Anti-lotus leaf effect: smearing millions of picoliter droplets on bio-inspired artificial lotus leaf

Compartmentalization of aqueous samples into micro-droplets enables to study single cells or detect single molecules by performing large number of independent experiments. However, as the droplet size becomes smaller, the surface energy becomes larger so that micro-channels or surface treatments are required to break droplets below the picoliter range, resulting in an increased experimental or chemical complexity. Herein, we study the spontaneous liquid film breakup observed after smearing a mother droplet on the surface of lotus leave reversal. As the film breaks and recedes, it is mechanically trapped in single wells formed by the natural bulges on the leaf and forms picoliter droplets, which we called anti-lotus effect droplet. The bio-inspired effect has the advantage of resisting capillary forces and releasing surface tension in the three-phase interface. Using soft-lithography and spin-coating, we obtain the bio-mimetic wells array with the corrugated surface analogous with the lotus leave, which enables a deeper liquid penetration during the smearing step that results in a more efficient filling of the wells. In the present experiments, we have obtained a regular array of 2 pL droplets with a density of 62,500 droplets/cm2, which equivalents to 5.4 million droplets on the surface a 96 well plate. This bio-inspired method of forming droplet will be a convenient way to operate with no chemical treatment and seal. It undoubtedly decreases the complexity of chip manufacturing and potential interference by chemical treatments. We believe that such a method contributes to the development of the efficient micro-droplet generator with a pure physical interface and thus is applicable to biological and medical analysis.