The origin of the idea of moving objects by acoustic vibration can be traced back to 1787, when Ernst Chladni reported the first detailed studies on the aggregation of sand onto nodal lines of a vibrating plate. Since then and to this date, the prevailing view has been that the particle motion out of nodal lines is random, implying uncontrollability. But how random really is the out-of-nodal-lines motion on a Chladni plate? Here we show that the motion is sufficiently regular to be statistically modelled, predicted and controlled. By playing carefully selected musical notes, we can control the position of multiple objects simultaneously and independently using a single acoustic actuator. Our method allows independent trajectory following, pattern transformation and sorting of multiple miniature objects in a wide range of materials, including electronic components, water droplets loaded on solid carriers, plant seeds, candy balls and metal parts.

Droplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10 µm and three orders of magnitude better force sensitivity than current tensiometers. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform.

In the case of low-surface-tension liquids, such as oils, wetting becomes a major concern if the liquid spreads easily at the interface and between the fibrils. Even though carefully controlled, thin layers of viscous oil (0.1-2.1 µm thick) applied on the fibril tips of artificial dry adhesives can enhance adhesion on both smooth and rough surfaces, [17-19] larger volumes of liquids (0.1-0.4 µL) at the solid-solid interface have been shown to drop adhesion to a fraction compared to dry conditions. [20] For overall adhesion performance in various wetting conditions, it would be advantageous to displace (push away) liquid from the contact interface and make a dry contact. Preferably all liquids, regardless of their surface tension, should remain in the Cassie state (i.e., the liquid droplet staying suspended on top of the fibrils), even during contact with the target surface. The transition barrier to the Wenzel state (i.e., the droplet fully wetting the substrate and fibrils) should also be sufficiently high to provide robust liquid repellency, since the adhesion would drop drastically in the Wenzel state. Combining high adhesion and low-surface-tension liquid repellency on the same fibrillar surface has not been possible yet as the two properties have fundamentally opposing requirements for solid fraction (the fraction of the solid surface in contact with the liquid or the solid surface to adhere)-it should be large for adhesion and small for liquid repellency. Furthermore, high liquid repellency has been traditionally achieved by a combination of surface chemistry and roughness modification, an approach which is typically incompatible with the goal of high adhesion. For example, sprayable coatings can be extremely effective at turning a surface superrepellent to all liquids, [21,22] but they rely on ultralow surface energy and hierarchical microand nanoscale roughness, both of which are detrimental to adhesion. Another prominent avenue for achieving repellency toward low-surface-tension liquids is based on arrays of microscale features with re-entrant geometry, [23] inspired by the skin of springtails. In recent years, this approach has been taken further by the introduction of double re-entrant structures, which can repel all liquids regardless of surface chemistry. [24,25] However, the fabrication techniques have mostly focused on rigid materials, which are not suitable for dry adhesives involving elastomeric compliant fibrils. Although rigid double reentrant structures for liquid repellency have been fabricated on flexible substrates, [25-27] and compliant double re-entrant structures have been demonstrated by shape-altering metal Bioinspired elastomeric fibrillar surfaces have significant potential as reversible dry adhesives, but their adhesion performance is sensitive to the presence of liquids at the contact interface. Like their models in nature, many artificial mimics can effectively repel water, but fail when low-surface-tension liquids are introduced at the contact interface. A bioinspired fibrillar a...

Controlled spreading of liquids deposited on a solid surface has seen an increasing research interest over the past decade. Since the groundbreaking work by Wenzel [ 1 ] and Cassie, [ 2 ] topographical and chemical surface structuring has been employed to create a multitude of new engineered surfaces for various applications. These surfaces are often inspired by nature (e.g., the lotus leaf [ 3 ] ) and can have wetting properties from superhydrophilic, [ 4 ] superhydrophobic, [ 5 , 6 ] to superoleophilic and superoleophobic, [7][8][9] and even omniphobic. [10][11][12] Surface structures can also induce anisotropic wetting [ 13 , 14 ] or directional wetting, [ 15 , 16 ] allowing a great degree of control over liquid spreading. On the other hand, inhibiting liquid spreading at one, well-defi ned boundary is the critical issue in a wide range of applications, from surface-directed liquid fl ow, [17][18][19] droplet-based microfl uidics, [ 20 ] capillary imbibition, [ 21 ] to passive fi lling of fl uidic channels by capillary action, [ 22 ] droplet shape control in biomedical applications, [ 23 ] microfl uidics for biological studies, [ 24 ] fl uidic optics, [ 25 ] and self-alignment of microchips. [26][27][28] In these applications, liquid overfl ow at the boundary would be catastrophic. Therefore, it is crucial to maximize the advancing contact angle at the boundary, regardless of whether the liquid is in the Cassie or the Wenzel state. In this paper, we report an easy-to-fabricate, purely topographical structure of undercut edge that can pin the triple contact line (TCL) of liquid on any single edge. By mere periodic repetition of such edges, we show that multiple droplets can be patterned in well-controlled shapes, and highly anisotropic wetting can also be achieved at large scale. The pinning property of the structure also does not depend on surface tension of the liquid. Our microfabrication method for the edges is simple, does not involve chemical surface modifi cation, and consequently can be applied to a wide range of materials, including silicon-based, glass and metals.The structure is designed based on a purely geometrical rule, known as the Gibbs inequality, that constrains the apparent contact angle of a spreading liquid meeting a sharp edge. The ability of sharp edges to impede liquid spreading has been long known, and studied both theoretically [ 29 ] and experimentally. [ 30 ] For the case where a liquid droplet meets a mathematically sharp edge, Gibbs [ 31 ] constructed a geometrical relation that describes the range of possible contact angles at the edge ( Figure 1 a):

Setae, fibrils located on a gecko's feet, have been an inspiration of synthetic dry microfibrillar adhesives in the last two decades for a wide range of applications due to unique properties: residue‐free, repeatable, tunable, controllable and silent adhesion; self‐cleaning; and breathability. However, designing dry fibrillar adhesives is limited by a template‐based‐design‐approach using a pre‐determined bioinspired T‐ or wedge‐shaped mushroom tip. Here, a machine learning‐based computational approach to optimize designs of adhesive fibrils is shown, exploring a much broader design space. A combination of Bayesian optimization and finite element methods creates novel optimal designs of adhesive fibrils, which are fabricated by two‐photon‐polymerization‐based 3D microprinting and double‐molding‐based replication out of polydimethylsiloxane. Such optimal elastomeric fibril designs outperform previously proposed designs by maximum 77% in the experiments of dry adhesion performance on smooth surfaces. Furthermore, finite‐element‐analyses reveal that the adhesion of the fibrils is sensitive to the 3D fibril stem shape, tensile deformation, and fibril microfabrication limits, which contrast with the previous assumptions that mostly neglect the deformation of the fibril tip and stem, and focus only on the fibril tip geometry. The proposed computational fibril design could help design future optimal fibrils with less help from human intuition.