Dynamic manipulation of droplets using mechanically tunable microtextured chemical gradients

Abstract

Materials and strategies applicable to the dynamic transport of microdroplets are relevant to surface fluidics, self-cleaning materials, thermal management systems, and analytical devices. Techniques based on electrowetting, topographic micropatterns, and thermal/chemical gradients have advanced considerably, but dynamic microdroplet transport remains a challenge. This manuscript reports the fabrication of mechano-tunable, microtextured chemical gradients on elastomer films and their use in controlled microdroplet transport. Specifically, discreet mechanical deformations of these films enabled dynamic tuning of the microtextures and thus transport along surface-chemical gradients. The interplay between the driving force of the chemical gradient and the microtopography was characterized, facilitating accurate prediction of the conditions (droplet radius and roughness) which supported transport. In this work, the use of microtextured surface chemical gradients in mechano-adaptive materials with microdroplet manipulation functionality was highlighted.

Programmable Droplet Transport Using Mechanically Adaptive Chemical Gradients with Anisotropic Microtopography

“…We previously demonstrated that the droplet transport properties (e.g., velocity) of microtextured chemical gradients (mechanically analogous to the gradient surfaces studied here) were unaffected by repeated cycles of stress (up to 100 cycles between ε c = 0 and ε c = 0.2). [ 10 ] We expected comparable mechanical durability in this work and indeed did not observe performance degradation throughout our tests, which typically included several stress cycles. Further, we did not observe residual liquid (in the form of microdroplets or films) using our optical tensiometer (which has an imaging resolution of ≈10 μm).…”

“…We previously reported data from confocal microscopy which gives wrinkle characteristics (e.g., amplitude) and surface roughness that agree with theoretical predications made using physical models of mechano‐induced wrinkles. [ 10 ] In these experiments the gradient orientation was always along the propagation axis of the wrinkled microtexture. The fundamental equation (Equation 1) that describes the spontaneous transport of droplets on surface energy gradients predict a critical radius ( R c ) associated with the condition where droplet transport ceases (i.e., velocity, v = 0 and F = 0).…”

“…As such, sequential transport of droplets on these surfaces was possible regardless of whether or not the surface was actively dried in between transport events. Finally, we acknowledge that droplet velocity is dynamic, [ 10 ] with variability associated with gradient intensity and the position of the droplet on the gradient. Therefore, in this study we adopted a standardized measurement protocol that controlled for droplet volume, placement, and measurement range in order to report average velocities which were convenient for direct comparison of different gradients/topographies.…”

“…In the Wenzel wetting state, a roughness factor ( r ) appears in the F h term. The net force ( F ) can thus be expressed aswhere C is a system‐specific fitting constant, [ 10 ] R is the droplet radius, γ is the surface tension of the liquid droplet, Φ is the gradient intensity (where Φ = ), and (cos θ r – cos θ a ) quantifies the contact angle hysteresis where θ r and θ a are the receding and advancing contact angles, respectively. Importantly, the magnitude of r was readily and reversibly tuned by applying mechanical tension providing a convenient method to modify the magnitude of F h and therefore the properties of droplet transport (Equation ()).…”

“…Importantly, the magnitude of r was readily and reversibly tuned by applying mechanical tension providing a convenient method to modify the magnitude of F h and therefore the properties of droplet transport (Equation ()). [ 10 ]…”