Mechanowetting drives droplet and fluid transport on traveling surface waves generated by light-responsive liquid crystal polymers

Abstract

In nature, capillary forces are often driving microfluidic propulsion and droplet manipulation, and technologies have been developed to utilize these forces in applications such as lab-on-a-chip biosensors and microfluidic systems. At the same time, responsive materials have been developed that can be activated by a variety of external triggers, including light, electric fields, and temperature, to locally deform and create dynamic surface structures, such as traveling waves. Here, we combine these developments into a system that enables capillary-driven droplet transport and fluid propulsion generated by light-induced surface waves in azobenzene-embedded liquid crystal polymers. We demonstrate that the traveling waves are able to efficiently propel fluids by means of mechanowetting. We couple the wave profiles to the fluid simulations using a multiphase computational fluid dynamics approach. We study three different fluid propulsion systems, i.e., peristaltic flow, liquid slug transport, and free-standing droplet transport. The first system operates on a fluid-filled single channel and achieves relative flow speeds of u=u wave < 0:01. In contrast, the slugs and droplets are transported at two orders of magnitude higher speed equal to the wave speed (u=u wave ¼ 1) by exploiting the mechanowetting effect. We quantify the capillary forces generated by the traveling surface waves. Our method opens new avenues in light-driven (digital) microfluidic systems with enhanced control of fluid flow.

Directional droplet transport on switchable ratchets by mechanowetting

“…4, i.e., This means that, when keeping d constant, the dimensionless force is strongest at Y = 90 • . This dependence is similar to that in our previous works de Jong et al (2020), De Jong et al (2021), de Jong et al (2019).…”

“…Compared to earlier work de Jong et al (2019de Jong et al ( , 2020, De Jong et al (2021), where we have investigated droplet and slug transport on traveling waves, we find a number of interesting differences. First, on waves, there are two (stable) possibilities for the relative droplet speed, which is usually 0 or 1, depending on the external forces (e.g., due to gravity or surface contamination) a droplet needs to overcome.…”

“…1. For sufficiently small values of A∕ ( < 0.1 ) and considering that the horizontal component gradient of the (piecewise linear) height function h∕ x ∝ A∕ , the force is proportional to the amplitude De Jong et al (2021), i.e., For larger values of A∕ the deformation under the droplet becomes so large, that the direct fluid displacement due to the activation of the sawtooth (i.e., ∼ 0.5Ad ) becomes significant, so that the actuation force no longer scales linearly with the amplitude. Additionally, the dependence of the forces on the contact angle (and the surface tension) follows directly from Eq.…”

“…Here, we use mechanowetting, i.e., the control of three-phase line motion of two-fluid systems (e.g., drops, slugs) by mechanically deforming surfaces. In previous work, we have shown that mechanowetting can be used to transport fluid slugs in channels (de Jong et al 2020;De Jong et al 2021), as well as individual droplets on surfaces, by creating mechanical traveling surface waves (de Jong et al 2019;De Jong et al 2021).…”

“…The introduction of stimuli-responsive materials such as azobenzene-enhanced polymers (White and Broer 2015), magnetic artificial cilia (Khaderi et al 2009;den Toonder and Onck 2013), responsive hydrogels (Kim et al 2011) and nanoporous materials with very high surface-to-volume ratio (Detsi et al 2011) has opened exciting new pathways to various applications, such as small cargo transport (Nistor et al 2016), soft robotics (Huang et al 2015;Li et al 2016;Diller et al 2014), magnetically driven fluid transport and swimmers (Khaderi et al 2009;Namdeo 2014), highprecision electrochemical actuators (Hai-Jun and Jörg 2010) and haptic feedback for Braille-type displays (Camargo et al 2012). However, it is very challenging to create traveling surface waves through coordinated action to break symmetry (Lv et al 2016;Gelebart et al 2017).…”