Dynamics of wrinkling in ultrathin elastic sheets

The wrinkling of thin elastic objects provides a means of generating regular patterning at small scales in applications ranging from photovoltaics to microfluidic devices. Static wrinkle patterns are known to be governed by an energetic balance between the object's bending stiffness and an effective substrate stiffness, which may originate from a true substrate stiffness or from tension and curvature along the wrinkles. Here we investigate dynamic wrinkling, induced by the impact of a solid sphere onto an ultra-thin polymer sheet floating on water. The vertical deflection of the sheet's centre induced by impact draws material radially inwards, resulting in an azimuthal compression that is relieved by the wrinkling of the entire sheet. We show that this wrinkling is truly dynamic, exhibiting features that are qualitatively different to those seen in quasi-static wrinkling experiments. Moreover, we show that the wrinkles coarsen dynamically because of the inhibiting effect of the fluid inertia. This dynamic coarsening can be understood heuristically as the result of a dynamic stiffness, which dominates the static stiffnesses reported thus far, and allows new controls of wrinkle wavelength.Comment: 8 pages, 4 figures. Please see published version for supplementary movies and SI Appendi

On-demand pitch tuning of printed chiral nematic liquid crystal droplets

Identifying facile means with which to tune the pitch of, and therefore the reflected colour from, chiral nematic liquid crystals (CLC) is of interest for many different photonics applications including optical filters, coloured displays, and mirrorless lasers. Precise control of the pitch of the helix, however, can be challenging. Here, we demonstrate the ability to tune the pitch, and consequently the reflection band, by depositing picolitre volumes of nematic LC into printed CLC droplets with a short pitch. Results are presented that demonstrate mixing of the nematic LC and CLC droplets such that the pitch elongates causing the reflection band located at blue wavelengths (430 nm) to redshift to longer wavelengths. The magnitude of the redshift can be controlled by varying the number of nematic LC droplets deposited into each CLC droplet. We consider the process of diffusion of these two separate mixtures using inkjet printing and showcase how this process of tuning the pitch can be employed to create coloured images in the form of an alphanumeric logo

Electrophoretic molecular communication with piecewise constant electric field

This paper studies a novel electrophoretic molecular communication (EMC) framework utilizing a piecewise constant electric field. EMC is a particular type of molecular communication that exploits electric fields to induce the movement of charged particles to enhance communication performance. Our previous work proposed an EMC framework utilizing a time-varying electric field that exponentially changes; however, the field with such a complicated shape might be challenging to be implemented in practice. Thus, this paper proposes a new EMC approach exploiting a piecewise constant electric field that can be readily implemented via, e.g., an on/off switch method. We formulate two optimization problems to design the electric field based on different objectives: minimizing a mean squared error and minimizing a bit interval. The solutions of each, such as optimal on-off timings and corresponding strengths of the constant electric fields, are obtained through the Lagrange multiplier approach and the geometric programming, respectively. The Monte Carlo simulation results verify that the proposed piecewise constant electric field significantly reduces the bit error rate relative to the constant field benchmark while performing less well, but not significantly, than the exponential field benchmark

Self-stimulated capillary jet

Inspired by a Savart’s pioneering work, we study the self-stimulated dynamics of a capillary jet. The feedback loop is realised by extracting surface perturbations from a section of the jet itself via a laser-photodiode pair, whose amplified signal drives an electromechanical actuator which, in turn, produces pressure perturbations at the exit chamber. Under specific conditions, this loop establishes phase-locked stimulation regimes that overcome the otherwise random natural breakup. For each laser position along the jet, the gain of the amplifier acts as a selector across a discrete set of observable frequencies. The main observed features are explained by a linear theory which combines the transfer function of each stage in the loop. Our findings are relevant to continuous inkjet technologies for the production of equally-sized droplets