeting fluid interfaces (elasto-capillary interactions). In this work we experimentally examine the case of a fluid resting on a thin film of rigid material which, in turn, is resting on a fluid substrate. To simplify complexity, we adapt the experiment to a one-dimensional contact geometry and examine the behaviour of polystyrene and polycarbonate films directly with confocal microscopy. We find that the fluid meets the film in a manner consistent with the Young-Dupré equation when the film is thick, but transitions to what appears similar to a Neumann-like balance when the thickness is decreased. However, on closer investigation we find that the true contact angle is always given by the Young construction. The apparent paradox is a result of macroscopically measured angles not being directly related to true microscopic contact angles when curvature is present. We model the effect with an Euler-Bernoulli beam on a Winkler foundation as well as with an equivalent energy-based capillary model. Notably, the models highlight several important lengthscales and the complex interplay of tension, gravity, and bending in the problem.

Polymer covered emulsion droplets have a considerable number of applications ranging from active cosmetics to advance drug delivery systems. In many of these systems the emulsion droplets do not exist in isolation but interact with other drops, surfaces and particles. In a step towards understanding how these complex mechanical interactions take place, we examine the interaction between a block copolymer covered emulsion droplet (polystyrene-b-poly(ethylene oxide) (PS-PEO) covered toluene) and a flat mica interface. As buoyancy pushes the droplet upwards, it buckles in as it nears the mica and traps a droplet of the surrounding fluid. The trapped outer fluid (water/glycerine in our experiment) drains out through an annular region of PEO brush. This study focuses on the late stage drainage, unique to large molecule surfactants, and examines the effects of the polymer and droplet size on the drainage rate. We introduce a scaling model of the drainage which highlights the importance of three lengthscales in the problem - the brush height, the slip length along the emulsion drop interface and the width of the annular contact region.

Wrinkles and strain localized features are observed in many natural systems and are useful surface patterns for a wide range of applications, including optical gratings and microfluidic devices. However, the transition from sinusoidal wrinkles to more complex strain localized features, such as delaminations or folds, is not well understood. In this paper, we investigate the onset of wrinkling and strain localizations in a model system of a glassy polymer film attached to a surface of an elastomeric substrate. We show that careful measurement of feature amplitude as a function of applied strain allows not only the determination of wrinkle, fold, or delamination onset but also allows clear distinction between each type of feature. We observe that amplitude increases discontinuously as delamination occurs; whereas, the amplitude for a fold deviates gradually compared to the amplitude for a nearby wrinkle as a function of applied strain. The folds observed in these experiments have an outward morphology from the surface, in contrast to folds that form into the plane for a film floating on a liquid substrate. A deformation mode map is presented, where the measured critical strain for localization is compared for films with thickness ranging from 5 nm to 180 nm.

Free-standing liquid films are generally unstable, failing whenever a hole or pore is created. The same is true of a polymer melt, although the details of the instability can be more complex and dependent on molecular architecture. Here, we compare the nucleation of holes in homopolymer films and films made from diblock co-polymers that can order into a cylindrical or lamellar phase. The different degrees of internal order (no long-range order, lamellar order, cylindrical order) has significant effects on the rate of hole formation. We find that lamellar order decreases the rate of film rupture by at least two orders of magnitude when compared to isotropic films. The hole formation is well described by a classical nucleation process. Notably, we find that the barrier to hole formation is identical for all samples studied here, favouring a simple capillary model. The vast differences in stability between films of differing internal structure is entirely quantified by the “attempt frequency” of barrier penetration and not the free energy barrier itself.