Among a growing body of mobile-assisted-language-learning (MALL) studies, vocabulary app studies have taken an unprecedented proportion. While previous research on mobile-assisted language learning has primarily focused on learning outcomes, content delivery, and user experiences of vocabulary apps, a limited number of studies have explored the underlying language learning theories and pedagogies in the design of apps. There is also a lack of understanding regarding the impact of app design on students' self-directed learning (SDL).

This study investigates the theoretical foundations and pedagogical approaches influencing the design of English vocabulary learning applications, specifically their impact on learners' self-directed learning behaviours in an out-of-classroom context. This study aims to fill the aforementioned gap by first conceptualising and theorising English vocabulary apps’ design with analysis of the features with relation to vocabulary learning strategies (VLS), language learning theories and pedagogies, categorising them into distinct four categories. Students’ self-directed vocabulary learning behaviours and outcomes were subsequently investigated through a quasi-experiment, daily self-reports, two questionnaires, and semi-structured interviews. The relationships between app design and learning outcomes, as well as the impact of self-directed technological usage on learning outcomes are analysed both quantitatively and qualitatively.

A purposive sampling strategy was employed to select three English vocabulary learning apps based on their download ranking and review ratings (Data sources: Qimai platform). The study included 61 participants: first-year non-English major undergraduates from mainland China who were preparing for a compulsory national English test. They engaged in a 90-day period of self-directed vocabulary learning. The collected data that is valid for further analysis consisted of 31 individuals’ data sets, including self-reported daily vocabulary learning forms (1317 reports), SDL questionnaires, apps usage perception questionnaires, and supplementary interviews.

The results reveal several key findings. Firstly, the analysis identified a total of 27 vocabulary learning features within the three selected English vocabulary learning apps. To facilitate vocabulary learning within the mobile-assisted context, Schmitt's Vocabulary Learning Strategies (VLS) taxonomy was developed and adapted. Moreover, these selected apps proved effective in teaching word form and meaning, although they lacked support for generative use.

The review of the three app designs demonstrated the incorporation of four prominent language learning theories: behaviourism, input-based emergentism, sociocultural theory, and information/cognitive processing theories.

Notably, students' learning outcomes and self-directed learning behaviours exhibited significant variations across different groups. The study delves into the relationships between app design and learning outcomes, as well as the impact of self-directed technological usage on learning outcomes.

Drawing from the findings, a proposed vocabulary learning app design model is presented, encompassing the VLS taxonomy, vocabulary learning features, pedagogies, and language learning theories.